Neglected X-ray discovered polars: III. RX J0154.0-5947, RX J0600.5-2709, RX J0859.1+0537, RX J0953.1+1458, and RX J1002.2-1925
K. Beuermann, V. Burwitz, K. Reinsch, A. Schwope, H.-C. Thomas
aa r X i v : . [ a s t r o - ph . H E ] O c t Astronomy&Astrophysicsmanuscript no. 38598 © ESO 2020October 23, 2020
Neglected X-ray discovered polars: III. RX J0154.0–5947,RX J0600.5–2709, RX J0859.1+0537, RX J0953.1+1458, andRX J1002.2–1925
Beuermann, K. , Burwitz, V. , Reinsch, K. , Schwope, A. , and Thomas, H.-C. ⋆ Institut f¨ur Astrophysik, Georg-August-Universit¨at, Friedrich-Hund-Platz 1, D-37077 G¨ottingen, Germany MPI f¨ur extraterrestrische Physik, Giessenbachstr. 6, 85740 Garching, Germany Leibniz-Institut f¨ur Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany MPI f¨ur Astrophysik, Karl-Schwarzschild-Str. 1, D-85740 Garching, GermanyReceived 8 June 2020; accepted 6 October 2020
ABSTRACT
We report results on the ROSAT-discovered noneclipsing short-period polars RX J0154.0-5947, RX J0600.5-2709, RX J0859.1 + + + + M = . × − M ⊙ yr − to 9 . × − M ⊙ yr − for white dwarf masses between 0.61 and 0.82 M ⊙ , in agreement with predictions basedon the observed e ff ective temperatures of white dwarfs in polars and the theory of compressional heating. Our analysis lends supportto the hypothesis that di ff erent mean accretion rates appply for the subgroups of short-period polars and nonmagnetic cataclysmicvariables. Key words.
Stars: cataclysmic variables – Stars: magnetic fields – Stars: binaries: close – Stars: individual: RX J0154.0-5947,RX J0600.5-2709, RX J0859.1 + +
1. Introduction
The discovery of hard and soft X-ray emission and of strong cir-cular polarization of the close binaries AM Her, VV Pup, andAN UMa (Hearn & Richardson 1977; Tapia 1977) identifiedthem as a very distinct type of object, for which Krzeminski &Serkowski (1977) proposed the name “polar”. They contain amass-losing late-type main-sequence star and an accreting mag-netic white dwarf (WD) in synchronous rotation. Of the morethan 1200 cataclysmic variables (CVs) in the catalog of Ritter& Kolb (2003, final online version 7.24 of 2016,) 114 are con-firmed polars. Many of these were discovered by their X-rayemission, which dominates the bolometric luminosity in highaccretion states. The basic physical mechanism underlying thehard and soft X-ray emission was described by Fabian et al.(1976), King & Lasota (1979), and Lamb & Masters (1979).Increasing the still moderate number of well-studied polars willshed light on incompletely understood aspects of the physics ofaccretion (Bonnet-Bidaud et al. 2015; Busschaert et al. 2015),close binary evolution (Knigge et al. 2011; Belloni et al. 2020),the origin of magnetic fields in CVs (Briggs et al. 2018; Belloni& Schreiber 2020), and the complex magnetic field structure ofaccreting WDs (Beuermann et al. 2007; Wickramasinghe et al.2014; Ferrario et al. 2015).Our optical identification program of high-galactic latitudesoft ROSAT X-ray sources led to the discovery of 27 new po- ⋆ Deceased 18 Jan 2012 lars (Thomas et al. 1998; Beuermann et al. 1999; Schwope et al.2002). Twenty sources have been described in previous publica-tions. In this series of three papers, we describe the remainingseven sources. In Papers I and II, we presented comprehensiveanalyses of V358 Aqr ( = RX J2316.1–0527) (Beuermann et al.2017, Paper I) and of the eclipsing polar HY Eri ( = RX J0501.7–0359) (Beuermann et al. 2020, Paper II). Here, we presentshorter analyses of the remaining five sources RX J0154.0–5947, RX J0600.5–2709, RX J0859 + + Table 1.
Short names, epoch 2000 coordinates, high-state V -band mag-nitude, orbital period, and Gaia distance with 90% confidence errors.Short Optical position V P orb d Gaia , DR2
Name RA,DEC (2000) l, b high (min) (pc)J0154 01 54 00.9 –59 47 49 289 . , − . + − J0600 06 00 33.3 –27 09 19 233 . , − . + − J0859 08 59 09.2 +
05 36 54 223 . , + . + − J0953 09 53 08.2 +
14 58 36 220 . , + . + − J1002 10 02 11.7 –19 25 37 257 . , + . + −
1. Beuermann et al.: Neglected X-ray discovered polars III.
Table 2.
Journal of X-ray observations (Cols. 1–6), interstellar extinction (Cols. 7–9), and the results of X-ray spectral fits with the sum of amultitemperature thermal component and a single-blackbody soft X-ray component with k T bb1 (Cols. 10–12). The bolometric flux in Col. (11)is three times that of the single-blackbody fit (see Sect. 3.1). The cyclotron flux in Col. (13) is an estimate based on nonsimultaneous data. Theluminosities in Cols. (14–16) are calculated with the Gaia distances in Table 1. The luminosity ratio in Col. (17) is R L = L sx / ( L hx + L cyc ). Theblackbody temperatures in Col. (10) are quoted with errors. The errors are omitted for the derived parameters in Cols. (12-20). The accretion ratesin Cols. (18) and (19) were calculated for the WD mass in Col. (20). Abbreviations: RASS = ROSAT All Sky Survey with the PSPC as detector,R = ROSAT pointed mode, X = XMM-Newton, P = PSPC as detector, H = HRI, pn = EPIC camera with pn as detector, M = MOS1&2, hi = highstate, in = intermediate state, and lo = low state. Numbers in brackets indicate errors in the last digits. A colon indicates an uncertain value.(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20)Short Start date Instr. Exp. CR HR N H , gal N H , E B − V N H , ad k T bb1 f sx , bol f th , bol f cyc L sx L hx L cyc R L ˙ M x ˙ M x + cyc M Name State (s) (1 / s) (10 H-atoms / cm ) (eV) (10 − erg / cm s) (10 erg / s) (10 − M ⊙ / yr) ( M ⊙ )J0154 26 Nov 1990 RASS hi 138 0.346 − .
78 1.96 1.0 (5) 1.0 24 + − .
82 0.25 . − .
57 1.0 24 + − .
76 0.20 . − .
92 2.20 1.5 (5) 1.5 42 + − . . . . . . − .
86 3.79 3.1 (8) 2.8 38 + − J0953 7 Nov 1990 RASS hi 335 0.430 − .
72 3.11 2.2 (6) 2.2 46 + − J1002 22 Nov 1990 RASS hi 474 0.690 − . − . − .
93 3.96 2.9 (9) > ∼ . < ∼
50 1 . . . . > ∼ . < ∼
50 0 . . . . N H , int =
0. 2) Lower limit based on d ≥
813 pc. 3) Mean of bright phase. 4) Lower limits based on N H ≥ . × H-atoms cm − . Table 3.
Journal of time-resolved spectroscopic observations.Short Name Date Wavelength Res. Number Exp. Tel.(Å) (Å) spectra (s)J0154–59 23 Aug 1993 3500–9500 10 6 600 (1)17 Dec 1993 3500–9500 8 18 480 (2)3–5 Jul 1995 3500–5400 6 / +
05 13 Dec 1993 3500–9200 20 1 1200 (1)5–7 Feb 1995 3810–5556 6 140 trailed (3)5–7 Feb 1995 5560–9200 6 150 trailed (3)7–8 Feb 1995 4173–5071 1.6 54 trailed (3)J0953 +
14 13 Dec 1993 3500–9200 20 1 1200 (1)5–7 Feb 1995 3810–5556 6 66 trailed (3)5–7 Feb 1995 5560–9200 6 78 trailed (3)J1002–19 24 Dec 1992 3600–9120 15 1 1800 (1)26 Dec 1992–1 Jan 1993 3500–5389 6 21 600 (1)1 Mar 1997 3300–10500 15 11 600 (1)2 Mar 1997 4070–7200 6 10 600 (1)(1) ESO / MPI 2.2-m EFOSC2, (2) ESO 1.5m B and C spectrograph, (3)Calar Alto 3.5m TWIN, trailed spectra, exposure times 25 −
60 min, (4)spectral resolution 6 A for 1 . ′′ . ′′
2. Observations
In Table 1 we summarize characteristic parameters of our targetssuch as the positions of the optical counterparts, the high-state
Table 4.
Journal of time-resolved photometric observations.Short Name Dates No. of Total Bands Exp. Tel.nights hours (s)J0154–59 16–17 Sep 1993 2 6.3 V 150 (1)8–9 Jul 1995 2 6.4 V 120 (2)29–31 Aug 2015 2 5.5 Sloan r 120 (3)1–3 Sep 2015 2 5.6 grizJHK 90 (3)May 2016–Oct 2018 22 39,9 WL 60 (4)J0600–27 5 Feb 1995 4.7 V + WL 300 (1)Sep 2017–Jan 2019 30 53.9 WL 60 (4)J0859 +
05 15 Jan 1996 9.0 V 30 (1)Feb 2010–Jan 2015 22 38.1 WL 60 (5)Feb 2018–Jan 2019 7 12.2 WL 60 (4)J0953 +
14 4 Feb 1995 7.5 V 240 (1)19 Mar 2002 7.0 WL 180 (6)Jan 2010–Feb 2015 14 19.4 WL 60 (5)Feb 2018–Jan 2019 5 8.6 WL 60 (4)J1002–19 1 Feb 1995 2.0 V 150 (1)Feb 2010–Feb 2015 11 25.6 WL 60 (5)Jun 2016–Jan 2019 16 42.3 WL 60 (4)(1) ESO / Dutch 90 cm, (2) ESO / Danish 1.5 m, (3) MPI / ESO 2.2 m,GROND, (4) MONET / S 1.2 m, (5) MONET / N 1.2 m, and (6)Observatorio Astron´omico de Mallorca, 30 cm; WL = white light. V -band magnitude, the orbital periods derived in this paper, andtrigonometric distances from the Gaia DR2 (Gaia Collaboration,
2. Beuermann et al.: Neglected X-ray discovered polars III.
Brown et al. 2018; Bailer-Jones et al. 2018) . Finding chartswere acquired from the PanSTARRS data archive (Chamberset al. 2016) and are provided in Appendix B. All targets discussed in this paper were discovered as variablevery soft high galactic latitude X-ray sources in the ROSAT AllSky Survey (RASS) and observed subsequently with ROSAT inthe pointed phase. The RASS covered the entire sky within onehalf year, starting in July 1990. Any target within its actual view-ing strip was visited every 96 min for an exposure time of up to30 s. All X-ray observations of our targets are summarized inCols. 1–6 of Table 2. Columns 7–9 contain information on theinterstellar extinction, and Cols. 10–12 list results of the X-rayspectral fits that are discussed in turn below. The hardness ra-tio HR . − . . − .
28 keV (pulse height channels 11-41) and ∼ . − . S and H in the soft and hard bands, respectively. The hardnessratio was defined as HR = ( H − S ) / ( H + S ). The PSPC spec-tra of the polars in this paper are dominated by soft X-rays andhave large negative HR
1. Subsequent pointed ROSAT observa-tions were made with the PSPC or, after its shutdown due tothe small amount of counter gas left, with the High ResolutionImager (HRI). The HRI lacked energy resolution and was lesssensitive than the PSPC, but had a higher spatial resolution andan even lower background. Some information on the relative re-sponse of PSPC and HRI and on the energy flux per PSPC unitcount rate are given in Appendix A. An additional observationwith XMM-Newton and the pn and MOS detectors of the EPICcamera is available for J1002 (Ramsay & Cropper 2003).
Time-resolved spectrophotometry was acquired between 1992and 1997, using the ESO 1.5 m telescope at La Silla in Chilewith the Boller and Chivensspectrograph and the ESO / MPI 2.2 m telescope withEFOSC2. The spectra were taken with di ff erent spectral resolu-tions and wavelength coverage. We refer to spectra covering theentire optical range with a full width at half maximum (FWHM)of 10 −
20Å as low resolution, spectra with reduced wavelengthcoverage and FWHM ≃
6Å as medium resolution, and with aFWHM = .
6Å as high resolution. Trailed simultaneous blueand red medium-resolution spectra of J0859 and J0953 were ac-quired with the TWIN spectrograph on the 3.5 m telescope of theCentro Astronomico Hispano Alem´an Calar Alto, Spain. Table 3summarizes the observations.
Time-resolved photometry was acquired between 1993 and2019. Details are provided in Table 4. Most of the data weretaken in white light (WL), using the two robotic 1.2 m MONETtelescopes , MONET / N at the University of Texas McDonaldObservatory and MONET / S at the South African Astronomical https: // vizier.u-strasbg.fr / viz-bin / VizieR?-source = I / https: // ps1images.stsci.edu / cgi-bin / ps1cutouts https: // monet.uni-goettingen.de Observatory. All images were corrected for dark current andflat-fielded in the usual way. All times were measured in UTCand converted into barycentric dynamical time (TDB), using thetool provided by Eastman et al. (2010) , which also accountsfor the leap seconds. All times that enter the calculation of theephemerides are listed in Appendix C.As described in Paper I, we performed synthetic photometryin order to tie the WL measurements into the standard ugriz sys-tem. We defined a MONET-specific WL AB magnitude w , whichhas its pivot wavelength λ piv = r band. For awide range of incident spectra, the synthetic color | ( w − r ) syn | . . w ≃ r is correct within 0.1 mag, except for very red stars.
3. General approach
All five targets were identified as polars by one or more of thefollowing properties: (i) variable soft X-ray emission, (ii) opti-cal spectroscopic and photometric variability, (iii) optical emis-sion lines with skewed profiles caused by streaming motions,(iv) strong He ii λ ff accretion. Living long-term V -band lightcurves of J0859, J0953, and J1002 are available from the moni-toring program of Ritter CVs in the Catalina Sky Survey (Drakeet al. 2009) . In a high state, the bolometric luminosity L bol of a polar is dom-inated by soft and hard X-ray emission. Our targets emit intensequasi-blackbody soft X-rays and thermal hard X-rays. The hardcomponent originates from the post-shock cooling flow in com-petition with cyclotron emission. The peak temperature in theflow is about 30 keV (20 keV) for a WD of 0.75 M ⊙ (0.60 M ⊙ )in a pure bremsstrahlung model with a shock near the WD sur-face, but lower for a tall shock or a strong magnetic field (Woelk& Beuermann 1996; Fischer & Beuermann 2001). Soft X-raysarise from the complex reprocessing of the energy carried intothe WD atmosphere in the spot and its vicinity (Lamb & Masters1979; King & Lasota 1979; Kuijpers & Pringle 1982). The bolo-metric fluxes of both spectral components are di ffi cult to mea-sure because the XUV flux is severely degraded by interstellarextinction and e ff ectively inaccessible below about 0.1 keV. Ofthe hard component, the ROSAT PSPC catches only a glimpseand the detectors of the EPIC camera on board XMM-Newtoncover it still incompletely.The soft component was conventionally modeled with a sin-gle blackbody with a temperature k T bb1 . A model like this is agross simplification, however, as shown by the highly resolvedoptically thick XUV spectrum of the prototype polar AM Her,taken with the Low Energy Transmission Grating Spectrometer(LETGS) on board Chandra. An improved model involves tem-peratures k T bb , ranging from about 0.5 to 2 . × k T bb1 . In the case http: // astroutils.astronomy.ohio-state.edu / time / http: // crts.caltech.edu /
3. Beuermann et al.: Neglected X-ray discovered polars III. of AM Her, the single-blackbody fit underestimated the bolomet-ric energy flux by a factor of 3 . ± . ff er for individual po-lars, but in the absence of further information, we corrected thebolometric energy fluxes f bb1 , bol of our single-blackbody PSPCfits upward by a factor c sx = f bb1 , bol fromPSPC spectra is the tradeo ff between the blackbody temperaturek T bb1 and the interstellar absorbing column density N H in frontof the source, which leads to large correlated errors in the fitparameters. This problem is relaxed by recent progress in theconstruction of 3D models of the galactic extinction E B − V (e.g.,Schlafly & Finkbeiner 2011; Lallement et al. 2018) , the conver-sion of E B − V into N H (Nguyen et al. 2018), and the availabilityof Gaia distances (Gaia Collaboration, Brown et al. 2018; Bailer-Jones et al. 2018). Combined, they allow an educated guess of N H that may be more trustworthy than the result of a free PSPCfit, except for particularly well-exposed PSPC spectra.In addition to absorption by interstellar gas, the emerging X-rays may su ff er internal absorption by atmospheric and infallingmatter, which we assume only a ff ects the hard X-ray component.The internal absorber probably fluctuates in space and time, asituation that is only approximately described by the conceptof a partial absorber with a covering fraction f pc and an unab-sorbed fraction 1 − f pc . Because the quality of the PSPC spectrais only moderate, we opted for a simple model that includes (i) amultitemperature thermal hard X-ray component with an addedpartial-absorber feature and (ii) a single-blackbody soft X-raycomponent with the energy flux corrected upward by c sx =
3. Thethermal component approximates a cooling-flow model by in-cluding a Mekal or bremsstrahlung component with a fixed hightemperature of 20 keV and one or two fitted low-temperatureMekal components with temperatures between 0.2 and 2 keV.For consistency reasons, we adopted the same simple model forthe single XMM EPIC pn spectrum we considered. Our ownmore complex cooling-flow model (Beuermann et al. 2012) didnot give significantly di ff erent integrated energy fluxes. The fitsto the ROSAT PSPC spectra are not sensitive to di ff erent lev-els of internal absorption, while the fit to the XMM-NewtonEPIC pn spectrum of J1002 improves substantially when inter-nal absorption is added, as noted already by Ramsay & Cropper(2003). Part of this study was facilitated by the availability of Gaia DR2distances for practically all known polars (Gaia Collaboration,Brown et al. 2018; Bailer-Jones et al. 2018). We show in Fig. 1,the frequency distribution of polars as a function of the separa-tion z = d sin b from the Galactic plane, with d the distance and b the galactic latitude, for systems listed in the final 2016 onlineversion 7.24 of the catalog of Ritter & Kolb (2003) . In PaperII and in the present paper, we found that HY Eri, J0600, andJ1002reside at | z | ≃
400 pc, raising the question whether theymight be halo objects. The three objects are marked by arrowsin the left panel of Fig. 1, and we conclude that their distancesfrom the plane are entirely compatible with their being members https: // irsa.ipac.caltech.edu / applications / DUST / http: // stilism.obspm.fr / The unusual magnetic CVs discovered by Hong et al. (2012) in awindow 1 . ◦ Fig. 1.
Left:
Frequency distribution of polars perpendicular to the galac-tic plane based on the distances in the Gaia DR2 for the stars listed in thefinal 2016 version 7.24 of the catalog of Ritter & Kolb (2003). The redhistogram shows all polars, and the blue histogram shows long-periodsystems with P orb > .
12 d.
Right:
Distribution of the absolute G-bandmagnitude. Arrows indicate individual objects discussed in the text. of what may be a single population of polars. Its standard devi-ation is σ z =
292 pc; the possible long-period polar V479 And(Gonz´alez-Buitrago et al. 2013) was excluded. When the distri-bution is split into short-period and long-period polars, the stan-dard deviations become 288 pc and 311 pc, respectively. Thesedistributions may obviously be a ff ected by an increasing incom-pleteness at higher d and | z | . Nevertheless, the value of 288 pcis compatible with the scale heights of 260 pc and 280 pc forshort-period magnetic CVs (mCVs) advocated by Pretorius etal. (2013) and Pala et al. (2020), respectively, but the value of311 pc disagrees with the 120 pc assigned to young (i.e., long-period) systems by Pretorius et al. (2013). The right-hand panelshows the distribution of the Gaia DR2 absolute G-band mag-nitudes, which cluster between M G = ff erences in the sys-tem parameters, notably the instantaneous accretion rate. Again,V479 And deviates from the well-defined sample of polars. We calculated the bolometric luminosity of component x as L x , bol = η π d f x , bol , where d is the distance, f x , bol the respec-tive bolometric energy flux, and η is a geometry factor, whichis η = θ , the geometry factor is η = / cos θ .Because θ is only approximately known, we used a conservativemean η = .
5, although Heise et al. (1985) argued for an ’emit-ting mound’ with η ≃ η -values in Table 2 ofBeuermann et al. 2012). We used η = η = c sx = L bol ≃ π d (1 . c sx f bb1 , bol + f th , bol + f cyc , bol ) ≃ G M ˙ M / R (1)and equated it to the gravitational energy released by matteraccreted from infinity at a rate ˙ M by a WD of mass M andradius R . We added the X-ray luminosity and an estimate ofthe cyclotron luminosity in an attempt to describe the accretion-induced luminosity. We disregarded the stream emission.The e ff ective temperature T of a su ffi ciently old accretingWD is thought to be largely determined by compressional heat-ing (Townsley & G¨ansicke 2009). This theory relates the equilib-
4. Beuermann et al.: Neglected X-ray discovered polars III.
Fig. 2.
2D representations of the phase-resolved He ii λ β spectra of J0859 (FWHM 1.6Å) and the H α spectra of J0953 (FWHM6Å); the flux increases from white to black. The NEL component isprominent over one half of the orbit centered on the superior conjunc-tion of the secondary star. The data are shown twice for better visibilityof the orbital structure. The ordinate is spectroscopic phase with φ = rium temperature T eq to the long-term mean accretion rate h ˙ M i in units of 10 − M ⊙ yr − by T eq = h ˙ M i / ( M / . M ⊙ ) K . (2)We here derived the accretion rate ˙ M for a measured or adopted M and quote the temperature T the WD would have if ˙ M wereidentified with h ˙ M i . We discuss to which extent the accretionrates derived by us fit into the general picture of short-periodpolars. No simultaneous X-ray and UV or optical observations are avail-able for the present targets. We therefore constructed the UV-optical-IR spectral energy distributions (SED) from all avail-able (nonsimultaneous) data in order to obtain an overview thatwould enable us to pick appropriate pairs of X-ray and op-tical flux levels. The upper envelope to the SED is taken asa measure of the UV-optical-IR flux in a high state of accre-tion. In favorable cases, the lower envelope provides informa-tion on the contributions by the stellar components. We col-lected the data using the Vizier SED tool provided by theCentre de Donn´ees astronomiques de Strasbourg . We searchedthe Galaxy Evolution Explorer (GALEX, Bianchi et al. 2017),the Sloan Digital Sky Survey (SDSS, Aguado et al. 2018) ,the Pan-STARRS Data Release 1 (Chambers et al. 2017), theSkyMapper catalog (Wolf et al. 2019), the Gaia catalog (GaiaCollaboration, Brown et al. 2018), the Two Micron All SkySurvey (2MASS, Skrutskie et al. 2006), the UKIRT InfraredDeep Sky Survey (UKIDSS, Lawrence et al. 2007), the VISTACatalog (McMahon et al. 2013), the Wide-field Infrared Survey(WISE, Cutri et al. 2012,2014), the PPMXL catalog (Roeser http: // vizier.unistra.fr / vizier / sed / Data Release 15, http: // / dr15 et al. 2010), and the NOMAD catalog (Zacharias et al. 2005).Harrison & Campbell (2015) studied the light curves of our tar-gets in the WISE W1 and W2 bands. SPITZER Space Telescopedata of J0154 were discussed by (Howell et al. 2006). Polars display complex Balmer and helium emission lines.Schwope et al. (1997) distinguished three components, a narrowemission line (NEL) from the heated face of the secondary star, abroad base component (BBC) from the magnetically guided partof the accretion stream, and a high-velocity component (HVC)from the ballistic part of the accretion stream in the orbital plane.The small velocity dispersion of the NEL represents the distri-bution of emission from the static stellar atmosphere, while thewidths of the two other components reflect the internal veloc-ity variation of the accelerating stream. Fig. 2 shows the trailedspectra of the emission lines of He ii λ β in J0859and of H α in J0953, taken on 7-8 and 5-7 February 1995, re-spectively. The lines are shown in the NEL (binary) phase withred-to-blue crossing at φ = .
5. As expected for a su ffi cientlyhigh inclination, the NEL is visible approximately from quadra-ture over superior conjunction to quadrature. The HVC crossesthe NEL near spectroscopic phase 0.50. The fuzzy excursions tohigh positive and negative velocities result from the combinedaction of HVC and BBC. The NEL for the other three targetsin our sample is well resolved in J1002, is perhaps marginallydetected in J0154, and remains undetected in J0600. We mea-sured the radial velocity for systems with resolved NEL by fit-ting a single Gaussian. For the combined BBC and HVC withits complex profile, we used the centroid of the cursor-definedfull extent of the line near zero intensity, which su ffi ces for ourpurpose because no quantitative argument is based on the mea-sured broad-line velocities. For the two systems in which theNEL was not resolved, we measured radial velocities by fittingsingle Gaussians to the total line profiles.The observed velocity amplitude K ′ of the NEL representsthe centroid of the emission from the secondary star and itsFWHM the distribution of the emission over the star. Theseneed not be the same for the NEL of di ff erent species (e.g.,Schwope et al. 2000). Transforming the observed K ′ into thevelocity amplitude K of the center of mass of the secondarystar therefore requires a model of the emission in the respectiveline. There is evidence that metal lines with low-ionization po-tentials are best suited to trace the secondary star (Schwope etal. 2000; Beuermann & Reinsch 2008; Beuermann et al. 2020).For this pilot study, we disregarded these complications and ap-plied the irradiation model BR08 (Beuermann & Reinsch 2008),which was devised for CaII λ ii λ K and a mass-radius re-lation R ( M ) of the Roche-lobe-filling secondary star, we calcu-lated the system parameters as a function of the unknown incli-nation i . We used the radii of main-sequence stars of solar com-position and an age of 1 Gyr of Bara ff e et al. (2015, henceforthBHAC) for masses between 0.072 and 0.200 M ⊙ , representedby a power law R BHAC / R ⊙ = A ( M / M ⊙ ) B with A = .
831 and B = . R = R BHAC f , where f = f f f and f = .
020 accounts for expansion by magnetic activity andspot coverage, f = .
045 for tidal and rotational deformation ofthe Roche-lobe-filling star, and f ≥ .
5. Beuermann et al.: Neglected X-ray discovered polars III.
Fig. 3.
Left:
WL light curve of the persistent one-pole system J0953from Kanbach et al. (2008). The accretion spot is visible for 60% ofthe orbital period, and the central depression is created preferentiallyby cyclotron beaming.
Right:
Derived light curves depicting the e ff ectof reduced beaming (red) and of adding a second accretion spot (blue).The latter mimics the light curves observed at times in J0859 and J1002. as functions of f and the inclination i for a given K ′ . With R and d known, the i -band magnitude of the secondary star isobtained, using the calibration of the i -band surface brightness S i = i AB + R / R ⊙ / d pc +
1) that we established as a functionof color or spectral type in Paper II. For spectral types dM4 todM8 in steps of one subclass, S i ≃ .
6, 8.2, 8.7, 9.2, and ∼ i -bandmagnitudes and those predicted at the Gaia distance agree well.None of the systems discussed in this paper is eclipsing, andobtaining information on i is often based on circumstantial evi-dence. If the primary accretion spot at colatitude β su ff ers a self-eclipse for a phase interval ∆ φ , the angles i and β are related bytan i tan β = / cos( π ∆ φ ) . (3)The colatitude ζ of the field vector in the spot usually exceeds β somewhat, but accounting quantitatively for the di ff erence re-quires a closer study. The emitted cyclotron radiation is mostintense perpendicular to and minimum along the field direction(cyclotron beaming). The cyclotron minimum is more than 10 ◦ wide, and we did not di ff erentiate explicitely between ζ and β . The shape of the minimum may be modified by ff - and bf-absorption in the infalling matter. For i > β , a narrow absorptiondip may occur when the line of sight crosses the magneticallyguided part of the accretion stream, or a wide depression, whenit is formed by an extended accretion curtain. The X-ray lightcurves are similarly shaped by geometric e ff ects and photoab-sorption. The interpretation of the optical light curves of noneclipsing po-lars varies from simple to complex. As a simple case, we showin Fig. 3 (left panel) the binned WL light curve of the stable one-pole emitter J0953 observed by Kanbach et al. (2008). The ac-creting pole is visible between orbital phases − .
30 and + . , and the light curve is shaped primarily by cyclotron beamingwith a central minimum at the instant of closest approach of theline of sight to the accretion funnel, chosen here to define or-bital phase φ =
0. The solid red curve shows the expected lightcurve formed by the varying aspect of the accretion spot with-out the beaming e ff ect. Using the observed light curve (with itsfluctuations) as a model, we constructed more complex cases.Increasing the minimum value of | β − i | or the optical depth of the emission region reduces cyclotron beaming. An exampleis shown by the red curve in the right panel. Adding a secondemission region in the lower hemisphere that is less influencedby beaming and is, for example, phase-shifted by 200 ◦ producesthe blue curve, in which the three orbital minima can no longeruniquely be assigned to the two poles without independent in-formation. Circular spectropolarimetry can provide the requiredinformation, as demonstrated for the case of HY Eri in Paper II,but is not available in the present study. The blue curve mimicsthe light curve 1996 V of J0859 in Fig. 6 and the light curves of25 February 2018 and 13-14 December 2010 of J1002 in Fig. 8.For each of our targets, we searched for an orbital featurethat reliably marks the orbital period. A detected period was ac-cepted as the orbital one if it agreed with the period of the radial-velocity variation, preferentially of the narrow component. Thespectroscopic and photometric periods agreed in all cases withinthe uncertainties and all objects were accepted as synchronizedpolars, in part with tight margins for a possible remaining asyn-chronism. The errors of the derived orbital periods are su ffi -ciently small to exclude alias periods over the time span of 30 yr,except for the faint object J0600, which exhibits a remaining un-certainty of one orbit in 150,000 cycles between 1995 and 2017.We applied several methods to determine the timings oforbital features. These included sinusoidal fits to light curves,Gaussian fits to the fluxes around minima, and graphical meth-ods. For instance, in double-humped light curves as that ofJ0953, we measured (i) the ingress to and egress from the brightphase individually, (ii) its center as the mean of the two tim-ings, or (iii) the position of the central minimum and chose theone that displayed the smallest long-term scatter. We determinedtimes of minima or maxima preferentially by the bisected-chordtechnique, marking the center between fall-o ff and rise at vari-ous intensity levels, and measuring the desired time and its errorfrom the mean and the scatter of the markings. Our approachminimizes timing errors for objects whose light curves are dis-torted by flickering or noise.
4. RX J0154.0–5947 (= J0154) in Hydrus
J0154 was discovered 1990 in the RASS as a moderately brightsoft X-ray source, spectroscopically identified by us as a polarand listed as such in Beuermann & Thomas (1993), Beuermann& Burwitz (1995), and Beuermann et al. (1999). With up to r = .
9, it is the brightest star in our sample and with a Gaiadistance of d = ± In the RASS, the star was detected with a PSPC count rateof 0 . ± .
05 cts s − and a hardness ratio HR = − . ± . − andan increased hardness ratio of − .
57, suggesting that only thesoft component had weakened. In an 11.2 ks HRI observation on3–7 January 1995, the system was found at a mean count rateof 0.080 HRI cts / s, which translates into about 0.64 PSPC cts s − (Table A.1), suggesting that the RASS observation representsonly a moderate high state. The count rates of both observationsin the bottom right-hand panel of Fig. 4 show little orbital varia-tion. Phases are from Eq. 4. The visible pole of J0154 had nearlystopped accreting, when XMM-Newton barely detected it with0.005(3) EPIC pn cts s − on 1 May 2002 (Ramsay et al. 2004).We fit the 1992 PSPC spectrum (not shown) with the X-rayspectral model described in Sect. 3.1. The fit gave k T bb1 =
24 eV,
6. Beuermann et al.: Neglected X-ray discovered polars III.
Fig. 4.
RX J0154.0–5947.
Left, top:
Mean spectra of of the observations on 23 August 1993 and 24 November 1995.
Second from top: He ii λ Third from top: He ii λ Bottom: O − C diagram for the times of orbital minimum. Right, top:
Overallspectral energy distribution (see text).
Bottom set of four panels:
Spectral flux in the B band of 17 December 1993 with peak magnitude B = . r light curve of 29 August 2015 with peak brightness r = .
9, WL relative flux on 5 May 2018, reaching magnitude w = .
2, and ROSATX-ray light curves taken with the PSPC in the intermediate state of 1–2 July 1992 and with the HRI in the high state of 3–7 January 1995. All lightcurves are phased on the ephemeris of Eq. 4. N H = . × H-atoms cm − , and a blackbody flux f bb1 , bol thattranslates to f sx , bol = c sx f bb1 , bol = . × − (Table 2), with c sx = E B − V ≃ .
011 (Lallement et al. 2018) or about half the galac-tic value of 0.0195 (Schlafly & Finkbeiner 2011), which corre-sponds to N H = . × H-atoms cm − (Nguyen et al. 2018),supporting the PSPC fit. In the brighter but statistically inferiorRASS spectrum, the count rate of the soft component is higherby a factor of five, and the hard component remains about thesame. For the same blackbody temperature of 24 eV, the soft X-ray flux in Table 2 is raised by a factor of five as well. A soft X-ray variability, exceeding that of the hard X-ray component, wasseen also in other polars and taken as evidence for the conceptof blobby accretion (Kuijpers & Pringle 1982), which carries thekinetic free-fall energy into subphotospheric layers and releasesthe reprocessed energy as soft X-rays, independent of more ten-uous sections of the flow that pass through a free-standing shock,radiating hard X-rays. Time-resolved optical photometry of J0154 was performedover a time span of 25 yr. Its brightness was measured rela-tive to a comparison star located at RA(2000) = h m . s = − ◦ ′ ′′ , or 0 ′′ W and 256 ′′ N of the target,which has V = . B − V = .
62, Sloan r = .
61, and w − r ≃ .
08. All light curves of J0154 are characterized bya quasi-sinusoidal modulation with a period of 89 min (Fig. 4,lower right-hand panels). The star reached r = . w = . T min = TDB 2457263 . + . E . (4)
7. Beuermann et al.: Neglected X-ray discovered polars III.
The O − C diagram is shown in the lower left panel of Fig. 4. Thetentative period P orb = . d Time-resolved low-resolution spectroscopy was collected on 23August 1993, 17 December 1993, and on 24–25 November1995. The top left panel shows the slightly smoothed mean low-resolution spectra of August 1993 and November 1995. Theycorrespond to mean AB magnitudes r = . ii λ W (He ii λ / W (H β ) ≃ .
79 (Table 5) is typical of the high-state emission-line spectrum of a polar. The line profiles ex-tend to − + − , more in the Balmer than inthe helium lines, but they are all peculiar in showing very lit-tle orbital variation. Single-Gaussian fits to lines observed inDecember 1993, July 1995, and November 1995 gave veloc-ity amplitudes between 60 and 80 km s − , with very little vari-ation in the phasing. The radial-velocity curve of the He ii λ φ br = . β results are very similar. We show examples of the He ii λ ffi ce to identify the NEL component, althoughthe variation in the line peak in 1993 may be an indication of itspresence. Assuming that the line nevertheless relates to a fixedstructure in the binary system (as the illuminated face of thesecondary), these events define the spectroscopic or binary or-bital period, P sp = . d P in Eq. 4,on the other hand, represents the rotational period of the WD.The di ff erence is consistent with zero and limits any asynchro-nism to a level of 2 × − . Because identification of the NELis required to locate the secondary star, it may be rewardingto study this bright system at higher spectral resolution. Further insight into the properties of the system is obtained fromthe nonsimultaneous overall SED in the upper right-hand panelof Fig. 4, which shows the mean spectrum of August 1993 (blackcurve) along with a model spectrum (green curve) for an isother-mal slab of hydrogen at a temperature of 17000 K, a pressure of10 dyne cm − , and a slab thickness of 10 cm. The model fitsthe observed 1993 spectrum and is consistent with the 2MASSnear-IR fluxes (green dots) and the Spitzer IRAC fluxes at 3.6and 4.5 µ m reported by Howell et al. (2006) (blue dots). Weinterprete this spectrum as the signature of a luminous accre-tion stream. There is no evidence for the associated accretionspot, however, which is evidently located on the far side of thewhite dwarf and is permanently out of view. On the other hand,our grizJHK photometry of August and September 2015 (yellowtriangles and yellow square), photometry of SkyMapper (red),2MASS (green), and Gaia 2, VISTA, and WISE (cyan blue)show higher fluxes that are probably associated with a spot onthe near hemisphere that accretes only temporarily. Our opticaland X-ray observations suggest that it was active in November 1990, July 1992, January 1995, and August and September 2015,but not in August and December 1993, July 1995, and May 2002.The persistent stream emission when the near spot is inactive ex-plains the preponderance of negative radial velocities in the 1993line profiles in Fig. 4 by the plasma motion toward the unseenpole.The variability of the source is also indicated by the GALEXfar-UV and near-UV fluxes of 0.050 mJy at 0.153 and 0.231 µ mthat belong to a low state, consistent with representing the WD.No spectral signature of the secondary star is detected. For P orb =
89 min, the evolutionary sequence of Knigge et al.(2011) with M = . M ⊙ predicts a Roche-lobe-filling sec-ondary star with M ≃ . M ⊙ , R ≃ . R ⊙ , and spectraltype dM6.6. For this spectral type, the i -band surface brightnessof the secondary star is S i ≃ . i -band magnitude be-comes i = . d =
320 pc (Table 1). Its K -band magnitude would be 16.9 mag (0.116 mJy). The ex-pected flux distribution of the secondary star is shown in theupper right-hand panel of Fig. 4 (crosses and dotted line). Forthe Knigge et al. component masses, the orbital velocity of thesecondary star is υ =
458 km s − . For an assumed NEL ampli-tude of up to ∼
100 km s − , interpreted by our irradiation modelBR08 ( K / K ′ = . ◦ . Fora WD of 0.75 M ⊙ , the secondary star is moderately bloatedwith f = .
12. As long as the inclination cannot be tightly con-strained, similar models can be constructed with primary massesfrom M = .
50 up to the Chandrasekhar mass and inclinationsbetween 20 ◦ and 12 ◦ . A promising path for progress involves aspectroscopic measurement of the WD radius, and thereby itsmass, when J0154 lapses into a low state or a measurement ofthe inclination by the identification of the NEL.The RASS soft and hard X-ray fluxes of Table 2 with thegeometry factors of Sect. 3 give a high-state bolometric X-rayluminosity of 2 . × erg cm − s − and an X-ray based accre-tion rate of ˙ M x = . × − M ⊙ yr − for an adopted WD massof 0.75 M ⊙ (Table 2). The optical level of 1 September 2015probably represents a high state as well, and we estimate thatabout 5 . × − erg cm − s − Å − arises from cyclotron radia-tion. When it is included in the energy balance, the accretion raterises to ˙ M x + cyc = . × − M ⊙ yr − . If this rate equals the long-term mean, the expected WD temperature due to compressionalheating would be 10400 K, near the lower end of the observedtemperature range (Townsley & G¨ansicke 2009). Its 4600Å fluxwould be 0.040 mJy, close to the observed GALEX fluxes. Inthe 1995 HRI observation, the accretion rate could have reached˙ M x + cyc ∼ × − M ⊙ yr − .
5. RX J0600.5–2709 (= J0600) in Lepus
J0600 was discovered 1990 in the RASS as a bright and verysoft X-ray source. The most distant and optically faintest objectin our sample (Table 1) was spectroscopically identified by usas a polar and is listed as such in Beuermann et al. (1999). Itsorbital period is close to the bounce period of CV evolution andits secondary is therefore close to substellar.
In the RASS, J0600 was detected with a mean PSPC count rateof 0 . ± .
03 cts s − and a hardness ratio of HR = − . ± .
8. Beuermann et al.: Neglected X-ray discovered polars III.
Fig. 5.
RX J0600.5–2709.
Left, top:
Identification spectrum taken on 16 November 1995.
Left, center:
Balmer line radial-velocity curve fromphase-resolved spectroscopy on 4 March 1997.
Left, bottom: O − C diagram for orbital maxima (green) and minima (cyan). Right, top:
Spectralenergy distribution, showing the identification spectrum, and a summary of nonsimultaneous photometry (see text).
Right, center:
X-ray lightcurve taken in the RASS between 10 and 13 September 1990.
Right, bottom:
Light curves taken in WL on 5 February 1995 and 28 December2018. The photometric phase is from Eq. 5. (Table 1). The rather long exposure time of 568 s in 35 satellitevisits allowed the construction of an orbital light curve, whichshows no evidence for a periodicity. The center right panel ofFig. 5 shows the light curve folded over the orbital period ofEq. 5. Judged by the X-ray luminosity (Table 2), J0600 was in ahigh state during the RASS, and the lack of orbital modulationsuggests that the accretion spot was permanently in view. In a13.5 ks ROSAT HRI observation between 12 and 28 September1995, it was found in a low state with a count rate of 0.0023cts / s or a soft X-ray flux roughly a factor of 30 below that of theRASS.The RASS PSPC spectrum (not shown) is dominated by softX-rays and appears only moderately absorbed despite the largedistance of J0600. No spectrally resolved follow-up X-ray ob-servation is available. The unconstrained blackbody fit to theRASS spectrum prefers an unrealistic N H ≃ T bb ≃
75 eVwith large errors (2RXS, Boller et al. 2016). At the positionof J0600, the total extinction is E B − V = . N H = . × H-atoms cm − (HI4PI Collaboration et al.2016). The extinction in front of J0600 is E B − V = . ± . N H = (1 . ± . × H-atoms cm − (Nguyen et al. 2018). Adopting this value of N H , the PSPC fitwith the X-ray spectral model of Sect. 3.1 yields T bb =
42 eVand the X-ray fluxes and the accretion rate listed in Table 2. TheGaia distance for J0600 (Table 1) has a large error, and the lu-minosity and accretion rate are quoted for the 90% confidencelower limit of 813 pc and marked as lower limits.
The optical counterpart of J0600 was identified as a 19–20 mag periodically variable star in 4.7 h of WL photom-etry taken on 5 February 1995 with the ESO-Dutch 90 cmtelescope. Its brightness was measured relative to a compar-ison star C1 that has AB magnitude r = .
89 and is lo-cated at RA(2000) = h m . ′′
2, DEC(2000) = − ◦ ′ ′′ ,40 ′′ E and 8 ′′ S of the target (Fig. B.2). The best period ofthe quasi-sinusoidal variation was 0 . d / MPI 2.2 m tele-scope at La Silla, Chile, revealed J0600 as a polar (top leftpanel). Strong He ii λ W (He ii λ / W (H β ) = .
86 (Table 5) indicated a high stateof accretion. Further nine low-resolution spectra of 15 min ex-posure were taken on 4 March 1997, when the source was ata similar brightness level of 19 −
20 mag. The emission-line ra-dial velocities were measured by cross-correlating the individualspectra with the mean spectrum on a log λ scale. The line pro-files display the asymmetries characteristic of polars, but are notsu ffi ciently well resolved to allow the separation of individualline components. The radial velocities in the left center panelof Fig. 5 yielded a period of 0 . d
9. Beuermann et al.: Neglected X-ray discovered polars III. found. We derived a long-term ephemeris from the photometryof 5 February 1995 and WL photometry of 25 nights betweenSeptember 2017 and January 2019 that resulted in 31 additionalmaximum and minimum times each. All times are reported inTable C in Appendix C. We fit the data by a linear ephemeris forthe orbital maxima, T max = TDB 2458015 . + . E , (5)allowing for a shift of the minima relative to the maxima. Theskewed light curves have their minima on average at φ = . + . − . χ ν = . P orb = . d The top right panel in Fig. 5 combines all available data intoan SED that includes the low-resolution spectrum of the topleft panel. The two yellow triangles indicate the full range ofthe MONET / S WL photometry, the blue dots the GALEX UVfluxes, the red dots fluxes from the Pan-STARRS DR1, and thecyan-blue ones the WISE W1 and W2 fluxes, all obtained viathe VizieR photometry viewer. Haakonsen & Rutledge (2009)misidentified the 2MASS image of a bright star 26 ′′ WNW withJ0600 (Fig.B.2). The WISE images of the region show severalfaint sources near the target position , of which the brightest islocated 7 ′′ SW. Harrison & Campbell (2015) may have mistakenthis 15 mag object for J0600. The 2MASS images show a faintequivalent to the WISE object, but no source at the position ofJ0600. The 3 σ upper limits are 0.15, 0.22, and 0.28 mJy in the J , H , and K s bands, respectively, still permitting a broad humpthat extends over the entire near-IR band. The hump looks sus-piciously like the SED of a late M-dwarf, but the secondary starwould be much fainter at the Gaia distance. A possible interpre-tation involves optically thick cyclotron emission in a magneticfield of B < ∼
20 MG. The identification of the GALEX sourcewith J0600 seems trustworthy because of the close positionalcoincidence. The origin of the UV emission remains uncertain.Phase-resolved observations in the di ff erent wavelength bandscould resolve the open questions. Of all polars, only CV Hyi and V4738 Sgr (Burwitz et al.1997) have orbital periods shorter than J0600. With less than79 min, all three binaries fall below the bounce period P bounce = . ± . ffi ciency, as may beappropriate for polars. The secondary mass at P bounce is about0.06 M ⊙ (Knigge et al. 2011), and as a given system evolvesthrough this point, the accretion luminosity drops rapidly. Thehigh observed X-ray luminosity of J0600 suggests that it is stillapproaching the minimum. The secondary is then expected to https: // irsa.ipac.caltech.edu / applications / wise https: // irsa.ipac.caltech.edu / applications / / IM / interactive.html be a very late star or a brown dwarf. For example, a Roche-lobe-filling secondary of M ≃ . M ⊙ with f = .
15 wouldhave R ≃ . R ⊙ , a spectral type of dM8, and an i -band surfacebrightness S i ≃
10. For the 90% confidence lower limit to thedistance of 813 pc, it would have i ≃ . . µ Jy), and K = . µ Jy), which is far below the observed fluxes.The narrow emission-line component could not be identifiedin J0600. When K ′ would tentatively be equated to the observedvelocity amplitude of 99 km s − , this would imply an inclinationof 14 ◦ for M = . M ⊙ . Any primary mass between 0.5 M ⊙ andthe Chandrasekhar limit can be accommodated.The bolometric fluxes and luminosities of the X-ray compo-nents are listed in Table 2. When the cyclotron flux is includedin the ˙ M calculation, the accretion rate rises for an 0.75 M ⊙ WD only minimally to ˙ M x + cyc > ∼ . × − M ⊙ yr − , placing thestar at the upper end of the range of accretion rates found inshort-period polars (Townsley & G¨ansicke 2009). The equiv-alent equilibrium temperature of the WD would be 14100 K,implying a 4600 Å flux of the compressionally heated WD of0.014 mJy, somewhat below the observed minimum MONETWL flux. Measuring the WD temperature and radius spectro-scopically in a low state appears feasible.
6. RX J0859.1+0537 (= J0859) in Hydra
J0859 was discovered 1990 in the RASS as a very soft X-raysource and was spectroscopically identified by us as a polar. Itis listed as such in Beuermann et al. (1999). Its orbital periodof 143.9 min, which places it at the lower edge of the remnantperiod gap of polars, as defined by Belloni et al. (2020) andSchwope et al. (2020).
J0859 was detected in the RASS with a mean PSPC count rateof 0 . ± .
03 cts s − and a hardness ratio HR = − . ± . − , equivalent to about 0.07 PSPC cts s − (TableA.1 in Appendix A). This observation provided nearly completephase coverage. The lower right-hand panel in Fig. 6 shows the Table 5.
Equivalent widths of prominent emission lines in Å for thehigh-state spectra of Figs. 1 to 5. The letters b and f refer to the brightand faint orbital phase intervals, respectively.Name Date H δ H γ HeII H β HeII HeI H α +
05 10 Jan 03 27.1 23.3 20.2 33.9 4.3 9.5 33.20953 +
05 6 / Fig. 6.
RX J0859.1 + Left, top:
SDSS spectra of RXJ0859 +
05 (black curve) and the dM5 star SDSS J101639.10 + ff erence spectrum (black) shows faint cyclotron lines. Second from top:
Modelcyclotron spectrum (red) fit to the observed di ff erence spectrum (black). Third from top:
Mean radial-velocity curves of the NEL (green) andthe combined BBC and HCV components (yellow) of H β , H γ , and He ii λ Bottom: O − C diagram for the times of the primary minimum. Green dots show this work, and open circles show results from Joshi et al. (2020)(see text). Right, top:
Overall spectral energy distribution, built from nonsimultaneous data (see text).
Center panels:
V-band light curve of 15January 1996 and WL light curves of on 4 March 2018, 17 January 2019, and 1 April 2011. The dashed lines schematically separate the emissionsof the primary and the secondary pole.
Bottom panel:
Rosat X-ray light curves taken 1990 with the PSPC and 1996 with the HRI. Phases are fromEq. 6. two light curves, with the HRI count rate multiplied by a factorof seven. The X-ray and optical bright phases coincide and thecentral X-ray dip, near phase zero on the ephemeris of Eq. 6,probably marks the instance at which the line of sight to theWD passes through the magnetically guided part of the accre-tion stream. We define the soft X-ray flux in the bright phaseby the two higher of the three RASS points with a mean of0.30 PSPC cts s − . This value is entered into Col. (5) of Table 2and is taken to represent the high state of J0859. The HRI obser-vation of 1996 has a bright-phase count rate of 0.020 HRI cts s − ,which converts into possibly as much as 0.14 PSPC cts s − orhalf the RASS count rate, representing an intermediate state.The only X-ray spectral information available is that of theRASS. A blackbody fit yields T bb1 ≃
38 eV and N H ≃ . × H-atoms cm − . The extinction in front of J0859 is E B − V ≃ . ± .
008 (Lallement et al. 2018), about 3 / N H = (3 . ± . × H-atoms cm − (Nguyen et al. 2018). Thecolumn densities from the RASS and from E B − V are compatible.We fit the adjusted RASS spectrum (not shown) with the X-rayspectral model described in Sect. 3.1, accepting the N H value ofthe fit. The X-ray flux, the luminosity, and the derived accretionrate in Table 2 are as expected for a short-period polar (Townsley& G¨ansicke 2009). The optical counterpart of J0859 was identified as a polarby a low-resolution spectrum taken on 13 December 1993.We determined the orbital period in 6.1 h of continuous V -band photometry, using the ESO-Dutch 90cm telescope at LaSilla on 13 January 1996. The trail was picked up using the
11. Beuermann et al.: Neglected X-ray discovered polars III.
MONET / N and MONET / S telescopes in 20 nights between2010 and 2019. Photometry was performed relative to the starSDSS085908.57 + ′′ W and 101 ′′ Sof the target and has Sloan r = .
83 and V ≃ .
17. All lightcurves possess a photometric primary minimum with a widthof about 20 min. J0859 exhibits substantial variability, which isillustrated by the light curves of 1996, 2011, 2018, and 2019(center right-hand panel of Fig. 6). The primary minimum re-sults from cyclotron beaming and marks the phase in which theline of sight approaches the accretion funnel most directly. Theemission from the primary pole is visible for ∆ φ ≃ .
65, indi-cating a location in the upper (near) hemisphere of the WD. Thedashed lines added to the 1996 and 2019 light curves indicate thesurmised emission from the primary pole. Excess emission be-tween φ = .
35 and 0.65 likely originates from a second accretionspot in the far hemisphere (compare the blue model light curvein Fig. 3). The system reached a peak brightness of V = . T min = BJD(TDB) 2455246 . + . E . (6)The O − C diagram is displayed in the bottom left panel of Fig. 6(green dots). A di ff erent period was published by Joshi et al.(2020). Their ephemeris is based on seven minimum times addedas open circles to our O − C diagram, of which four timings of2015 agree perfectly with our data, while their three 2014 tim-ings are 6 min and more than 10 min early. Their published tim-ings still yield a most probable period that agrees with that ofEq. 6 within the errors, but their published period is a less likelyalias that involves a cycle count error of one orbit over one year. We obtained trailed medium- and high-resolution optical spec-tra of J0859 between 5 and 8 February 1995 with 6 Å and 1.6 ÅFWHM resolution, respectively, using the blue and red arms ofthe TWIN spectrograph of the 3.5 m telescope on Calar Alto,Spain (Table 3). With exposure times of 60 min, these spectraextend over a sizeable part of the CCD chip. As a consequence,the Meinel OH bands in the red arm do not subtract well, com-plicating the flux calibration. The Balmer and helium emissionlines in the blue spectra show well-defined narrow NEL andBBC + HCV components, with examples shown as gray plots inFig. 2. We measured their radial velocities and show the meanvelocities of H β , H γ , and He ii λ K ′ = ± − . The blue-to-red zero crossing occursat photometric phase φ br = − . ± .
01 and defines spectroscopicphase as φ sp = φ ph + .
06. It has its zero point bona fide at inferiorconjunction of the secondary star and represents the true binaryphase. The broad (BBC + HVC) component has a velocity ampli-tude K broad ∼
540 km s − with a γ ∼
83 km s − . Maximum posi-tive radial velocity is attained at photometric phase φ broad ∼ . W around 30Å for the Balmer linesand He ii λ W (He ii λ / W (H β ) ≃ .
60 (Table 5).We also show in Fig. 6 the SDSS spectrum of the dM5 star SDSS J101639.10 + ±
10 tofit the strength of the TiO bands of J0859. It defines the i -bandmagnitude and flux of the secondary star as i = . ± .
15 and0 . ± .
009 mJy, respectively. Subtracting the adjusted dM5spectrum reveals waves in the di ff erence spectrum that we in-terpret as cyclotron harmonics. The second left-hand panel fromthe top in Fig. 6 shows the cyclotron line spectrum obtained bysubtracting, in addition, a smooth representation of the summedcontinua of the WD, the stream, and the cyclotron component.The cyclotron line model (red curve) was calculated with the the-ory of Chanmugam & Dulk (1981) for a field strength of 36 MG,an angle θ = ◦ between the line of sight and the field, a plasmatemperature of 10 keV, and a thickness parameter log Λ =
2. Thefit identifies the observed humps as the emission in the fourthto sixth cyclotron harmonic. A dip centered at 5800 Å could bethe H α σ − Zeeman absorption trough in a field of 34 MG, whichmay correspond to the mean field in an accretion halo. Spectralstructure in the continuum shortward of 5000 Å is likely due tothe Zeeman absorption components of the higher Balmer lines.We do not confirm the occurrence of cyclotron emission lines inthis part of the spectrum suggested by Joshi et al. (2020).
The top right-hand panel of Fig. 6 shows the overall SED ofJ0859. It includes the SDSS spectrum (black curve) and theSDSS u -band flux (magenta dot). The Pan-STARRS data points(green), the NOMAD and PPXML data (red), and the GALEXdata of two epochs (blue) indicate the variability of the system,as do the two yellow triangles that describe the full range ofthe MONET WL measurements. The 1996 brightening (opensquare, peak value) may represent a rare event. An optical fluxlevel of ∼ . B =
36 MG, a plasma temper-ature of 10 keV, a large viewing angle, and a thickness parameterlog Λ > ∼
6, the theory predicts an optically thick cyclotron spec-trum that extends into the near-UV. At the lower end of the fluxscale, the SED of the dM5 star adjusted in the i band is shownby the dotted curve. A dM4 star adjusted in the same way wouldhave slightly lower IR fluxes. For a CV with P orb =
144 min, the evolutionary model of Kniggeet al. (2011) assumes that it entered the period gap from longerorbital periods, causing its bloated secondary star to return tothermal equilibrium ( f = .
0) and mass transfer to cease. Intheir model, M = . M ⊙ , R = . R ⊙ , and the spectral typeis dM4.0. The i -band surface brightness is S i = . i = . M ⊙ and the mass limit with inclinations between 75 ◦ and30 ◦ and either an unbloated secondary star of 0.21 M ⊙ ( f = . M ⊙ ( f = . ∆ φ ≃ .
30, in Eq. 3 of Sect. 3.5. The deep pri-mary minimum suggests that it is preferentially shaped by cy-clotron beaming, which leads to i ∼ − ◦ , and with themeasured K ′ , to M = . − . M ⊙ . If the wide X-ray dip at φ =
12. Beuermann et al.: Neglected X-ray discovered polars III. to i to 55 − ◦ and M to 0 . − . M ⊙ . The primary mass of0.75 M ⊙ preferred by Knigge et al. (2011) in their evolutionarysequence requires i ≃ ◦ . The derived inclination is consistentwith the visibility of the NEL for about half an orbit around supe-rior conjunction of the secondary star. There are ways to improveon i and M . In addition to a more accurate measurement ofthe variation in NEL, phase-dependent cyclotron spectroscopyor spectropolarimetry can provide information on i . Finally, abetter X-ray light curve may confirm or disprove the existence ofthe absorption dip and limit i . Alternatively, it should be feasibleto measure the temperature and radius of the WD spectroscopi-cally in a low state and thereby infer its mass.The accretion rate obtained from the X-ray fluxes in Table 2for M = . M ⊙ and ˙ M x = . × − M ⊙ yr − is typical of short-period polars, confirming that the RASS observation representeda high state. We estimated the cyclotron flux in Table 2 from theNOMAD and PPXML optical fluxes (red dots), which representa moderate high state as well. Including this component yields˙ M x + cyc = . × − M ⊙ yr − . Interpreted as the long-term meanaccretion rate, the WD would have an equilibrium temperatureof 10500 K and a spectral flux of 0.031 mJy at 4600 Å, consis-tent with the lower pair of GALEX points in Fig. 6 (upper rightpanel) representing the WD.
7. RX J0953.1+1458 (= J0953) in Leo
J0953 was discovered in the RASS as a soft X-ray source, spec-troscopically identified by us as a polar, and is listed as such inBeuermann & Burwitz (1995) and Beuermann et al. (1999). Thesystem shows comparatively weak He ii λ In the RASS, the star was detected in November 1990 with amean PSPC count rate of 0 . ± .
03 cts s − and a hardness ra-tio HR = − . ± .
07 (Table 1). The RASS light curve (Bolleret al. 2016) in Fig. 7, lower right-hand panel, revealed a well-defined bright phase with a mean count rate of 0 . ± .
03 cts s − .The phase convention refers to the center of the optical brightphase (Eq. 7). Because the flux goes to zero during the faintphase, the orbital mean RASS spectrum is readily scaled up-ward to that of the bright phase. The extinction in front of thesource is E B − V = . ± .
006 (Lallement et al. 2018) and cor-responds to N H = (2 . ± . × H-atoms cm − (Nguyen et al.2018). When this value is adopted, the spectral fit with the modelof Sect. 3.1 yields the bolometric X-ray fluxes and luminositieslisted in Table 2. In spite of the rather low equivalent width ofHe ii λ The orbital period of J0953 was measured on 4–6 February1995 by phase-resolved V -band photometry with the ESO-Dutch 90 cm telescope that extended over four consecu-tive orbital periods. Photometry was performed relative toSDSS J095309.26 + ′′ E and 162 ′′ N of the target and has AB magnitudes g = . r = .
55, and i = .
33. J0953 was observed in WL by Kanbach et al. (2008)in 2002 and repeatedly by us between 1995 and 2019 (Table 4).The light curves of J0953 (Fig. 3 and 7) are good examples of a single-pole accretor with strong cyclotron beaming. In searchof an appropriate fiducial mark for the period measurement, weopted for the center of the bright phase, calculated as the meanof the ingress and egress times. A linear fit to 18 center-brighttimes yielded the alias-free ephemeris T cb = BJD(TDB) 2455217 . + . E . (7)The bottom left panel of Fig. 7 shows the O-C diagram. All mea-sured times are listed in Table C.4 in Appendix C. We obtained trailed optical spectra of J0953 on 5–8 February1995, using the same setup as for the previous target (Table 3).In this case, the correction for the Meinel OH-bands was lessproblematic. We show the mean faint-phase spectrum and themean spectrum for intervals around the orbital maxima in Fig. 7,top left panel. The system reached r ≃ . . + i = .
12 astemplate and show its spectrum, adjusted by a factor of 160 ± i -bandmagnitude and flux of the secondary star as 20 . ± .
21 and0 . ± . α , seen more clearly in the di ff erence spectrum of bright andfaint phases (second left-hand panel from the top). The H α π and σ − components are undisturbed, while the σ + component co-incides with the uncorrected atmospheric B band. These rathersharp lines with the resolved π -components are of nonphoto-spheric origin and of a type usually referred to as halo lines(Ferrario et al. 2015, their Table 2). They occur in cool matterin the vicinity of the hot plasma emitting the cyclotron radiation.In V834 Cen, this is the free-falling pre-shock matter (Schwope& Beuermann 1990), and in BL Hyi more stationary matter atan uncertain location (Schwope et al. 1995). In both cases theobserved lines indicate the field strength in the vicinity of the ac-cretion region. With 19 MG, the derived field strength in J0953is rather low for a polar.We measured the radial velocities of the narrow and broadcomponents of the Balmer lines and show the results for H α in the third left-hand panel of Fig. 7. The narrow componenthas a velocity amplitude K ′ = ± − and a blue-to-redzero crossing at photometric phase φ br = . ± .
01, defin-ing spectroscopic phase as φ sp = φ ph − .
11, which is bona fidealso the true binary phase. The broad component has a some-what uncertain K broad ∼
920 km s − with γ broad ∼
400 km s − . Itreaches maximum positive radial velocity at photometric phase φ ph = . ± .
02, almost coincident with the closest approachto the accretion funnel, when the BBC is moving away from theobserver.The question of a possible asynchronism was raised byOliveira et al. (2020), who considered J0953 as an intermedi-ate polar. Their argument was based on a single double-peakedspectrum, which they interpreted as originating from an accre-tion disk. The gray plot in Fig. 2 shows that J0953 may display
13. Beuermann et al.: Neglected X-ray discovered polars III.
Fig. 7.
RX J0953.1 + Left, top:
Combined blue and red flux-calibrated medium-resolution spectra at orbital maximum and minimum.
Secondfrom top:
Zoom into a di ff erence spectrum emphasizing the H α Zeeman features.
Third from top:
Mean radial-velocity curves of the narrow andbroad emission-line components of H α . Bottom: O − C diagram for the center of the bright phase of the optical light curves. Right, top:
Overallspectral energy distribution, involving nonsimultaneous data (see text).
Center:
Optical light curves taken in WL, taken in 1995, 2010, 2015, and2019, shifted by multiples of 0.25 units in the ordinate.
Bottom:
PSPC X-ray light curve taken in the RASS. The photometric phase is from Eq. 7. a double-peaked line profile near φ sp = . P n = . d P b = . d × − . Because the simple light curve did notchange over more than 20 years, however, this places the limit atlower than 10 − . The overall SED is shown in the upper right panel of Fig. 7.The observed spectra at orbital maximum and minium (blackcurves) and the MONET fluxes (yellow triangles) delineate therange of the orbital and temporal variability over the years. TheGaia, Pan-STARS, and a 2MASS J -band point (red dots) fallwithin this range. The GALEX point (blue), the SDSS photom- etry (cyan), part of the 2MASS data (green), and the WISE W1point (blue) belong to a low or an intermediate state.The SED of the adjusted dM6 secondary star is shown by thedashed curve. An earlier star, adjusted to the same i -band flux,would have lower IR fluxes. The secondary accounts for partof the red and IR flux observed in the SDSS (cyan), the 2MASS(green) and WISE (blue). The remaining flux in the IR representscyclotron radiation or stream emission. The low fluxes measuredin the SDSS (cyan) and with GALEX (blue) can be accountedfor by a WD of 0.75 M ⊙ with radius 7 . × cm and e ff ectivetemperature 12000 K, placed at the Gaia distance of 448 pc (bluecurve). Measuring the temperature and radius of the WD spec-troscopically should be feasible. At the orbital period of J0953, the evolutionary model of Kniggeet al. (2011) predicts a moderately bloated secondary star witha mass of 0.118 M ⊙ , radius of 0.161 R ⊙ , and a spectral typedM5.5 with an i -band surface brightness S i ≃ .
4. The predicted
14. Beuermann et al.: Neglected X-ray discovered polars III. i -band magnitude and flux at the Gaia distance are 20.62 and0.0205 mJy, respectively, in agreement with the observed quan-tities (Sect. 7.3). For further analysis, we converted the radial-velocity amplitude K ′ =
254 km s − into K with our irradia-tion model BR08. The well-defined length of the self-eclipse, ∆ φ ≃ .
40, places the accretion region in the upper hemisphereof the WD. The deep cyclotron minimum and the lack of an ab-sorption dip require i < ∼ β ≃ ζ (Sect. 3.5). Eq. 3 gives i ≃ − ◦ and the measured value of K ′ gives M ≃ . − . M ⊙ . Thestandard primary mass of Knigge et al. (2011) of 0.75 M ⊙ wouldrequire an inclination of 50 ◦ .The accretion rate derived from the X-ray luminosity is˙ M x = . × − M ⊙ yr − . The estimate of the cyclotron luminosityin Table 2 is based on the observed optical spectrophotometry,extrapolated into the near-IR. When it is included, the requiredaccretion rate rises to ˙ M x + cyc = . × − M ⊙ yr − for a WDof 0.63 M ⊙ . The corresponding equilibrium temperature of thecompressionally heated WD would be 10400 K. The predicted4600Å flux of the WD of 0.027 mJy agrees closely with thedereddened SDSS photometric g-band flux of 0 . ± .
02 mJy(Fig. 7, upper right panel, cyan dots). The SDSS ugr points andthe GALEX flux with 0 . ± .
03 mJy (blue dot) define a flatspectrum, which likely represents the magnetic WD. The agree-ment between predicted and observed spectral fluxes suggeststhat the current e ff ective temperature of the WD in J0953 and itsequilibrium temperature do not di ff er substantially.
8. RX J1002.2–1925 (= J1002) in Hydra
J1002 was discovered in the RASS as the brightest and softestX-ray source in our sample, spectroscopically identified by us asa polar, and it is listed as such in Beuermann & Thomas (1993),Beuermann & Burwitz (1995), and Thomas et al. (1998). Despiteits high degree of variability, it seems to be a tightly synchro-nized polar.
J1002 was detected in the RASS with a mean count rate of0 . ± .
04 cts s − and a hardness ratio HR = − . ± .
03, imply-ing that 98% of the photons had energies below the carbon edgeat 0.28 keV (Table 2). J1002 was reobserved with ROSAT andthe PSPC in 1992 and 1993 and with the HRI in 1995. In 1992,it was in a low state, with − . ± . − , andin 1993 again in a high state, with 0.71 PSPC cts s − and 97%soft photons. Ramsay & Cropper (2003) observed it with XMM-Newton in 2001 in an intermediate state. We reanalyzed their ob-servation for the present purpose. The lower left-hand panels ofFig. 8 show the orbital light curves of the RASS PSPC, the 1995ROSAT HRI, and the 2001 XMM-Newton EPIC pn and MOS12observations placed on the ephemeris of Eq. 8. Common prop-erties are a bright phase that lasts for ∼
75% of the orbital periodand a narrow absorption dip near its center. This repetitive fea-ture marks the instance when the line of sight to the WD passesthrough the magnetically guided part of the accretion stream.The XMM-Newton EPIC pn observation and the 1993ROSAT PSPC observation both cover exclusively the brightphase. The PSPC spectrum is not shown. A graph of the EPIC pnspectrum can be found in Fig. 7 of Ramsay & Cropper (2003).We fit both spectra with the model of Sect. 3.1. Table 6 summa-rizes the results. The model in line 1 is a moderately successfulfit to the pn spectrum with N H , int =
0. With the model in line 2,we confirm the findings of Ramsay & Cropper (2003) that the fit (i) prefers an interstellar column density close to zero and(ii) benefits from the inclusion of an internal absorber. A verylow value of N H is unrealistic, however, given the Gaia distanceof 797 pc. The total galactic column density at the position ofJ1002 is N H , gal = . × H-atoms cm − (HI4PI Collaborationet al. 2016) and the total extinction is E B − V = . E B − V ≃ . ± . N H = (2 . ± . × H-atoms cm − (Nguyen et al. 2018). Because of tradeo ff s between the param-eters N H and k T bb1 , reasonably good fits are obtained for anycolumn density up to N H , gal (lines 2 to 4). Fitting the 1993 PSPCand the 2001 pn spectrum with the same N H –k T bb1 combinationrequires N H = . × H-atoms cm − and k T bb1 =
50 eV (lines3 and 5). The blackbody fluxes for the two observations di ff erby about a factor of two, which is a measure of the di ff erentbrightness levels during the two runs. We adopted these fits, butconsider the fluxes of lines 3 and 5 in Table 6 as approximatelower limits and marked them as such in Table 2. The RASS observation has suggested a periodicity with P orb = . d P orb = . d ffi ce for an alias-free ephemeris, how-ever. Phase-resolved optical photometry and spectrophotometryof J1002 was performed in 1992, 1995, and 1997. We added WLphotometry with the MONET telescopes in 27 nights between2010 and 2019 (Tables 3, 4, and C.5). In the photometric runs,the brightness of the target was measured relative to a compar-ison star C1 located at RA(2000) = h m . s
4, DEC(2000) = − ◦ ′ ′′ or 5 ′′ W and 35 ′′ S of the target (Fig B.3). It has anAB magnitude r = . and colors g − r = .
66 and r − i = . w ≃ . https: // panstarrs.stsci.edu Table 6.
Fit parameters for the XMM-Newton pn and the ROSAT PSPCbright-phase X-ray spectra of J1002. The letter “f” denotes a frozenparameter. The quantity f sx , bol = c sx f bb1 , bol with c sx = f bb1 , bol thebolometric flux of the single-blackbody fit.Fit Detector N H N H , int f pc k T bb1 f sx , bol f th , bol χ (dof)(10 cm − ) (eV) (10 − erg / cm s)1 pn 2.15 38.1 2.05 0.11 81.9 (56)2 pn 0.01 544 0.69 53.8 0.53 0.26 45.6 (53)3 pn 1.00 f 536 0.69 50.1 0.86 0.27 46.4 (53)4 pn 2.90 f 520 0.69 43.2 2.34 0.29 49.1 (54)5 PSPC 1.00 f 100 f 1.0 f 50.0 1.78 0.22 33.7 (39)15. Beuermann et al.: Neglected X-ray discovered polars III. Fig. 8.
RX J1002.2–1925.
Left, top:
Flux-calibrated low-resolution spectra on 24 December 1992 and at orbital maximum and minimum on 1March 1997. Cyclotron harmonics are indicated.
Left, second from top:
Balmer-line radial velocities of the narrow and broad components fromspectra of 1 and 2 March 1997.
Left, third from top:
Spectral flux in the B band of 1 and 2 March 1997 in units is 10 − ergs cm − s − Å − . Left,next three panels:
Soft X-ray light curves of 1990, 1995, and 2001.
Right, top:
Overall nonsimultaneous SED.
Right, second from top:
Cyclotronspectra on 24 December 1992 and 1 March 1997 with models for a field strength of 33 MG.
Right, next four panels:
Samples of optical lightcurves taken in WL, illustrating the variability of primary and secondary minima.
Left, bottom: O − C diagram for the primary minimum, fromlight curves with peak orbital WL AB magnitude w < ∼ Right, bottom: O − C for the primary minimum vs. w . The photometric phase is fromEq. 8.16. Beuermann et al.: Neglected X-ray discovered polars III. common reference. They show that (i) the cyclotron-dominatedbright phase and the X-ray bright phase coincide, (ii) the primaryoptical minimum at φ ≃ φ ≃ .
5. The lightcurves in the right-hand panels are highly variable. The cases of18 February 2010 and 28 June 2016 suggest that either the pri-mary spot wanders in latitude or the minimum is filled up by theemission of an independent second accretion region. Repeatedlyover the years, we observed light curves in which the separationof the primary and the secondary minimum di ff ered from halfan orbital period (e.g., 13 +
14 December 2010 and 25 February2018). This apparent shift is probably caused by a second emis-sion region that appears near φ = . + ◦ (e.g., April–May 2018). The light curves of 4 January 2012 and 17 February2010 lack any trace of the primary minimum.Our search for a long-term ephemeris is based on a total offive X-ray dips and 70 times of optical minima, not all measuredfrom complete orbital light curves. Because there is no uniqueway to distinguish primary and secondary minima observation-ally, we started our search for a long-term ephemeris by con-sidering the mixed bag of primary and secondary minima andcalculating a periodogram in the vicinity of P orb /
2. This proce-dure yielded a unique (alias-free) ephemeris. We assigned cyclenumber E = P orb / P orb basisfor the subset of all primary minima with redefined cycle num-bers. We kept the definition of E = O − C values of all primary minima based onthe ephemeris of Eq. 8 in the bottom left panel of Fig. 8. Thisephemeris satisfies all minima of 1990–2001 and also selectedminima of 2010, 2014, 2016, and 2018 (green dots, shown witherror bars), but fails to meet other groups of minima of 2010,2017, 2018, and 2019 (yellow dots, shown without error bars toavoid clutter). The likely physical cause of the discrepant O − C values becomes clear from the bottom right panel, where weshow the same data plotted versus the brightness of the system,measured by the peak orbital WL magnitude w . In high stateswith w < O − C averages zero, while in intermediate or lowstates with w > ∼ O − C reaches up to ∼
12 min. Obviously, nolinear ephemeris can describe the up and down of O − C .Selecting the early data (five X-ray dips and four primaryoptical minima) and the seven primary minima from high-statelight curves with w <
18 defines the alias-free ephemeris T min = BJD(TDB) 2452254 . + . E , (8)which served as our reference and is represented by the dashedline in the bottom left panel of Fig. 8. The X-ray data repre-sent high states (ROSAT) or a moderately high state (XMM-Newton). The spectrophotometry of 1992 and 1997 is charac-terized by strong He ii λ w >
18 (yellow dots in the bottom panels of Fig. 8), supplemented by the seven timings of 1990–1997, but excluding the now discrepant 2001 XMM timings, weobtain the longest period compatible with part of the data, T min = BJD(TDB) 2452254 . + . E . (9)This ephemeris has an orbital period 4.9 ms longer than that ofEq. 8 and is represented by the dotted line in the bottom leftpanel of Fig. 8. Both ephemerides assign the same cycle num-bers to all minima of the 30 yr covered by our data. Despite theremaining uncertainty, our ephemerides are therefore alias-free.The times of all primary minima used for Eq. 8 or Eq. 9 are listedin Table C.5 in Appendix C. The high-state spot position is sta-ble over at least about 1.5 mag in w , and we argue that the periodin Eq. 8 more likely represents the true binary period.The O − C variations in J1002 are unlike anything observed insynchronized polars. Does J1002 lack synchronism? It showedno evidence for a shorter, intermediate-polar like periodicity.Between 1990 and 2001, X-ray period, optical photometric pe-riod, and spectroscopic period agreed, severely limiting the per-mitted degree of asynchronism. The data of 26 December 1992to 1 January 1993 and of 1 − . d . d − . ± . − . ± . . d O − C . We are therefore left with the classical explanations:(i) At a reduced accretion rate, the stream penetrates less deeplyinto the magnetosphere, the spot moves closer to the line con-necting the two stars, and the closest approach to the spot occurslater. (ii) At a reduced accretion rate, the stream switches from aballistic trajectory in the orbital plane to a magnetically guidedpath starting from the secondary star, a possibility considered inPapers I and II. (iii) In a complex field geometry, the stream maybe directed to di ff erent positions on the WD surface dependingon the accretion rate and the ram pressure it exerts. All polarsexperience drastic variations in the accretion rate, yet none hasso far displayed apparent or real spot movements similar to thoseobserved in J1002. Low- and medium-resolution phase-resolved spectrophotometrywas performed in December 1992 and March 1997 (Table 3).The top left panel in Fig. 8 shows the single spectrum of 1992(blue) and the 1997 spectra at orbital maximum and minimum(black). They are characterized by a blue continuum, strongBalmer and He ii λ ii λ β (Table 5) fallin the general area populated by polars (Oliveira et al. 2020, theirFig. 2). Furthermore, the bright-phase line ratio near unity indi-cates that J1002 was in a high state in December 1992 and March1997. The Balmer lines in the medium-resolution spectra of bothyears are fairly wide, as noted already by Oliveira et al. (2017).Their FWHM varies between 1100 and 2300 km s − over the or-bit. Balmer and helium lines consist of a broad component, rep-resenting a mixture of the BBC and HVC, and a well-defined
17. Beuermann et al.: Neglected X-ray discovered polars III. narrow component, representing the NEL. The second left panelin Fig. 8 shows the radial-velocity curves of the narrow com-ponent, measured from the 2 March 1997 spectra, and of thebroad component, obtained from 1 and 2 March. The narrowcomponent has a radial-velocity amplitude K ′ = ±
30 km s − ,with a blue-to-red zero crossing at photometric phase φ br , n = . ± . K broad = ±
47 km s − , reaching maximum positiveradial velocity at φ broad = . ± . φ sp = φ ph − . φ ph = ◦ ± ◦ in azimuth before inferior conjunction. Maximumpositive broad-line radial velocity occurs 4 ◦ ± ◦ before inferiorconjunction, but primary minima that are 10 min late occur atbinary phase φ sp ≃ .
05 or 5 min past inferior conjunction. Verysimilar numbers were obtained for 1992. Whether they still ap-plied between 2010 and 2019, when the pronounced O − C vari-ations occurred, remains uncertain.The spectrum of 24 December 1992 displays weak cyclotronlines superposed on the blue optically thick cyclotron contin-uum and the optically thin stream emission. Lines at the samepositions are also detected in the di ff erence between the 1997orbital maximum and minimum spectra. They are displayed inFig. 8 (second right-hand panels from the top) with an estimatedcontinuum interactively subtracted. They were fit by constant-temperature models calculated with the theory of Chanmugam& Dulk (1981) (red curves) for a field strength of 33 MG withk T e ≃ θ = − ◦ , and athickness parameter log Λ ≃ .
8. For these parameters, the linesrepresent the fourth to seventh harmonic. We cannot entirely ex-clude that the observed lines are harmonics 3 – 6 in a field of40 MG.
The top right-hand panel of Fig. 8 shows the overall SEDof J1002 that includes the 1 March 1997 bright and faint-phase spectra (black solid curves), a summary of nonsimulta-neous photometry, and a representation of the secondary starfrom our dynamical model presented below. The photometryis from GALEX (blue), the XMM-Newton UV monitor (ma-genta), Gaia, SkyMapper, and the XMM-Newton V monitor(red), PanSTARRS (green), VISTA (cyan), and ALLWISE W1(magenta). The full range of the brightness variations of our ex-tensive MONET WL observations (yellow triangles) spans a fac-tor of 50. None of the observations represents a low state, al-though such drops in brightness exist, as evidenced by the 1992ROSAT PSPC observation and the long-term light curve of theCatalina Sky Survey (Drake et al. 2009) , which shows a drop to >
20 mag at the end of 2007 from a general level of 18 ± At the orbital period of J1002, the evolutionary model of Kniggeet al. (2011) predicts a mildly bloated secondary star with a massof 0.108 M ⊙ , a radius of 0.153 R ⊙ , and a spectral type dM5.6.With an i -band surface brightness S i ≃ .
5, the i -band magnitudeand flux at the Gaia distance are 22.08 and 0.0053 mJy. It is notsurprising that the secondary star is not detected in the observedspectrum. The occurrence of the soft X-ray absorption dip re-quires that the inclination i of the system exceeds the inclinationof the accreting field line ζ in the accretion spot. In the optical http: // crts.caltech.edu / and X-ray light curves, the primary accretion spot is visible for3 / ∆ φ ≃ .
25, implying i > ◦ (Eq. 3 in Sect. 3.5). On the otherhand, if the deep minima in some light curves are produced bycyclotron beaming, i cannot be too large, suggesting i ≃ − ◦ .When we convert the observed NEL radial-velocity amplitudeof K ′ =
307 km s − into K with our irradiation model BR08, thecorresponding mass range is M ≃ . − . M ⊙ . For i = ◦ ,just avoiding an eclipse, the minimum primary mass consistentwith the measured K ′ is M ≃ . M x > ∼ . × − M ⊙ yr − . When the cyclotron fluxfrom the high photometric data points in the SED of Fig. 5 (seeTable 2 column 13) is included, the required accretion rate risesto ˙ M x + cyc > ∼ . × − M ⊙ yr − . Both ˙ M values are quoted aslower limits because they are based on an X-ray fit with a lower-than-standard interstellar absorbing column density (Sect. 8.1).Correspondingly, for the XMM-Newton observation in a mod-erately high or intermediate state complemented by an estimateof the cyclotron luminosity from the orbital mean of our spec-trophotometry, ˙ M x + cyc > ∼ . × − M ⊙ yr − . If either one ofthese rates equals the secular mean, the corresponding equilib-rium temperatures of the WD from compressional heating wouldfall between > ∼ > ∼ M ⊙ . Thepredicted 4600Å flux of the WD in the high state is 0.014 mJy,about a factor of two below the lowest MONET flux (yellowtriangle). A spectroscopic temperature measurement and massestimate of the WD in a low state appears feasible.
9. Discussion
In this last paper of a series of three, we report results on fiveROSAT-discovered polars collected over three decades. Papers Iand II (Beuermann et al. 2017, 2020) contained in-depth analy-ses of V358 Aqr and the eclipsing polar HY Eri. The results onthe present five objects are less complete, but they are accom-panied by accurate linear ephemerides, which allow the correctphasing of past and future observations. There is no evidence ofa variation in orbital period or for an asynchronism in any ofthe five targets, and we consider them as bona fide synchronousrotators.Two of our targets, J0154 and J0859, belong to the leagueof bright polars that reach 15 or 16 mag. At the other end ofthe brightness scale, the distant object J0600 resides near thebounce period and does not appear to exceed 19 mag. J0154 isthe second polar after VY For (Beuermann et al. 1989), in whichthe main active pole appears to be permanently hidden behindthe WD. Cropper (1997) advocated polarimetry of VY For toinvestigate its accretion geometry. Studying the brighter systemJ0154 may be more profitable.The evolution of polars di ff ers from that of nonmagneticCVs. The concept of reduced magnetic braking of Li et al.(1994) was devised to explain the e ff ective disappearance of theperiod gap for polars and was successfully employed by Belloniet al. (2020) in their binary stellar evolution code. Polars showat most a remnant gap, and systems with P orb < ∼
150 min be-have e ff ectively as short-period polars (Schwope et al. 2020).In the current sample, this applies to J0859. There is agreementthat all short period CVs su ff er larger angular momentum losses(AML) than predicted by gravitational radiation alone. Kniggeet al. (2011) included the additional AML as a numerical scal-ing factor, while Belloni et al. (2020) implemented the empiricalconsequential angular momentum loss eCAML of Schreiber etal. (2016) in their code. CAML describes an accretion-related
18. Beuermann et al.: Neglected X-ray discovered polars III. process, for instance, the time-averaged Bondi-Hoyle-type fric-tional energy loss that the secondary star experiences in the novashells that are expelled at intervals by the WD. Both authors ef-fectively raised the AML and thereby ˙ M in an attempt to bettermatch observed quantities as the bounce period or the space den-sity of CVs.The concept of an ’observed’ accretion rate was consideredquestionable for a long time, but was placed on firmer ground by(i) the advent of the Gaia trigonometric distances (Bailer-Joneset al. 2018) and (ii) the progress in constructing a 3D extinc-tion map for the solar neighborhood (Lallement et al. 2018),which helps to derive reliable optical and X-ray luminosities.Calculating the accretion rate requires knowledge of the WDmass. For three of our targets, we measured the radial-velocityamplitude K ′ of the narrow component of the Balmer lines,which is thought to originate on the irradiated surface of the sec-ondary star. Converting K ′ into the amplitude K of the centerof mass of the secondary star requires knowledge of the incli-nation i of the system and an irradiation model for the emissionline in question. In this pilot study, we have applied our irradi-ation model BR08 (Beuermann & Reinsch 2008; Beuermann etal. 2017) although it was not optimized for the Balmer lines andHe ii λ i have been noted in the relevantsubsections. The derived primary masses depend only weaklyon M because the stellar models and the moderate bloating inshort-period polars leave little freedom. All five systems, how-ever, are open for a more direct determination of M by a spec-troscopic measurement of the temperature and radius of the WDin a low state of the respective system.For five of the seven targets in Papers I-III of this series, wemeasured the field strength in the accretion region spectroscop-ically. The mean field strength for the present sample of 30 MGdoes not di ff er much from the 33.4 MG of the complete sam-ple of our 27 ROSAT soft and hard X-ray discovered polarsand the 38 MG of all polars in Table 2 of Ferrario et al. (2015).Measuring the WD temperature and thereby its radius requiresmodels of magnetic atmospheres. Such models were calculatedby Jordan (1992), but significant uncertainties related to Starkbroadening in the presence of a magnetic field remain and weemphasize again the need for calculations of the shifts of indi-vidual Stark components (see Paper II).The polars discussed in this paper feature X-ray spectrathat consist of a soft quasi-blackbody and a hard thermal X-ray component. Hard X-rays originate primarily in the coolingflow of the accretion stream that develops downstream of thestrong shock that is set up above the stellar surface. Its tem-perature distribution is determined by the competition betweenbremsstrahlung and cyclotron cooling and the associated radia-tive transfer (Woelk & Beuermann 1996; Fischer & Beuermann2001). Soft X-rays originate from the heated stellar atmosphereof the spot and its surroundings. Insight into the temperaturedistribution of the optically thick soft X-ray emission was ob-tained from the high-resolution spectrum of the prototype po-lar AM Her measured down to 92 eV (135 Å) with the LowEnergy Transmission Grating Spectrometer (LETGS) on boardChandra (Beuermann et al. 2012). The analysis revealed the ex-pected spread in temperature and demonstrated that modelingthe soft X-ray component by a single blackbody is a severe ap-proximation. In the case of AM Her, the single-blackbody modelunderestimated the bolometric flux of the XUV and soft X-raycomponent by a factor of 3 . ± .
7. We approximately accountedfor this e ff ect by raising the bolometric energy flux obtained fora single-blackbody fit by a factor c sx =
3. Townsley & G¨ansicke (2009) and Pala et al. (2020) reportedreliable e ff ective temperatures of nine polars and of 42 non-mCVs with P orb < . ±
570 K and 103 min for the polars and 14350 ±
370 K and92 min for the non-mCVs. According to the theory of compres-sional heating of the WD (Eq. 2 in Sect. 3.3), these tempera-tures translate into long-term mean accretion rates of h ˙ M polar i = . × − M ⊙ yr − and h ˙ M non − mCV i = . × − M ⊙ yr − , respec-tively, assuming a WD mass of 0.75 M ⊙ . The mean accretion rateof the five polars in Table 2 with a mean WD mass of 0.72 M ⊙ and a mean period of 103 min is h ˙ M x + cyc i = . × − M ⊙ yr − intheir normal high states. We consider the close agreement with h ˙ M polar i coincidental, but the general agreement supports our up-ward correction of the single-blackbody flux by the factor c sx = T bb = . c sx therefore appearsplausible, but in general, employing a multitemperature model ispreferable.Di ff erent mean accretion rates for short-period polars andnon-mCVs are currently not predicted by the evolutionary mod-els. Knigge et al. (2011) do not distinguish between the sub-types of CVs. In their best-fit model, they predict ˙ M = . × − M ⊙ yr − at 100 min for M = . M ⊙ , which correspondsrather to the temperature-derived value for polars than to thatfor non-mCVs. Belloni et al. (2020) explicitly accounted forthe reduced magnetic braking in polars, but at short orbital pe-riods or M < . M ⊙ , the fraction Φ of open field lines ofthe secondary star vanishes for a CV with M = . M ⊙ and B =
30 MG (Belloni et al. 2020, their Fig. 2). From that pointon, their code assigns the same AML to polars and non-mCVsand predicts ˙ M ≃ . × − M ⊙ yr − at 100 min, which is cor-rect for non-mCVs, but exceeds h ˙ M polar i . Our targets with knownfield strengths, J0859, J0953, and J1002, lie in the Φ = ff erently, but the physical cause remains elusive so far.The question of the energy balance between the soft and hardX-ray emission of polars was intensely debated at the time whenthe ROSAT and XMM-Newton soft X-ray measurements be-came available (e.g., Ramsay et al. 1994; Beuermann & Burwitz1995; Ramsay & Cropper 2004a). The background was the pre-diction by King & Lasota (1979) and Lamb & Masters (1979)that one-half of the accretion energy escaped as hard X-rays andthe other half was intercepted by the photosphere of the WD andreprocessed into soft X-rays. The high sensitivity of ROSAT forsoft X-rays and its very limited hard X-ray response led to thenotion that intense soft X-ray emission was one of the hallmarksof polars, and the luminosity ratio of soft to hard X-rays seemedto exceed unity by a large factor. The theoretical description ofshocks buried in the photosphere provided an understanding ofthe dominance of soft X-rays (Kuijpers & Pringle 1982). Theinternal absorption of hard X-rays (Ramsay & Cropper 2004a)showed that the hard X-ray flux had been severely underesti-mated and that cyclotron radiation needs to be included in theenergy balance of the post-shock cooling flow. Furthermore, thecase of AM Her showed that the soft X-ray luminosity had beenunderestimated as well. Taking all these caveats into account,the best estimate for AM Her is L sx / ( L hx + L cyc ) = . ± . L sx / ( L hx + L cyc ) = . ± .
1. Furthermore, at the
19. Beuermann et al.: Neglected X-ray discovered polars III. reduced accretion rate in intermediate and low states the temper-ature of the heated surrounding of the spot drops and the emis-sion moves out of the soft X-ray band, causing the system toturn into a (apparently) hard X-ray source (Ramsay et al. 2004;Beuermann et al. 2008; Schwope et al. 2020).We have analyzed five previously neglected polars and de-rived accretion rates based primarily on ROSAT PSPC data. Amodern instrument such as eRosita (Predehl et al. 2020) will pro-vide a more adequate data base for an up-to-date study of a largerpopulation of CVs.
Acknowledgements.
We thank the anonymous referee for a constructive andhelpful report that improved the presentation. Our fifth author, Hans-ChristophThomas, analyzed a large part of the early data before his untimely death. Most ofthe more recent photometric data were collected with the MONET telescopes ofthe Monitoring Network of Telescopes, funded by the Alfried Krupp von Bohlenund Halbach Foundation, Essen, and operated by the Georg-August-Universit¨atG¨ottingen, the McDonald Observatory of the University of Texas at Austin, andthe South African Astronomical Observatory. The spectroscopic and part of thephotometric observations were made at the European Southern Observatory LaSilla, Chile, and the Calar Alto Observatory, Spain. We made use of the SloanDigital Sky Survey (SDSS), the Two Micron All Sky Survey (2MASS), theWide-field Infrared Survey Explorer (WISE), the Galaxy Evolution explorer(GALEX), the PanSTARRS data base, and further sources accessed via theVizieR Photometric viewer operated at CDS, Strasbourg, France. We thank JanKurpas of the Astronomisches Institut Potsdam for producing the finding charts.
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20. Beuermann et al.: Neglected X-ray discovered polars III.
Appendix A: ROSAT count rates and energy fluxes
The polars discussed in this paper were discovered as soft highgalactic latitude X-ray sources in the ROSAT All Sky Survey(RASS). The survey was performed with the position-sensitiveproportional counter (PSPC) as detector, which possesses mod-erate energy resolution with an FWHM of 0.4 keV at 1 keV pho-ton energy. Subsequent pointed ROSAT observations were madewith either the PSPC or the High Resolution Imager (HRI). TheHRI lacked energy resolution. For quick reference, examples ofan integrated PSPC count rate of 1.0 cts s − converted into anHRI count rate are given in Table A.1 for selected incident black-body spectra and thermal APEC spectra absorbed by a columndensity N H of neutral matter of solar composition. They werecalculated with the NASA HEASARC tool PIMMS . For theblackbodies absorbed by N H < ∼ × H-atoms cm − , the PSPCis typically more sensitive than the HRI by a factor 7 ± . − . − . The unabsorbed flux is quoted in the sec-ond and the bolometric flux in the third line. For a temperaturek T bb1 =
60 eV and a low column density, the PSPC measures anenergy flux that is not far from the bolometric flux. For 20 eV,however, it recovers only a small fraction of the flux and the cor-rections become very large. https: // heasarc.gsfc.nasa.gov / cgi-bin / Tools / w3pimms / w3pimms.pl Table A.1.
HRI count rates equivalent to 1 PSPC cts s − for blackbodyand thermal APEC spectra with the quoted temperatures and neutralabsorbers N H in erg cm − s − for 1 PSPC cts s − . N H Blackbody APEC(cm − ) 20 eV 40 eV 60 eV 1.0 keV 3.1 keV 9.7 keV 27.3 keV1e19 0.112 0.127 0.143 0.319 0.242 0.270 0.2711e20 0.117 0.138 0.162 0.362 0.315 0.310 0.3133e20 0.129 0.163 0.203 0.392 0.360 0.352 0.3521e21 0.147 0.280 0.379 0.407 0.385 0.375 0.3731e22 0.494 0.350 0.377 0.388 0.362 0.346 0.346 Table A.2.
Blackbody energy fluxes in units of 10 − erg cm − s − forthree values each of k T bb1 and neutral absorber N H in H-atoms cm − nor-malized to produce 1 PSPC cts s − . The three lines refer to the absorbedand unabsorbed fluxes in the interval 0 . − T bb1 =
20 eV k T bb =
40 eV k T bb =
60 eVRange 1e19 1e20 3e20 1e19 1e20 3e20 1e19 1e20 3e20Absorbed 3.9 2.4 2.2 2.7 2.6 3.4 3.3 3.8 6.1Unabsorbed 5.3 24.3 88.2 3.3 8.1 32.0 3.6 7.6 24.1Bolometric 21.4 97.4 217.0 4.6 11.3 44.6 4.1 8.9 27.6
Appendix B: Finding charts in Sloan g
Fig. B.1.
Finding chart for RX J0154.0–5947. Size is 2 ′ × ′ . The com-parison star is 2 ′′ E and 256 ′′ N of the target and outside the image. 21. Beuermann et al.: Neglected X-ray discovered polars III.
Fig. B.2. L eft: Finding chart for RX J0600.5–2709. Size is 2 ′ × ′ . The comparison star is 40 ′′ E and 8 ′′ S of the target and marked C1. R ight: Finding chart for RX J0859.1 + ′ × ′ . The comparison star is 15 ′′ E and 162 ′′ N of the target and outside the image.
Fig. B.3. L eft: Finding chart for RX J0953.1 + ′ × ′ . The comparison star is 9 ′′ W and 101 ′′ S of the target and outside the image. R ight: Finding chart for RX J1002.2–1925. Size is 2 ′ × ′ . The comparison star is 5 ′′ W and 36 ′′ S of the target and marked C1.
Appendix C: Observed times in BJD (TDB)
22. Beuermann et al.: Neglected X-ray discovered polars III.
Table C.1.
Observed times of optical minima of RX J0154.0–5947transformed from UTC to BJD (TDB).Cycle BJD(TDB) Error O − C Expos Band Instr.2400000 + (min) (min) (min)-129778 49247.672100 1.4 1.8 2.5 V (1)-129777 49247.732600 1.4 0.0 2.5 V (1)-129776 49247.795000 1.4 0.9 2.5 V (1)-128306 49338.592000 4.3 -0.0 8.0 Spec (2)-119106 49906.855400 1.4 -1.1 2.0 V (3)-119105 49906.917700 1.4 -0.3 2.0 V (3)-119090 49907.844570 1.4 0.2 2.0 V (3)-119089 49907.906600 1.4 0.6 2.0 V (3)-116841 50046.760000 1.4 -0.1 5.0 Spec (2)0 57263.764246 0.7 -0.7 2.0 Sloan r (4)1 57263.825894 0.7 -0.9 2.0 Sloan r (4)2 57263.888474 0.7 0.3 2.0 Sloan r (4)32 57265.740385 0.7 -1.3 1.5 Sloan r (4)49 57266.790959 0.7 -0.6 1.5 Sloan r (4)50 57266.852630 0.7 -0.7 1.5 Sloan r (4)82 57268.830126 0.7 0.6 1.5 Sloan r (4)83 57268.892077 0.7 0.9 1.5 Sloan r (4)4434 57537.642297 0.7 -0.9 1.0 U (5)4514 57542.584319 0.7 -0.0 1.0 Sloan g (5)4515 57542.646022 0.7 -0.1 1.0 Sloan g (5)4580 57546.661802 1.0 1.2 1.0 WL (5)4595 57547.586244 0.9 -1.8 1.0 WL (5)4596 57547.648947 0.9 -0.5 1.0 WL (5)4675 57552.529255 0.9 0.5 1.0 WL (5)4676 57552.590257 0.9 -0.6 1.0 WL (5)4774 57558.644130 0.9 0.3 1.0 WL (5)11909 57999.356810 1.0 0.1 1.0 Sloan g (5)12105 58011.463446 1.0 0.3 1.0 WL (5)12168 58015.354383 1.0 -0.3 1.0 WL (5)12169 58015.416603 1.0 0.3 1.0 WL (5)12170 58015.477993 1.0 -0.2 1.0 WL (5)13544 58100.347313 0.7 0.4 1.0 WL (5)13546 58100.470187 0.7 -0.5 1.0 WL (5)17515 58345.626916 1.0 0.3 1.0 WL (5)18273 58392.447349 0.9 1.0 1.0 WL (5)18321 58395.411922 0.9 0.6 1.0 WL (5)18336 58396.338846 0.9 1.2 1.0 WL (5)18337 58396.399895 0.9 0.1 1.0 WL (5)18352 58397.327088 0.6 1.1 1.0 WL (5)(1) ESO / Dutch 0.9 m, (2) MPI / ESO 2.2m, EFOSC 2, (3) ESO / Danish2.5 m, (4) ESO 2.2 m, GROND, (5) SAAO MONET / S 1.2 m.
Table C.2.
Observed times of optical maxima ( m = +
1) and minima( m = −
1) for RX J0600.5–2709 transformed from UTC to BJD(TDB).A maximum and the following minimum have been assigned the samecycle number. The maxima define φ = φ = .
55. There may be a cycle count error by one orbit at E = − m Error O − C Expos Band Instr.2400000 + (min) (min) (min)-151194 49753.5883336 + − + − + m Error O − C Expos Band Instr.2400000 + (min) (min) (min)145 58023.4810547 + − + − + − + + + − − + − + + − + − + − + − − + − + − + − + − + + + − − + − − + + − + − − + − − + − + − + − + − − + − / Dutch 0.9 m, (2) SAAO MONET / S 1.2 m 23. Beuermann et al.: Neglected X-ray discovered polars III.
Table C.3.
Observed times of optical minima for RX J0859.1 + O − C Expos Band Instr.2400000 + (min) (min) (min)-51517 50097.645900 2.9 1.3 0.5 V (1)-51516 50097.744854 1.0 -0.1 0.5 V (1)-51515 50097.843176 2.9 -2.4 0.5 V (1)0 55246.837490 3.6 0.3 1.0 WL (2)2643 55511.009128 1.8 0.7 1.0 WL (2)2662 55512.908370 2.3 1.0 1.0 WL (2)2663 55513.008100 2.7 0.7 1.0 WL (2)2722 55518.904510 4.5 -0.4 1.0 WL (2)2723 55519.006940 2.9 3.2 1.0 WL (2)2841 55530.798980 1.4 -0.0 1.0 WL (2)2842 55530.898890 1.4 -0.1 1.0 WL (2)2872 55533.895720 2.7 -2.5 1.0 WL (2)2962 55542.893854 2.7 1.1 1.0 WL (2)3132 55559.885170 1.4 0.5 1.0 WL (2)4061 55652.740434 2.3 1.2 1.0 WL (2)6454 55891.925500 2.9 3.4 1.0 WL (2)16989 56944.910430 2.9 0.2 1.0 WL (2)18028 57048.758480 1.4 -1.8 1.0 WL (2)18058 57051.757280 1.4 -1.4 1.0 WL (2)29250 58170.412259 1.7 -1.7 1.0 WL (3)29369 58182.306969 1.4 -1.0 1.0 WL (3)32282 58473.466899 1.4 1.5 1.0 WL (3)32552 58500.452806 1.4 0.1 1.0 WL (3)32562 58501.453029 1.4 1.1 1.0 WL (3)(1) ESO / Dutch 0.9 m, (2) McDonald Observatory MONET / N 1.2 m,(2) SAAO MONET / S 1.2 m.
Table C.4.
Observed times of the center of the optical bright phase forRX J0953.1 + O − C Expos Band Instr.2400000 + (min) (min) (min)-75884 49752.612917 0.9 -0.4 4.0 V (1)-40714 52285.641017 0.4 -0.0 1.0 WL (2)-40713 52285.713156 0.4 0.2 1.0 WL (2)-39773 52353.413600 0.6 -0.7 3.0 WL (3)0 55217.961107 0.4 0.1 1.0 WL (4)388 55245.906033 0.4 0.4 1.0 WL (4)389 55245.977962 0.4 0.3 1.0 WL (4)444 55249.939017 0.4 0.0 1.0 WL (4)4193 55519.951229 0.4 0.4 1.0 WL (4)4430 55537.020418 0.4 0.2 1.0 WL (4)5803 55635.906705 0.4 -0.5 1.0 WL (4)9261 55884.959682 0.6 -1.2 1.0 WL (4)24353 56971.922940 0.6 0.5 1.0 WL (4)25616 57062.886273 0.6 -0.8 1.0 WL (4)41021 58172.391979 0.4 0.0 1.0 WL (5)41175 58183.482879 0.6 -0.7 1.0 WL (5)45730 58511.545461 0.4 0.0 1.0 WL (5)45758 58513.562168 0.4 0.2 1.0 WL (5)(1) ESO / Dutch 0.9 m, (2) Calar Alto 3.5 m OPTIMA (Kanbach et al.2008), (3) Observatorio Astron´omico de Mallorca, 30-cm, (4)McDonald Observatory MONET / N 1.2 m, (5) SAAO MONET / S1.2 m.
Table C.5.
Observed times of the X-ray dips and the primary opticalminima of RX J1002.2–1925 transformed from UTC to BJD(TDB). Theminima included in the fit for Eq. 8 have i =
1, those for Eq. 9 have i =
1. The O − C values are correlated with the WL AB magnitude w .Cycle BJD(TDB) Error i7 O − C i8 O − C Exp Band w Instr.2400000 + Eq. 8 Eq. 9(min) (min) (min) (min) (mag)-58114 48219.66788 1.9 1 -1.4 1 0.7 0.5 X (1)-47122 48982.81668 2.5 1 0.3 1 1.5 10.0 Sp (2)-36074 49749.85143 1.4 1 -1.0 1 -0.7 2.5 V (3)-34287 49873.91908 0.9 1 -0.1 1 0.0 0.9 X (4)-34286 49873.98852 0.9 1 -0.1 1 0.1 0.9 X (4)-25144 50508.69555 1.8 1 0.4 1 -0.2 10.0 Sp (2)-25130 50509.66776 2.5 1 0.7 1 0.1 10.0 Sp (2)0 52254.38165 0.7 1 -0.0 0 -2.7 0.3 X (5)0 52254.38211 0.7 1 0.6 0 -2.1 1.0 X (6)43087 55245.80718 1.7 1 0.7 0 -5.5 1.0 WL 16.69 (7)47381 55543.93358 2.9 0 7.2 1 0.6 1.0 WL 19.17 (7)47396 55544.97553 1.7 0 8.0 1 1.4 1.0 WL 19.09 (7)47754 55569.82950 1.4 0 6.4 1 -0.2 1.0 WL 19.07 (7)47757 55570.03700 1.4 0 5.3 1 -1.4 1.0 WL 18.76 (7)48157 55597.81030 1.9 0 8.6 1 1.9 1.0 WL 18.87 (7)48947 55652.65544 1.4 0 4.8 1 -2.0 1.0 WL 17.96 (7)68353 56999.96355 2.3 1 0.5 0 -7.9 1.0 WL 17.57 (7)76308 57552.25820 1.4 1 -1.7 0 -10.8 1.0 WL 17.60 (8)76309 57552.32788 1.4 1 -1.4 0 -10.4 1.0 WL 17.48 (8)76538 57568.22837 1.2 1 0.9 0 -8.1 1.0 WL 17.53 (8)76610 57573.22657 1.2 1 0.1 0 -9.0 1.0 WL 17.48 (8)80528 57845.24718 1.4 0 5.0 1 -4.3 1.0 WL 18.60 (8)85210 58170.31063 1.7 0 10.3 1 0.5 1.0 WL 18.84 (8)85282 58175.31047 1.7 0 11.8 1 2.1 1.0 WL 18.74 (8)85283 58175.37911 1.4 0 10.7 1 0.9 1.0 WL 18.78 (8)85958 58222.24217 1.7 0 9.9 1 0.1 1.0 WL 18.89 (8)85987 58224.25517 1.4 0 9.3 1 -0.5 1.0 WL 18.80 (8)86045 58228.28154 1.4 0 8.7 1 -1.1 1.0 WL 18.90 (8)86261 58243.28102 1.4 0 13.2 1 3.4 1.0 WL 18.95 (8)86277 58244.39114 1.4 0 12.2 1 2.3 1.0 WL 18.87 (8)86289 58245.22339 2.0 0 10.9 1 1.1 1.0 WL 18.77 (8)86290 58245.29243 2.0 0 10.3 1 0.5 1.0 WL 18.90 (8)89174 58445.51451 2.9 1 0.3 0 -9.8 1.0 WL 17.99 (8)89994 58502.45117 2.0 0 9.0 1 -1.1 1.0 WL 18.45 (8)90008 58503.42288 2.0 0 8.6 1 -1.5 1.0 WL 18.44 (8)90009 58503.49290 2.0 0 9.5 1 -0.7 1.0 WL 18.40 (8)90010 58503.56170 2.0 0 8.6 1 -1.6 1.0 WL 18.40 (8)(1) RASS PSPC, (2) MPI / ESO / / Dutch 0.9 m, (4) ROSAT HRI, (5) XMM-Newton EPIC pn,(6) XMM-Newton EPIC MOS12, (7) McDonald ObservatoryMONET / N 1.2 m, (8) SAAO MONET //