Polarimetric and spectroscopic optical observations of the ultra-compact X-ray binary 4U 0614+091
M. C. Baglio, D. Mainetti, P. D'Avanzo, S. Campana, S. Covino, D. M. Russell, T. Shahbaz
AAstronomy & Astrophysics manuscript no. 4U0614˙polarimetry˙sub˙arxiv c (cid:13)
ESO 2018October 7, 2018
Polarimetric and spectroscopic optical observations of theultra-compact X-ray binary 4U 0614+091 (cid:63)
M. C. Baglio , , D. Mainetti , , P. D’Avanzo , S. Campana , S. Covino , D. M. Russell , and T. Shahbaz , Universit`a dell’Insubria, Dipartimento di Fisica, Via Valleggio 11, I22100 Como, Italye-mail: [email protected] INAF, Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate (Lc), Italy Universit`a di Milano-Bicocca, Dipartimento di Fisica G. Occhialini, Piazza della Scienza 3, I-20126 Milano, Italy New York University Abu Dhabi, P.O. Box 129188, Abu Dhabi, United Arab Emirates Instituto de Astrof´ısica de Canarias (IAC), E-38200 La Laguna, Tenerife, Spain Departamento de Astrof´ısica, Universidad de La Laguna (ULL), E-38206 La Laguna, Tenerife, Spain
ABSTRACT
Aims.
We present a polarimetric and spectroscopic study of the persistent ultra compact X-ray binary 4U 0614 +
091 aimed at searchingfor the emission of a relativistic particle jet and at unveiling the orbital period P orb of the system. Methods.
We obtained r -band polarimetric observations with the Telescopio Nazionale Galileo (TNG) equipped with the PAOLOpolarimeter and with the Nordic Optical Telescope (NOT) equipped with the ALFOSC instrument, covering ∼ ∼ ∼ Results.
The polarimetric analysis performed starting from the TNG dataset revealed a polarisation degree in the r -band of 3% ± / N ratio, we could obtain only a 3 σ upper limit of 3.4%. From the joining of a spectroscopicand photometric analysis, through the study of the equivalent width variations of the CII 7240 Å line and the r -band light curve, wecould find a hint of a ∼
45 min periodicity.
Conclusions.
A polarisation degree P of ∼
3% in the r -band is consistent with the emission of a relativistic particle jet, which issupposed to emit intrinsically linearly polarised synchrotron radiation. Since no variations of P with time have been detected, and theaccretion disc of the system does not contain ionised hydrogen, scattering by free electrons in the accretion disc has been rejected.The period of ∼
45 min obtained through the analysis of the system light curve and of the equivalent width variations of the selectedspectral line is probably linked to the presence of a hot spot or a superhump in the accretion disc, and lead to an orbital period (cid:38)
Key words.
1. Introduction
About thirty ultra-compact X-ray binaries (UCXBs) candidateshave been identified to date (in’t Zand et al. 2007). These sys-tems are a subclass of low mass X-ray binaries (LMXBs) withvery short orbital period, typically (cid:46)
80 min. The compact natureof UCXBs implies that the companion star’s density is higherthan typical main sequence stars, and so can be excluded (Nelson (cid:63)
Based on observations made with the Italian Telescopio NazionaleGalileo (TNG) operated on the island of La Palma by the Fundaci´onGalileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at theSpanish Observatorio del Roque de los Muchachos of the Instituto deAstrofisica de Canarias and with the Nordic Optical Telescope, op-erated by the Nordic Optical Telescope Scientific Association at theObservatorio del Roque de los Muchachos, La Palma, Spain, of theInstituto de Astrofisica de Canarias and on observations made with ESOTelescopes at the Paranal Observatory under programme ID 079.D-0884(A). et al. 1986; Savonije et al. 1986). Because their sizes are compa-rable to the donor Roche lobe dimension, the most likely com-panion star candidates for UCXBs are white dwarfs or heliumburning stars (Nelson et al. 1986). This inference was later con-firmed through the observation of strong helium and carbon-oxygen lines in their spectra (Schulz et al. 2001; Nelemans et al.2004; Nelemans et al. 2006).The UCXB candidate 4U 0614 +
091 is a persistent LMXB(Forman et al. 1978) that has been identified in the optical with afaint ( ∼
18 mag), blue, variable star in the galactic plane (vanParadijs & van der Klis 1994). After the detection of Type Ibursts (Kuulkers et al. 2009), the compact object of the sys-tem was identified as a neutron star. Assuming the Eddingtonluminosity for the bursts, a first estimate of the system dis-tance was obtained ( < a r X i v : . [ a s t r o - ph . S R ] O c t . C. Baglio et al.: Polarimetric and spectroscopic optical observations of 4U 0614 + oxygen lines, but no hydrogen or helium, leading to the conclu-sion that the system possesses a carbon-oxygen accretion disc(Nelemans et al. 2004; Nelemans et al. 2006). The observationof a broad emission line associated to O VIII Ly α emission inthe X-ray band led to the identification of the companion starwith an oxygen-rich donor star (Nelemans et al. 2004). However,the observation of type I bursts remained unexplained, sincethe presence of high quantities of helium is needed to accountfor these thermonuclear reactions. To solve this puzzle Juett &Chakrabarty (2003) supposed that spallation reactions at the neu-tron star surface could split C and O nuclei into H and He whichcan then recombine again into heavier elements, explaining boththe absence of H and He in the spectra and the occurrence oftype I bursts.Several attempts have been made to determine the orbital pe-riod of the system. Modulations in the optical / NIR band can bedue to di ff erent phenomena, like the heating of one side of thecompanion by X-ray irradiation from the neutron star, but also tothe presence of superhumps or hot spots from the impact pointof the accretion stream with the disc. Firstly O’Brien (2005) re-ported a ∼
50 min orbital period from high-speed optical data ob-tained with ULTRACAM. A ∼
50 min period was also suggestedfrom Zhang et al. (2012) thanks to the observation of a quasi-periodic oscillation. Shahbaz et al. (2008b) found evidence forthree di ff erent periods based on optical photometry (42, 51.3 and64 min) with the 51.3 min being the clearest modulation. Hakalaet al. (2011) was not able to find any periodicity for the systemdespite their extensive observations. Madej et al. (2013) finallynoted a weak periodical signal modulated at a period of ∼
30 minin the red-wing / blue-wing flux ratio of the most prominent emis-sion feature at ∼ + +
091 particularly in-triguing, since synchrotron emission is known to be intrinsicallylinearly polarised (Rybicki & Lightman 1979).The paper is organised as follows: in Section 2 we presentthe results of the first optical polarimetric study ( r -band) per-formed on the persistent LMXB 4U 0614 + ff erentlystated.
2. TNG and NOT optical polarimetry
The system 4U 0614 +
091 was observed on January 27, 2013with the 3.6 m FGG TNG telescope at La Palma, equipped withthe PAOLO polarimeter. A set of 28 images of 240 s integrationeach was taken, with the optical r filter (6200 Å). The night wasclear, with seeing degrading with time (from 1 . (cid:48)(cid:48) to 1 . (cid:48)(cid:48) ). 4U0614 +
091 was then observed a year later (March 11, 2014) in thesame optical band with the ALFOSC instrument in polarimet-ric mode (using the Wedged Double Wollaston configuration, WeDoWo) mounted at the 2.5 m NOT telescope at La Palma,for a totality of 2 images of 900 seconds integration each. Imagereduction was carried out following standard procedures: sub-traction of an averaged bias frame, division by a normalized flatframe. Flux measurements have been performed through aper-ture photometry with daophot (Stetson 1987) for all the objectsin the field.Both the polarimeter PAOLO (Covino et al. 2014) and theWeDoWo device (Oliva 1997) consist of a double Wollastonprism (DW) mounted in the filter wheel, which produces foursimultaneous polarisation states of the field of view. The imagesare then separated by a special wedge, producing four imageslices on the CCD that correspond to four di ff erent position an-gles with respect to the telescope axis (0 ◦ , 45 ◦ , 90 ◦ and 135 ◦ ).Such images possess all the information needed in order to pro-vide linear polarisation measurements. This is a fundamental re-quirement for PAOLO, since the instrument is mounted on theNasmyth focus of the TNG, introducing varying instrumentalpolarization of the order of 2-3 % (Giro et al. 2003).The normalised Stokes parameters for linear polarisation, Q and U , are defined as follows: Q = f (0 ◦ ) − f (90 ◦ ) f (0 ◦ ) + f (90 ◦ ) ; U = f (45 ◦ ) − f (135 ◦ ) f (45 ◦ ) + f (135 ◦ ) , (1)where f corresponds to the measured flux of the source.An estimate of the observed linear polarisation degree of theincoming radiation can then be achieved from: P obs = (cid:112) Q + U , (2)which should be corrected for a bias factor (Wardle & Kronberg1974; di Serego Alighieri 1998) in order to account for the non-gaussianity of the statistic describing the polarisation distribu-tion. In particular: P = P obs (cid:115) − (cid:32) σ P P obs (cid:33) , (3)where σ P is the r.m.s. error on the polarisation degree.The polarisation angle θ can be obtained from the relation: θ = . − ( U / Q ) . (4) We considered first of all the set of images obtained in 2013with the PAOLO instrument. In order to reach a high S / N ratio,that is fundamental in polarimetry, we tried to estimate the linearpolarisation degree in the r -band starting from the average of allthe images with good seeing. In particular, we defined a limitto the seeing of (cid:46) . (cid:48)(cid:48) , which allowed us to use 12 images (48min).The Stokes parameters Q and U of all the objects in the fieldhave been evaluated by means of the instrumental polarizationmodel extensively described in Covino et al. (2014). Since thefield stars we chose as references cluster rather well around acommon value in the Q − U plane (Fig. 1), we could safely as-sume them to be intrinsecally unpolarised and that the interstel-lar polarisation for these bright stars in the field is probably low.This hypothesis is supported from the evaluation of the instru-mental contribution only to the polarisation of the field stars, thatwas possible thanks to the tools described in Covino et al. (2014)and that was consistent with the results obtained for the Stokes
2. C. Baglio et al.: Polarimetric and spectroscopic optical observations of 4U 0614 + Table 1.
Values of the Q and U Stokes parameters not correctedfor interstellar or instrumental e ff ects represented in Fig. 1 for4U0614 +
091 and for five reference field stars. The weightedmean of the reference stars Stokes parameters has been reportedin the last raw and corresponds to the amount of correction thatwe applied to Q and U of the target. Object Q (%) U (%)4U0614 + − . ± .
93 0 . ± . − . ± . − . ± . . ± . − . ± . . ± . − . ± . − . ± . − . ± . . ± . − . ± . . ± . − . ± . parameters of the chosen reference stars. In particular, the mod-elling of the instrumental polarization by means of observationsof a suitable set of polarimetric (polarized and / or unpolarized)standard stars has shown routinely a rms residual at about 0.2%or even better if the observations cover a rather limited range inHour Angles, as this is the case. This fact allowed us to correctthe values of Q and U of the target for the average Q and U obtained for the field stars, in order to account for the not neg-ligible e ff ects of instrumental polarisation. The amount of thiscorrection is reported in Tab. 1 as the weighted mean. Fig. 1. U vs. Q for the averaged image of the optical r filter,for 4U0614 +
091 (blue triangle) and for five reference field stars(black squares), Tab. 1. With a red dot we indicated the weightedmean of the reference stars Stokes parameters. The parametersreported are not corrected for interstellar or instrumental e ff ects.Starting from these Stokes parameters (that can be supposedto be normally distributed), we used a Monte Carlo simulation toobtain the probability distribution that describes the polarisationdegree P of the radiation (Rice distribution, Wardle & Kronberg1974). Passing from a Cartesian coordinate system with axis Q and U to a polar system, where P is the radius (eq. 2), we had tocorrect the distribution for the geometrical factor 1 / P , obtainedevaluating the Jacobian determinant of the coordinate change.The probability density function obtained after this correctioncan be demonstrated to be at first approximation a Gaussian,simply supposing the errors of Q and U to be similar betweeneach others in the Rice distribution. From the fit of this new dis-tribution with a Gaussian function, we evaluated the most prob-able value of P and its uncertainty to be P = . ± .
96% that is significant at a 3 σ level. This value does not need any bias cor-rection (eq. 3), since it has been evaluated without supposing P to be normally distributed. Furthermore we obtained an estimateof the polarisation angle θ (eq. 4) of 85 . ◦ ± . ◦ . FollowingSerkowski et al. (1975) we could evaluate the maximum ex-pected interstellar contribution to the linear polarisation P max of4U 0614 + P max ≤ A V , where A V = . V -band Galacticextinction expected along the source line of sight . Accordingto this relation, the maximum contribution to the linear polari-sation for 4U 0614 +
091 due to interstellar e ff ects should remainunder 4%. For this reason it is not possible to fully rule out thepossible involvement of the interstellar dust in the observed po-larisation degree of the target. Future multi-wavelength observa-tions will allow us to obtain a polarimetric spectral energy dis-tribution, that, if fitted with the Serkowski law (Serkowski et al.1975), will permit us to verify whether the observed polarisationdegree has an interstellar origin or is intrinsic to the target.We then performed a similar analysis on the two imagestaken with the WeDoWo device in 2014, summing them togetherin order to achieve a higher significance. The Stokes parametershave been extracted thanks to another custom set of command-line tools developed for the WeDoWo instrument, and were cor-rected through the observation of the standard non-polarised starG191B2B . The polarisation degree P was obtained through thesame analysis described above. Unfortunately a combination be-tween the faintness of the source and the bad seeing of the night( > . (cid:48)(cid:48) ) did not permit us to obtain a significant polarisationdetection for the source. A 3 σ upper limit to P of the 3.4% wasobtained, that is consistent with the result achieved for the 2013dataset. With the aim of detecting some kind of variability in the polari-sation degree of the source, we analised the 28 images obtainedwith the PAOLO polarimeter, extracting the normalised Stokesparameters Q and U as above. The polarisation degree trend asfunction of time for the target and for one of the reference starsis shown in Fig. 2. For ∼ the 50% of the images the target did notpossess a polarisation degree di ff erent from 0 at a (cid:62) σ level; inthat cases we decided to report in Fig. 2 the 3 σ upper limits on P . The polarisation degree of 4U 0614 +
091 does not show anyparticular trend with time, remaining almost constant around acommon value P mean . In order to take into account both thepolarisation detections and upper limits in Fig. 2 ( top panel),and maintaining the same cut to seeing (cid:46) . P mean by summing together the distributionsof polarisation of the selected images and by fitting the ob-tained distribution with a Gaussian function. With this method, P mean = . ± . P ob-tained in Sec. 2.1.
3. The orbital period of 4U 0614+091 r -band light curve of 4U0614+091 As stated in Sec. 1, no decisive measure of the orbital period P orb of 4U 0614 +
091 has been obtained to date. A possibil- http://ned.ipac.caltech.edu/forms/calculator.html
3. C. Baglio et al.: Polarimetric and spectroscopic optical observations of 4U 0614 + Fig. 2.
Top panel: r -band polarisation curve of 4U0614 + P /σ P (cid:62)
1) with their error bars, whereas with red arrows werepresented all the upper limits that we evaluated when a polar-isation measure was not possible, due for example to lower S / Nratios. The superimposed horizontal line represents the value P mean obtained by fitting the polarisation detections and upperlimits with a constant. The gray band represents the 1 σ level.Note that a better determination of the polarisation level is ob-tained from the probability density function (as discussed in thetext), resulting in a similar central value but a much smaller er-ror (0 . Bottom panel: r -band polarisation curve of one fieldstar chosen as reference (black squares: polarisation detections;red arrows: upper limits).ity for measuring P orb is to observe a periodic modulation inthe optical flux emitted from the source. In fact in the case ofLMXBs, where the dominant optical / IR emission from a sys-tem in quiescence is the companion star, they are often sub-ject to ellipsoidal modulations. This e ff ect derives from the factthat the companion star su ff ers for a tidal distortion due to thelarge gravitational field of the compact object, and for this rea-son the projected surface area of the distorted star is di ff erent atquadratures than at conjunctions, resulting in maxima and min-ima of the observed flux, respectively. When the systems possessshorter orbital periods and smaller orbital separations (as in caseof UCXBs), supposing the companion star emission to be themost relevant component, the e ff ects of irradiation should domi-nate the observed fluxes, causing the light curves to have a singlemaximum and a single minimum around the phases of superiorand inferior conjunction, respectively. Observing these patternswould allow us to measure precisely the orbital period of thesource. However, such methods are usually e ff ective with quies-cent LMXBs, where the companion star is expected to dominatethe optical emission.The photo-polarimetric images of 4U 0614 +
091 taken withPAOLO allowed us to compute the system r -band light curve.Infact, not considering a negligible loss in flux, the sum of theintensities measured in the four slices for each image results inthe total flux coming from our target of observation. We there-fore summed the fluxes corresponding to the four di ff erent posi- Fig. 3.
Top panel: r -band light curve obtained for the system4U0614 + Bottom panel: residual light curve after the subtraction ofthe straight line obtained with the fit of the light curve in the top panel from the dataset itself. Superimposed, the fit of the curvewith a sinusoidal function + a constant. With a dashed line weindicated in both panels the fraction of the day that correspondsto the possible polarisation flare (Fig. 2.tion angles for the system 4U 0614 +
091 and for 5 isolated starsin the field of view. We performed di ff erential photometry withrespect to this selection of reference stars, in order to minimizeany systematic e ff ect. The calibration of the magnitudes was per-formed using the R2 magnitudes of the USNO B1.0 catalogue ,whose magnitudes were transformed into r -band magnitudes us-ing the transformation equations of Jordi et al. (2006). However,a systematic error of a few tenths of a magnitude should be takeninto account due to the inaccuracy of the USNO magnitudes.The light curve (Fig. 3, top panel) shows a general decreas-ing trend with time, probably due to accretion variations in thedisc, with superimposed a significant short term variability. Thefit with a straight line y = mx + q produces a reduced χ of ∼ .
04, with q = . ± .
01 and m = . ± .
13. We couldthen subtract from the light curve the straight line obtained fromthe fit, ending up with the residuals in Fig. 3 ( bottom panel) thatsuggest a sinusoidal modulation (with significance probability of ∼
98% given by an f-test) with a periodicity of 44 . ± .
4. C. Baglio et al.: Polarimetric and spectroscopic optical observations of 4U 0614 + and a semi-amplitude of (2 . ± . × − mag (reduced χ of ∼ . A set of 16 low resolution ( ∼
160 km s − ) spectra of 300 s in-tegration each was taken of 4U 0614 +
091 on 3 September 2007with the ESO VLT (Very Large Telescope), equipped with theFORS1 spectrograph. All the spectra were taken with the 300Vgrism with a 1-arcsec slit, using 2 × ∼ . (cid:48)(cid:48) . These ob-servations cover ∼ . ∼
50 minutes orbitalperiod of the system. The extraction of the spectrum was per-formed with the ESO-MIDAS software package. Wavelengthand flux calibration of the spectra were achieved using helium-argon lamp and observing spectrophotometric stars. In Fig. 4 wereport the average spectrum of the source. The spectrum showsboth absorption and emission lines, the latter possibly due to thepresence of partially ionised carbon and oxygen, that possess alot of lines in the investigated wavelength range. Fig. 4.
Average spectrum of 4U 0614 +
091 with indicated themost prominent identified emission lines (Tab. 2).We could not find any trace of hydrogen and helium lines inour average spectrum, leading to the conclusion that the compan-ion star of the system must be a hydrogen-poor star. The identi-fication of the emission lines was made by the comparison withthe accurate results of Nelemans et al. (2004), that in turn usedthe local thermodynamic equilibrium (LTE) model proposed byMarsh et al. (1991). In particular, our results are in agreementwith the companion star being a C / O-rich white dwarf as statedin Nelemans et al. (2004). The most prominent identified fea-tures in our spectrum are reported in Tab. 2.In order to determine the orbital period of the source, westudied the variations of the equivalent width (EW) of the emis-sion lines CII / OII 6580 Å and CII 7240 Å, indicated in Fig. 4.We performed sinusoidal fitting of the EW variations, averagingthe spectra two by two with the aim of enhancing the signal tonoise ratio. The results of the fits are reported in Tab. 3: the mostsignificant periodicity is the obtained with the 7240 Å line (Fig.5), for which P = . ± . r -band light curve (Fig. Table 2.
Strongest features identified in the spectrum of 4U0614 +
091 (Fig. 4). For details about the transitions, see Tab. 3in the work of Nelemans et al. (2004).
Feature (Å) Ion4650 CIIIOII4700 OII4935 OII5700 CIII5810 CIIICIV?6580 CII(OII)6790 CII7240 CII
Table 3.
Results of the sinusoidal fit of the EW variations for thetwo selected lines. In the last column we reported the confidencelevel of the fit, obtained with an F-test with respect to a fit with aconstant. All the uncertainties are reported at the 90% confidencelevel. λ (Å) Period (min) Reduced χ CL6580 37 . ± . . ± . bottom panel). We also tried to measure the radial velocity ofthe CII / OII 6580 Å and CII 7240 Å emission lines. We found noevidence for periodical Doppler motions, likely due to the poorresolution of our spectra.
Fig. 5.
EW variation curve of CII 7240 Å line, obtained by aver-aging the spectra two by two, with superimposed a sinusoidal fitwith period 41 min.
4. Discussion
Relativistic particle jets have been observed to be emitted fromdi ff erent kinds of systems, like Active Galactic Nuclei (AGN),Super Soft Sources and X-ray binaries (XRBs), all subject tothe phenomenon of accretion (disc-jet coupling, Fender 2001).
5. C. Baglio et al.: Polarimetric and spectroscopic optical observations of 4U 0614 + The existence of jets in XRBs in particular does not seem to de-pend on the type of compact object hosted in the system, thatcan be both a neutron star and a black hole (recently evidencefor a transient jet in a binary system containing a white dwarfhas been found, K¨ording et al. 2008). However, XRBs hostingblack holes are among the most studied jet emitters sources, alsobecause they are brighter then, e.g, neutron stars systems in thewavelength range where the jet contributes the most to the emis-sion, i.e. the radio band. Generally, XRBs that emit jets are char-acterised by an optically thick, flat synchrotron radio spectrum,interpreted as a signature of the presence of the compact jet (forsome galactic black hole XRBs this has been confirmed by ra-dio imaging of the jet itself; Stirling et al. 2001; Dhawan et al.2000). Because the size scale of the emitting region in the jet isexpected to scale inversely with frequency (Nowak et al. 2005;Blandford & K¨onigl 1979), the jet break frequency marks thestart of the particle acceleration in the jet (Polko et al. 2010).A break from an optically thick to optically thin synchrotronspectrum is then expected in the IR / optical range (Falcke et al.2004). The break is detected in 4U0614 +
09 (Migliari et al. 2010)and also seen in the black hole candidates GX 339–4 (Corbel& Fender 2002; Gandhi et al. 2011; Corbel et al. 2013), XTEJ1118 +
480 (Hynes et al. 2006), XTE J1550-564 (Chaty et al.2011), V404 Cyg (Russell et al. 2013a) and MAXI J1836-194(Russell et al. 2013b).Synchrotron emission is expected to be intrinsically linearlypolarised at a high level, up to tens of per cent, especially incase of ordered magnetic fields. A polarimetric signature ofsynchrotron-emitting jets can be observed both in the NIR andin the optical in LMXBs; however, due to tangled and turbulentmagnetic fields at the base of the jets, polarisation degrees inthese sources have never been found to exceed a few per cent, asstated in Russell et al. 2011. The only evidence of ordered mag-netic fields in LMXBs jets has been observed for the persistentsystem Cyg X-1 (Russell & Shahbaz 2014). Only a few LMXBshave been observed with polarimetric techniques to date, bothpersistent and transient ones (Charles et al. 1980; Dolan & Tapia1989; Gliozzi et al. 1998; Hannikainen et al. 2000; Schultz et al.2004; Brocksopp et al. 2007; Shahbaz et al. 2008a; Russell &Fender 2008; Russell et al. 2011; Baglio et al. 2014).The aim of our work was to obtain additional evidence forthe emission of a compact jet from the UCXB 4U 0614 + . ± . + +
091 can be due to synchrotron radiation from a
Table 4.
RADIO and IR fluxes (uncorrected for the negligibleGalactic reddening) obtained from the ALLWISE catalogue and from Migliari et al. (2010)and r -band de-reddened flux ( A V = .
4) from the TNG data used tobuild the SED in Fig. 6.Band Flux (mJy)4.86 GHz (Migliari et al. 2010) 0 . ± . . ± . . × Hz (24 µ m , Migliari et al. 2010) 0 . ± . . × Hz (8 µ m , Migliari et al. 2010) 0 . ± . . × Hz (5.8 µ m , Migliari et al. 2010) 0 . ± . . × Hz (4.5 µ m , Migliari et al. 2010) 0 . ± . . × Hz (3.6 µ m , Migliari et al. 2010) 0 . ± . . × Hz (ALLWISE W1) 0 . ± . . × Hz (ALLWISE W2) 0 . ± . . × Hz (ALLWISE W3) < . × Hz ( r -band, this work) 0 . ± . relativistic particle jet emitted from its central regions, which isinferred from the SED of the system (Migliari et al. 2010).Following Migliari et al. (2010), we built the infrared SEDof 4U 0614 +
091 adding to the infrared and radio points re-ported in that paper and the archival data in the ALLWISE cat-alog (Tab. 4; Fig. 6). These data are not contemporary to thatof Migliari et al. (2010), but they fit well with the previousdataset, meaning that the average spectrum of the target doesnot vary significantly with time. From the fit with a linear func-tion we obtained a spectral index α of ∼ α ∼ − .
43, consistent with Migliari et al. (2010). The breakfrequency ν break between optically thin and optically thick syn-chrotron emission remains in the same range of Migliari et al.2010 (1 . × Hz < ν break < . × Hz).
Fig. 6.
Spectral energy distribution of the system 4U 0614 + r -flux obtainedin this work (black x). With an orange arrow we indicated theW3-band WISE upper limit obtained from the ALLWise catalog.Superimposed, the two linear fits of the SED. All the points arenormalized to the fluxes of Migliari et al. (2010).Under the hypothesis of jet emission, starting from the lin-ear fit to the infrared part of the SED in Fig. 6 we could esti-mate the expected r -band flux due to the jet emission ( F jet ∼ .
6. C. Baglio et al.: Polarimetric and spectroscopic optical observations of 4U 0614 + mJy). Since we know that the total de-reddened r -band flux ofthe source at the time of the TNG polarimetric observation was F r ∼ F jet and F r an estimate of the intrinsic linear polarisation degree of thejet only of ∼
15% .The polarisation light curve (Fig. 2, top panel) shows an al-most constant trend with time, as stated in Sec. 2.2. Neverthelessit has to be noted that for higher times a hint of a small polarisa-tion flare seems to be present; if we suppose the emission of thejet to be constant with time, the decrease of the flux that is ob-served in Fig. 3 contemporary to the possible polarisation flare(indicated with a dashed line) makes the percentile flux of the jetincrease, and this could explain the possible higher polarisationdegree observed.
Shahbaz et al. (2008b) found evidence of three di ff erent orbitalperiods for 4U0614 + r -band light curve of the system starting from the polarimetricmeasurements, and we could observe a slight decrease of theflux with time (Fig. 3, top panel), with a possible superimposedsinusoidal modulation with a periodicity of ∼
45 min (Fig. 3, bottom panel). This modulation could arise from the X-ray irra-diation of the inner face of the companion star from the compactobject (in this case, giving direct information about the orbitalperiod of the binary) or from the presence of hot spots or super-humps in the accretion disc, that can create a modulation in thelight curve due to the rotation of the system.We then tried to identify any periodicity in the systemthrough the analysis of the EW variation curves of the CII 7240Å line. From a sinusoidal fit (Fig. 5) we obtained a periodicalmodulation at a 40 . ± . bottom panel). This suggests that both the modula-tions could be caused by the same phenomenon; specifically, wepropose that the EW variation of the CII 7240 Å emission linecould be due to the modulation of the continuum emission, thatwe observe indeed in our light curve, and that should be linkedto the accretion disc, as our spectroscopic analysis pointed out.Following this hypothesis, we could rule out the X-ray irradia-tion as the origin of the observed modulation in the light curve,that can be thus explained e.g. referring to the presence of hotspots or superhumps in the accretion disc. Unfortunately the cal-ibration precision of our spectra is too low in order to detect amodulation in the continuum emission as small as the one thatwe observed in the light curve (i.e. ∼ . × − mag), whoseamplitude of oscillation does not exceed 10%.Since the periodicity that we measured from the EW varia-tion is linked to the outer regions of the disc, in order to obtainan estimate of the orbital period of the source we approximatedthe outer radius of the disc with its tidal radius R tid (King et al.1996): R tid = . R L , (5) where R L is the Roche lobe radius of the compact object of thesystem, and is given by: R L a = . q / . q / + ln (cid:0) + q / (cid:1) , (6)where q = M NS / M ∗ is the ratio between the neutron star andthe companion star mass ( M NS and M ∗ , respectively) and a isthe binary separation, that is an upper limit to the radius of thecompanion star orbit.From the Kepler law, we know that: P ( R tid ) P orb ∝ (cid:18) R out a (cid:19) / , (7)where P ( r tid ) and P orb are, respectively, the orbital period of theouter region of the disc and of the companion star of the system.Considering a mass ratio of at most ∼ .
1, that is typical of aUCXB system, we obtain P orb (cid:38) P ( r tid ), depending on the valueof q that we consider, and meaning that the orbital period of oursystem should exceed 1 hour.
5. Conclusions
In this work, we presented the results of a r -band polari-metric and a spectroscopic analysis of the persistent UCXB4U0614 + r -band polarisation degree of 2 . ± .
96% from the TNGdataset, and a 3 σ upper limit of 3.4% from the NOT data. A po-larisation degree of a few per cent in the optical is exactly what isexpected in case of a jet emission; for this reason we can confirmthe presence of this further component in the system emitted ra-diation. Under this hypothesis, we could moreover estimate anintrinsic linear polarisation degree of the jet only in the r -bandof ∼ +
091 starting from the polari-metric images obtained with the PAOLO polarimeter, observinga decreasing trend of the flux with time, possibly due to accre-tion variations in the disc, with superimposed a sinusoidal mod-ulation at a ∼
45 min period. From the spectroscopic FORS1images we could then extract the EW variation curve of the CII7240 Å line, and from its sinusoidal fit we could obtain a peri-odicity of 40 . ± . +
091 an orbital period (cid:38)
Acknowledgements.
MCB acknowledges S. Crespi for helpful discussions andT. Pursimo and I. Andreoni for their support during her observing run inLa Palma. DM acknowledges Prof. M. Colpi (Universit`a di Milano-Bicocca)for supportive discussions and the INAF-Osservatorio Astronomico di Brerafor kind hospitality during her bachelor thesis. TS was supported by theSpanish Ministry of Economy and Competitiveness (MINECO) under the grantAYA2010-18080.
7. C. Baglio et al.: Polarimetric and spectroscopic optical observations of 4U 0614 + References
Baglio, M. C., D’Avanzo, P., Campana, S., & Covino, S. 2014, A&A, 566, A9Blandford, R. D. & K¨onigl, A. 1979, ApJ, 232, 34Brandt, S., Castro-Tirado, A. J., Lund, N., et al. 1992, A&A, 262, L15Brocksopp, C., Miller-Jones, J. C. A., Fender, R. P., & Stappers, B. W. 2007,MNRAS, 378, 1111Brown, J. C., McLean, I. S., & Emslie, A. G. 1978, A&A, 68, 415Charles, P. A., Thorstensen, J. R., Bowyer, S., et al. 1980, ApJ, 237, 154Chaty, S., Dubus, G., & Raichoor, A. 2011, A&A, 529, A3Cheng, F. H., Shields, G. A., Lin, D. N. C., & Pringle, J. E. 1988, ApJ, 328, 223Corbel, S., Aussel, H., Broderick, J. W., et al. 2013, MNRAS, 431, L107Corbel, S. & Fender, R. P. 2002, ApJ, 573, L35Covino, S., Molinari, E., Bruno, P., et al. 2014, Astronomische Nachrichten, 335,117Dhawan, V., Mirabel, I. F., & Rodr´ıguez, L. F. 2000, ApJ, 543, 373di Serego Alighieri, S. 1998, Cambridge University Press, 199, 287Dolan, J. F. 1984, A&A, 138, 1Dolan, J. F. & Tapia, S. 1989, PASP, 101, 1135Falcke, H., K¨ording, E., & Marko ff , S. 2004, A&A, 414, 895Fender, R. P. 2001, MNRAS, 322, 31Forman, W., Jones, C., Cominsky, L., et al. 1978, ApJS, 38, 357Gandhi, P., Blain, A. W., Russell, D. M., et al. 2011, ApJ, 740, L13Giro, E., Bonoli, C., Leone, F., et al. 2003, in Society of Photo-OpticalInstrumentation Engineers (SPIE) Conference Series, Vol. 4843, Polarimetryin Astronomy, ed. S. Fineschi, 456–464Gliozzi, M., Bodo, G., Ghisellini, G., Scaltriti, F., & Trussoni, E. 1998, A&A,337, L39Hakala, P. J., Charles, P. A., & Muhli, P. 2011, MNRAS, 416, 644Hannikainen, D. C., Hunstead, R. W., Campbell-Wilson, D., et al. 2000, ApJ,540, 521Hynes, R. I., Robinson, E. L., Pearson, K. J., et al. 2006, ApJ, 651, 401in’t Zand, J. J. M., Jonker, P. G., & Markwardt, C. B. 2007, A&A, 465, 953Jordi, K., Grebel, E. K., & Ammon, K. 2006, A&A, 460, 339Juett, A. M. & Chakrabarty, D. 2003, ApJ, 599, 498King, A. R., Kolb, U., & Burderi, L. 1996, ApJ, 464, L127K¨ording, E., Rupen, M., Knigge, C., et al. 2008, Science, 320, 1318Kuulkers, E., in’t Zand, J. J. M., Atteia, J.-L., et al. 2010, A&A, 514, A65Kuulkers, E., in’t Zand, J. J. M., & Lasota, J.-P. 2009, A&A, 503, 889Madej, O. K., Jonker, P. G., Groot, P. J., et al. 2013, MNRAS, 429, 2986Marsh, T. R., Horne, K., & Rosen, S. 1991, ApJ, 366, 535Migliari, S., Tomsick, J. A., Miller-Jones, J. C. A., et al. 2010, ApJ, 710, 117Nelemans, G., Jonker, P. G., Marsh, T. R., & van der Klis, M. 2004, MNRAS,348, L7Nelemans, G., Jonker, P. G., & Steeghs, D. 2006, MNRAS, 370, 255Nelson, L. A., Rappaport, S. A., & Joss, P. C. 1986, ApJ, 311, 226Nowak, M. A., Wilms, J., Heinz, S., et al. 2005, ApJ, 626, 1006O’Brien, K. 2005, presented at “A life with stars” (Conference in Honour of Ed.van den Heuvel), AmsterdamOliva, E. 1997, A&AS, 123, 589Polko, P., Meier, D. L., & Marko ff , S. 2010, ApJ, 723, 1343Russell, D. M., Casella, P., Fender, R., et al. 2011, ArXiv e-printsRussell, D. M. & Fender, R. P. 2008, MNRAS, 387, 713Russell, D. M., Marko ff , S., Casella, P., et al. 2013a, MNRAS, 429, 815Russell, D. M., Russell, T. D., Miller-Jones, J. C. A., et al. 2013b, ApJ, 768, L35Russell, D. M. & Shahbaz, T. 2014, MNRAS, 438, 2083Rybicki, G. B. & Lightman, A. P. 1979, Radiative processes in astrophysicsSavonije, G. J., de Kool, M., & van den Heuvel, E. P. J. 1986, A&A, 155, 51Schultz, J., Hakala, P., & Huovelin, J. 2004, Baltic Astronomy, 13, 581Schulz, N. S., Chakrabarty, D., Marshall, H. L., et al. 2001, ApJ, 563, 941Serkowski, K., Mathewson, D. S., & Ford, V. L. 1975, ApJ, 196, 261Shahbaz, T., Fender, R. P., Watson, C. A., & O’Brien, K. 2008a, ApJ, 672, 510Shahbaz, T., Watson, C. A., Zurita, C., Villaver, E., & Hernandez-Peralta, H.2008b, PASP, 120, 848Stetson, P. B. 1987, PASP, 99, 191Stirling, A. M., Spencer, R. E., de la Force, C. J., et al. 2001, MNRAS, 327, 1273van Paradijs, J. & van der Klis, M. 1994, A&A, 281, L17Wardle, J. F. C. & Kronberg, P. P. 1974, ApJ, 194, 249Zhang, Y., Hynes, R. I., & Robinson, E. L. 2012, MNRAS, 419, 2943, S., Casella, P., et al. 2013a, MNRAS, 429, 815Russell, D. M., Russell, T. D., Miller-Jones, J. C. A., et al. 2013b, ApJ, 768, L35Russell, D. M. & Shahbaz, T. 2014, MNRAS, 438, 2083Rybicki, G. B. & Lightman, A. P. 1979, Radiative processes in astrophysicsSavonije, G. J., de Kool, M., & van den Heuvel, E. P. J. 1986, A&A, 155, 51Schultz, J., Hakala, P., & Huovelin, J. 2004, Baltic Astronomy, 13, 581Schulz, N. S., Chakrabarty, D., Marshall, H. L., et al. 2001, ApJ, 563, 941Serkowski, K., Mathewson, D. S., & Ford, V. L. 1975, ApJ, 196, 261Shahbaz, T., Fender, R. P., Watson, C. A., & O’Brien, K. 2008a, ApJ, 672, 510Shahbaz, T., Watson, C. A., Zurita, C., Villaver, E., & Hernandez-Peralta, H.2008b, PASP, 120, 848Stetson, P. B. 1987, PASP, 99, 191Stirling, A. M., Spencer, R. E., de la Force, C. J., et al. 2001, MNRAS, 327, 1273van Paradijs, J. & van der Klis, M. 1994, A&A, 281, L17Wardle, J. F. C. & Kronberg, P. P. 1974, ApJ, 194, 249Zhang, Y., Hynes, R. I., & Robinson, E. L. 2012, MNRAS, 419, 2943