A search for radio pulsations from neutron star companions of four subdwarf B stars
aa r X i v : . [ a s t r o - ph . GA ] J un Astronomy&Astrophysicsmanuscript no. 16098˙arxiv c (cid:13)
ESO 2018November 15, 2018
A search for radio pulsations from neutron star companions of foursubdwarf B stars
Thijs Coenen , Joeri van Leeuwen , , and Ingrid H. Stairs Astronomical Institute ”Anton Pannekoek,” University of Amsterdam, P.O. Box 94249, 1090 GE, Amsterdam, The Netherlands Stichting ASTRON, PO Box 2, 7990 AA Dwingeloo, The Netherlands Dept. of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, B.C., V6T 1Z1 Canada
ABSTRACT
We searched for radio pulsations from the potential neutron star binary companions to subdwarf B stars HE 0532-4503, HE 0929-0424, TON S 183 and PG 1232-136. Optical spectroscopy of these subdwarfs has indicated they orbit a companion in the neutron starmass range. These companions are thought to play an important role in the poorly understood formation of subdwarf B stars. Usingthe Green Bank Telescope we searched down to mean flux densities as low as 0.2 mJy, but no pulsed emission was found. We discussthe implications for each system.
1. Introduction
The study of millisecond pulsars (MSPs) enables several typesof research in astrophysics, ranging from binary evolution (e.g.Edwards & Bailes, 2001), to the potential detection of back-ground gravitational radiation using a large set of pulsars withstable timing properties (Ja ff e & Backer, 2003). Furthermore,high-precision timing provides opportunities for fundamentalphysics research: by measuring and modeling pulse arrival timesfrom binary systems, one can constrain neutron star masses andequations of state (Demorest et al., 2010), or study strong-fieldgeneral relativity (Taylor & Weisberg, 1989).Finding such millisecond pulsars is done through wide-areasurveys or through targeted surveys, each with specific advan-tages: with limited telescope time, a directed search can a ff ordto dwell on a specifically interesting part of the sky for longerthan an undirected search. A directed search is therefore moresensitive to faint pulsars in specific locations than an undirectedsearch would be that only incidentally scans over such a location.An undirected search however covers a larger area. Directedsearches have targeted sources associated with MSPs, such asglobular clusters (Ransom et al., 2005), steep spectrum sources(Backer et al., 1982), unidentified Fermi sources (Ransom et al.,2011) and low mass-white dwarfs (van Leeuwen et al., 2007;Ag¨ueros et al., 2009). Here we report the results of a directedsearch for radio pulsations from four short-orbit sub-luminous Bdwarf (sdB) binaries identified through spectroscopy as possiblycontaining neutron stars (Geier et al., 2008).Sub-luminous B dwarfs are some of the most abundant faintblue objects (Green et al., 1986). They are thought to be light(about 0 . M ⊙ ) core helium burning stars with very thin hy-drogen envelopes. After the helium burning in the core stops,sdB stars evolve directly to the white dwarf cooling sequence(cf. a recent review by Heber, 2009). In their survey of sub-dwarf B stars Maxted et al. (2001) find that 2 / < ∼
10 days) binaries. Taking into ac-count the insensitivity of their survey to longer-period binaries,Maxted et al. (2001) conclude that binary star evolution is fun-damental to the formation of sdB stars. Several such binary formation channels have been hypoth-esized. For an sdB to form, a light star must lose most of itshydrogen envelope and ignite helium in its core. In these binarysystems, sdB stars can be formed through phases of CommonEnvelope evolution where the envelope is ejected, or throughstable Roche Lobe overflow stripping the donor star of its hydro-gen envelope. Binary population synthesis models for the abovescenarios in the case of white-dwarf companions were recentlycompared by Han et al. (2003) and Hu et al. (2007).Specifically interesting for finding new millisecond pul-sars, is the channel that leads to short-period binary systemscontaining an sdB star and a neutron star or a black hole(Podsiadlowski et al., 2002; Geier et al., 2010). Here, the binarysystem contains a star that is massive enough to evolve into aneutron star or a black hole and a secondary, lighter star that isthe progenitor of the sdB star. The secondary is in a wide or-bit with a period of about 20 years. The system undergoes twophases of common envelope evolution, a supernova and a shortX-ray binary phase. A first common envelope phase starts soonafter the primary becomes a red supergiant and starts overflow-ing its Roche Lobe. During this first common envelope phasethe orbit of the binary shrinks. After the subsequent supernova,the primary leaves a neutron star or black hole. This neutronstar remains visible as a normal radio pulsar for on average10 − years, and turns o ff . Once the secondary evolves o ff themain sequence the second phase of mass transfer starts when thesecondary begins to overflow its Roche Lobe. The system un-dergoes a short X-ray binary phase, in which the neutron staris recycled (Bhattacharya & van den Heuvel, 1991). The secondcommon-envelope phase starts shortly thereafter, further shrink-ing the orbit of the binary and dissipating the envelope of thesecondary. The secondary, which is now mostly stripped of itshydrogen envelope, ignites helium in its core and continues itsevolution as a sdB star (Podsiadlowski et al., 2002; Geier et al.,2010). From population modeling, Pfahl et al. (2003) find thatabout 1% of sdB stars evolve as outlined above and orbit a neu-tron star or black hole. In such systems the secondary is an sdBstar for ∼ years (Heber, 2009), before evolving to a whitedwarf. Thijs Coenen et al.: A search for radio pulsars in four subdwarf B star binaries
While low-mass companions (white dwarfs, main se-quence stars) to sdB stars have been optically confirmed (e.g.Geier et al., 2010), the unambiguous identification of a high-mass companion in the form of a radio pulsar would providefurther constraints on the relative likelihood of di ff erent binaryevolution models (as was similarly done for companions of low-mass white dwarfs in van Leeuwen et al., 2007). Furthermore,timing of a possible MSP companion will likely provide an sdBmass measurement, either through the modeling of the orbitalparameters, or more directly and precisely through a Shapirodelay measurement (e.g. Ferdman et al., 2010). As millisec-ond pulsars can turn on shortly after the cessation of accretion(Archibald et al., 2009) and can shine for more than the ∼ year sdB star lifetime, MSPs will be active throughout the sdBstar phase. Thus, non-detections of radio pulsations from thesesystems could mean the absence of any neutron star, the pres-ence of only a very weak radio pulsar, or of a brighter one thatis beamed away from Earth. Non-detections in a large enoughsample of sdB stars eventually provide statistics on sdB forma-tion channels.In Section 2 we describe how sources were selected; inSection 3 our observing and data reduction setup is outlined.Section 4 contains our results. In Section 5 we discuss theseresults and compare to recent optical results on some of oursources.
2. Source selection
Our source selection was based on the report by Geier et al.(2008) of the detection of the first four sdB systems that possiblycontain a neutron star or black hole companion: HE 0532-4503,HE 0929-0424, TON S 183 and PG 1232-136.These candidate sdB - neutron star (sdB-NS) systems wereidentified through spectroscopic observations of sdB binaries(Geier et al., 2008). In sdB-NS systems, the optical spectrumis single lined and the compact companion cannot be detecteddirectly optically. Assuming tidal synchronization between theorbital period and the sdB rotational period, it is possible to con-strain the binary inclination from measurements of the sdB sur-face gravity, projected rotational velocity and sdB mass. In their2010 follow-up paper Geier et al. give a detailed description ofthis method. In binaries with periods shorter than 1 . .
3. Observations and data reduction
On 2008 Oct 15, 18 and 20 we observed these four sdB bina-ries with the Robert C. Byrd Green Bank Telescope (GBT). Weused the lowest-frequency receiver PF1, which provided a band-width ∆ f =
50 MHz centered around 350 MHz. Every 81.92 µ sthe Spigot backend (Kaplan et al., 2005) recorded 2048 spectralchannels in 16-bit total intensity. Over the three sessions, threetest pulsars (Table 2) and four sdB-star binaries (Table 3) wereobserved. Due to scheduling and weather constraints, integra-tion times of the binaries varied, ranging from 12 to 68 minutes(Table 3). Object t int P DM
Peak S / N (s) (ms) (pc cm − )PSR J0034-0534 60 1.877 13.8 22PSR B0450-18 30 549.0 39.9 115PSR B1257 +
12 30 6.219 10.2 15
Table 2.
List of the previously known pulsars that were used as acheck of the telescope, back-end and pulsar-search systems. Testsource PSR J0034-0534 is in a short-orbit 1.6-day binary with a0 . M ⊙ companion (Bailes et al., 1994).For each system in our sample, we derived the expecteddispersion measures (DM) from their measured distance, us-ing the NE 2001 free electron model (Cordes & Lazio, 2002).These expected DMs, listed in Table 1, are all well below100 pc cm − . Up to 200 pc cm − the trial DMs were spacedsuch that our search remains sensitive to pulsars with rota-tional periods down to 1 ms; faster pulsars are not generallyexpected (Chakrabarty et al., 2003). Beyond 200 pc cm − theintra-channel smearing begins to dominate, so trial DMs up to1000 pc cm − where created at a lower time resolution. We nextsearched these trial DMs for pulsar signals, in both the time andfrequency domain, using the PRESTO data reduction package(Ransom, 2001). In the time domain we searched for single dis-persed pulses of radio emission. In the frequency-domain searchwe have to take into account the acceleration present in thesebinary systems. One could potentially derive the neutron-star or-bital phase from the optical modulation of the sdB star, and thusestimate the acceleration at the time of the observations. Thishowever relies on the assumption that there is no lag betweenthe optical light curve and the orbital motion. We have taken theconservative approach to search over the full range of possibleaccelerations throughout the orbit. As each integration time t int was shorter than 10 % of the binary period, our search in pe-riod and period-derivative space su ffi ced (Johnston & Kulkarni,1991). Using standard PRESTO routines, for each observationthe radio-frequency interference was flagged, candidate periodsignals were sifted to remove harmonics, and the top 30 can-didates we folded and further refined by searching nearby dis-persion measures, periods and period derivatives to maximizesignal-to-noise ratio. All candidate plots were subsequently vi-sually inspected.At the start of each observing session a known pulsar wasobserved; as listed in Table 2, these were all blindly re-detectedby the above sdB-NS search pipeline, thus confirming the e ff ec-tiveness of the search method.
4. Results
No new pulsars were detected toward any of the four sdB starsin our sample.We next investigate how strongly these non-detections ruleout the presence of an MSP. We derive the minimum detectablemean flux density S min for each observation from the pulsar ra-diometer equation (Dewey et al., 1985; Bhattacharya, 1998): S min = ( S / N min ) T sys G p n p t int ∆ f r WP − W In this equation S / N min is the minimum signal-to-noise ratioat which a pulse profile can clearly be recognized as a pulsar pro-file. The system temperature T sys is defined as T sys = T sys , GBT + hijs Coenen et al.: A search for radio pulsars in four subdwarf B star binaries 3Object l b M comp P orb T sky d DM ( M ⊙ ) (d) (K) (kpc) (pc cm − )HE 0532-4503 251.01 -32.13 1.4 - 3.6 0.2656 ± ± ± ± Table 1.
Pulsar search targets based on sdB-NS candidate binaries identified by Geier et al. (2008). Companion mass and binaryperiod taken from Geier et al. (2008) and references therein. Sky temperature at 408 MHz extracted from Haslam et al. (1982).Distance determinations taken from Lisker et al. (2005) and Altmann et al. (2004). Dispersion measure estimate based on the NE2001 electron model (Cordes & Lazio, 2002). T sky where T sys , GBT =
46 K . For each source, T sky was extractedfrom Haslam et al. (1982) and scaled from 408 to 350 MHz us-ing the T sky ∝ ν − . scaling law from Lawson et al. (1987). Thegain G for the GBT is 2 K Jy − (Prestage et al., 2006), the num-ber of polarizations n p , added for these total intensity observa-tions, is 2. Integration time t int and bandwidth ∆ f for each obser-vation are described in Section 3. Finally, W is the pulse widthand P is the rotational period. As these are unknown for non-detections, we use the average pulse duty cycle W / P = . ± . W / P duty cycles in theATNF pulsar database (Manchester et al., 2005). Here and be-low, we define MSPs in the ATNF database as those sources withperiods shorter than 50 ms (thus, moderately to fully recycled)and magnetic fields lower than 10 Gauss. For each source, welist the resulting S min value in Table 3.The errors on our values of S min have contributions from theerror on the average pulse duty cycle and from the systematicerror on the minimum signal-to-noise ratio S / N min . The S / N min at which a pulsar can be detected depends on the shape of itsprofile: a strongly peaked profile at high DM is more easily iden-tified as a pulsar than a same-S / N sinusoidal profile at low DM,as the latter can also be terrestrial interference. We estimate thissystematic error as follows. To set a lower limit to the possi-ble ( S / N min ) range we inspected the pulse profiles of the testpulsars for increasing fractions of the total integration time. Weconcluded that starting from a peak ( S / N min ) of 7 the pulsar re-detections become unambiguous. As an upper limit to the rangeof ( S / N min ) values we adopt a value of 10.To estimate the completeness of our search we derivedpseudo luminosity L = S d limits for each of the binaries in oursample and compared them with the pseudo luminosities ofknown MSPs. We define the completeness of our pulsar searchas the percentage of known millisecond pulsars that have pseudoluminosities higher than our derived pseudo luminosity upperlimits, i.e., as the percentage of known MSPs that would be de-tected if placed at the distance of the candidate. We searchedthe literature for distances d to the binaries in our sample andused the most recent values. For HE 0532-4503 and HE 0929-0424 these are from Lisker et al. (2005), while the distances toTON S 183 and PG 1232-136 are from Altmann et al. (2004),where TON S 183 is known as SB 410. The former article claimsan error on the distance of 10%. In the latter article distance er-rors are not estimated, so for those sources we also propagate a10% error. All distances were derived from models of sdB atmo-spheres combined with the magnitude measurement.In Figure 1 we compare our limits to the pseudo luminositiesof the millisecond pulsar population. For this statistical compar-ison, we have selected all 50 MSPs with known 400 MHz fluxesin the ATNF database (Manchester et al., 2005). We scale these http://science.nrao.edu/gbt/obsprop/GBTpg.pdf Object t int S min L C (s) (mJy) (mJy kpc ) (%)HE 0532-4503 720 0 . ± .
21 3 . ± . . ± .
18 1 . ± . . ± .
19 0 . ± .
07 100TON S 183 4080 0 . ± .
09 0 . ± .
03 100
Table 3.
Integration time t int , flux upper limit S min derived fromradiometer equation, pseudo luminosity limit L calculated fromthe flux upper limit, and completeness C compared to the knownMSPs in the ATNF database. Fig. 1.
Cumulative histogram of pseudo luminosities for theknown MSP population in the ATNF database. The left axis islabelled with the cumulative fraction of MSPs; the right axis islabelled with survey completeness for each candidate. The er-rors on the pseudo luminosity upper limits contain errors on thedistance, pulse duty cycle and our estimate of the relevant rangeof ( S / N min ) values used with the pulsar radiometer equation (seeSection 4).400 MHz fluxes to our central observing frequency of 350 MHz,using the − . C .
5. Discussion
As no radio pulsations were found in any of our observations, weplace upper limits on the pulsed radio emission from the putativeneutron stars in these systems. Assuming the known MSP lumi-
Thijs Coenen et al.: A search for radio pulsars in four subdwarf B star binaries nosity distribution, we can exclude the presence of such pulsedradio emission with 100% certainty for 2 systems: PG 1232-136and TON S 183 (Figure 1). For 2 other systems, HE 0532-4503and HE 0929-0424, we were not able to put as strict upper lim-its on the pseudo luminosity due to their larger distance and theshorter integration time for these pointings. For HE 0929-0424,our survey would have detected 96% of MSPs at the HE 0929-0424 distance. For HE 0532-4503, this completeness C is 88%(Table 3, Figure 1).In relation to these non-detections we now discuss two se-lection e ff ects: the fraction of the sdB lifetime during which anMSP is on; and the beaming fraction, which is the fraction of skyover which an MSP beam sweeps, and thus from which the MSPis in principle detectable. As outlined in Section 1, the MSP andsdB star are formed simultaneously; as MSPs have ages up of to10 − years and sdBs have ages up to 10 years, an MSP formedin a binary with an sdB star will shine for the entire age of thesdB, and longer. We thus conclude there is no age bias againstdetecting MSPs around sdB stars. There is however a non-zerochance that for some of our non-detections a bright MSP emis-sion beam is present, but missing the Earth. The beaming frac-tion of MSPs is 0.7 ± ff before the formation of the sdB began (cf. Section1). For any non-recycled pulsars that are in the beam by chance,the S min in Table 3 list the minimum detectable flux for thatpointing. The pseudo-luminosity distributions of non-recycledand millisecond pulsars are similar (Manchester et al., 2005), butas the distance to any chance-coincident non-recycled pulsar isunknown, their pseudo luminosity is unknown too.We now compare the results of our search, triggered bythe sample from Geier et al. (2008), with the follow-up re-sults of further spectroscopic investigation recently published inGeier et al. (2010). For PG 1232-136, where we put a very strict(100% complete) limit on the pseudo luminosity, Geier et al.(2010) now report the mass of the unseen companion M comp to be higher than 6 M ⊙ . This indicates the compact compan-ion is a black hole; therefore no pulsed radio emission is ex-pected from that system, in agreement with our non-detection.For TON S 183 the new spectroscopic results point to a M comp of 0 . M ⊙ but the error bars allow for a low mass white dwarf(WD). Our non-detection of pulsed radio emission is compat-ible with a WD companion, and since the constraint we puton pulsed radio emission from TON S 183 is strict (100% com-plete) we rule out an MSP beamed towards Earth. For HE 0929-0424 a M comp slightly above the Chandrasekhar limit is reported,but with error bars that also allow for a high mass WD. Our96%-complete non-detection in this case cannot exclude eitherpossibility but an MSP is unlikely. The system with the leaststrict pseudo luminosity upper limit, HE 0532-4503, is reportedto contain a companion to the sdB star of 3 M ⊙ . Even if oneallows for an sdB star as light as 0 . M ⊙ the companion re-mains more massive than the Chandrasekhar limit. Given ournon-detection corresponding to the 12th percentile of the knownMSP population (88% complete), an MSP is not likely, but thissystem remains an interesting candidate for deeper radio follow-up. Further observations of this source and of new candidatesdB-NS systems from Geier et al. (2010) are currently ongoing.
6. Conclusions
We have searched for pulsed emission from potential pulsarcompanions of four subdwarf B stars. No pulsed emission wasfound down to luminosities corresponding to, on average overthe 4 sources, the 4th percentile of the known millisecond pulsarpopulation.
Acknowledgements.
We thank Jason Hessels for help with the search pipeline.This work was supported by the Netherlands Research School for Astronomy(Grant NOVA3-NW3-2.3.1) and by the European Commission (Grant FP7-PEOPLE-2007-4-3-IRG
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