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arXiv:1106.2053v1 [astro-ph.GA] 10 Jun 2011 erhfrrdoplain rmnurnsa opnoso companions neutron from pulsations radio for search A atde l 20)cnld htbnr treouini f is . evolution sdB of binary the that to bi conclude damental longer-period (2001) to al. survey et their Maxted of insensitivity the (P count short-orbit in are stars wr tr atde l 20)fidta 2 sub- that of find survey (2001) their al. In et 2009). Maxted sequenc Heber, stars cooling by B dwarf review stops dwarf white recent core the a the to (cf. in directly evolve burning stars sdB the After envelopes. drogen leojcs(re ta. 96.Te r huh ob ligh be to thought are They 1986). 0 al., (about et (Green objects blue 2008). al., et (Geier stars pos as neutron direct spectroscopy containing a through identified of sub-lumi binaries short-orbit (sdB) results 2007; four dwarf the from al., pulsations report et radio for Leeuwen we search (van Here dwarfs 2009). mass-white al., Ag¨ueros et low et (Ransom and sources sou Fermi 2011) unidentified spectrum 1982), a steep al., such 2005), et Directed (Backer al., MSPs, et area. with (Ransom larger associated clusters globular sources a targeted covers have however searches search loca a undirected such over undi scans An incidentally an mo only than that therefore locations be long would is specific for search in search sky directed faint the A to of sensitive search. part a undirected interesting can an specifically search than a directed on a adva dwell time, specific to telescope with limited each with surveys, tages: targeted through or surveys strong- study 1989). or Weisberg, 2010), & (Taylor al., relativity a et general masses (Demorest star state neutron of constrain equations can one t systems, arrival binary pulse fundame from modeling and for measuring by opportunities research: physics provides timing back- pulsars (Ja of high-precision of properties set detection large timing a potential stable using the radiation gravitational to ground evolution 2001), binary Bailes, from typ & ranging several Edwards astrophysics, enables in (MSPs) research pulsars of millisecond of study The Introduction 1. oebr1,2018 15, November srnm Astrophysics & Astronomy u-uiosBdaf r oeo h otaudn faint abundant most the of some are dwarfs B Sub-luminous wide-area through done is pulsars millisecond such Finding 2 1 h re akTlsoew erhddw oma u densiti flux mean to importan down searched an we play Telescope to Bank thought Green are the thes companions of spectroscopy These Optical neutron range. 1232-136. mass potential PG and the 183 from S TON pulsations 0424, radio for searched We 3 h mlctosfrec system. each for implications the tctn SRN OBx2 90A wneo,TeNetherlan The Dwingeloo, AA 7990 2, Box PO ASTRON, Stichting srnmclIsiue”no anke, nvriyo A of University Pannekoek,” ”Anton Institute Astronomical et fPyisadAtooy nvriyo rts Colum British of University Astronomy, and Physics of Dept. . 5 M ⊙ oehlu unn tr ihvr hnhy- thin very with stars burning helium core ) < ∼ ff aucitn.16098˙arxiv no. manuscript 0dy)bnre.Tkn noac- into Taking binaries. days) 10 akr 03.Furthermore, 2003). Backer, & e hj Coenen Thijs 1 / udafBstars B or a Leeuwen van Joeri , ffil sdB field of 3 osB nous naries, rected sibly ff imes tion. with (e.g. field ABSTRACT rces ntal un- ord al., trbnr opnost udafBsasH 5240,H 0 HE 0532-4503, HE stars B subdwarf to companions sedm ..Bx929 00G,Asedm h Netherlan The Amsterdam, GE, 1090 94249, Box P.O. msterdam, nd ed es n- re er e s oei h oryudrto omto fsbwr stars B subdwarf of formation understood poorly the in role t i,62 giutrlRa,Vnovr .. 6 Z Canad 1Z1 V6T B.C., Vancouver, Road, Agricultural 6224 bia, t , udaf a niae hyobtacmaini h neutr the in companion a orbit they indicated has subdwarfs e sa o s02my u ople msinwsfud ediscu We found. was emission pulsed no but mJy, 0.2 as low as es trrmisvsbea omlrdopla o naverage on for neutron radio This normal hole. a black as or 10 visible star neutron remains supern subsequent a star phase the leaves envelope After primary common shrinks. binary first the the this of overflo During orbit starts Lobe. the and Roche supergiant its red soon a starts ing becomes phase envelope primary common short the two first after a or- undergoes A and wide system phase. binary The a a evolution, X-ray years. in envelope 20 is common about of secondary of phases The tha period star. star a a lighter sdB hole with secondary, into bit the a evolve black of and to hole a progenitor enough black the massive or a is or star that star b the neutron Here, a 2010). syste a al., contains et binary system and Geier short-period 2002; star al., to et (Podsiadlowski sdB leads that an channel containing the is sars, (2007). al. et Hu and recent (2003) were al. et companions Han white-dwarf by of compared case ab the the for in models scenarios hyd synthesis throug its population of Binary or star envelope. donor ejected, gen Common the stripping is of overflow its Lobe envelope phases Roche of the stable through most where formed lose bin evolution be must these Envelope can In star stars core. light its sdB in a systems, helium form, ignite and to envelope sdB an For esized. rnsa rbakhl.I uhssestescnayi nsd an is secondary dwarf. the neu- systems a such orbit for In and star hole. above black tha outlined or find as star evolve (2003) tron stars al. sdB et of et Pfahl Geier 1% about 2002; modeling, al., population et From (Podsiadlowski continues star 2010). and sdB core it th a its of of as in evolution stripped envelope helium mostly the ignites now dissipating envelope, is hydrogen and which binary secondary, the The secondary. of orbit star the ing neutron secon further thereafter, The the shortly 1991). starts which Heuvel, phase den in common-envelope van un- & phase, system (Bhattacharya binary recycled The is X-ray Lobe. short Roche a its t dergoes overflow when starts to transfer begins mass of secondary phase second the sequence main ds 7 − pcfial neetn o nignwmlieodpul- millisecond new finding for interesting Specifically hypoth- been have channels formation binary such Several 2 8 , 1 er,adtrso turns and years, n nrdH Stairs H. 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c Using . nstar on S 2018 ESO a four f ds 929- ss shrink- ff inary ova, ove ary is t the al., ms ro- w- its he ly B d h e s t 2 Thijs Coenen et al.: A search for radio pulsars in four binaries

While low-mass companions (white dwarfs, main se- Object tint P DM Peak S/N −3 quence stars) to sdB stars have been optically confirmed (e.g. (s) (ms) (pc cm ) Geier et al., 2010), the unambiguous identification of a high- PSR J0034-0534 60 1.877 13.8 22 PSR B0450-18 30 549.0 39.9 115 mass companion in the form of a radio pulsar would provide PSR B1257+12 30 6.219 10.2 15 further constraints on the relative likelihood of different binary evolution models (as was similarly done for companions of low- Table 2. List of the previously known pulsars that were used as a mass white dwarfs in van Leeuwen et al., 2007). Furthermore, check of the telescope, back-end and pulsar-search systems. Test timing of a possible MSP companion will likely provide an sdB source PSRJ0034-0534 is in a short-orbit 1.6-day binary witha mass measurement, either through the modeling of the orbital 0.16 M⊙ companion (Bailes et al., 1994). parameters, or more directly and precisely through a Shapiro delay measurement (e.g. Ferdman et al., 2010). As millisec- ond pulsars can turn on shortly after the cessation of 8 (Archibald et al., 2009) and can shine for more than the ∼ 10 For each system in our sample, we derived the expected year sdB star lifetime, MSPs will be active throughout the sdB dispersion measures (DM) from their measured distance, us- star phase. Thus, non-detections of radio pulsations from these ing the NE 2001 free electron model (Cordes & Lazio, 2002). systems could mean the absence of any neutron star, the pres- These expected DMs, listed in Table 1, are all well below ence of only a very weak radio pulsar, or of a brighter one that 100pc cm−3. Up to 200pccm−3 the trial DMs were spaced is beamed away from Earth. Non-detections in a large enough such that our search remains sensitive to pulsars with rota- sample of sdB stars eventually provide statistics on sdB forma- tional periods down to 1 ms; faster pulsars are not generally tion channels. expected (Chakrabarty et al., 2003). Beyond 200pc cm−3 the In Section 2 we describe how sources were selected; in intra-channel smearing begins to dominate, so trial DMs up to Section 3 our observing and data reduction setup is outlined. 1000pc cm−3 where created at a lower time resolution. We next Section 4 contains our results. In Section 5 we discuss these searched these trial DMs for pulsar signals, in both the time and results and compare to recent optical results on some of our frequency domain, using the PRESTO data reduction package sources. (Ransom, 2001). In the time domain we searched for single dis- persed pulses of radio emission. In the frequency-domainsearch 2. Source selection we have to take into account the acceleration present in these binary systems. One could potentially derive the neutron-star or- Our source selection was based on the report by Geier et al. bital phase from the optical modulation of the sdB star, and thus (2008) of the detection of the first four sdB systems that possibly estimate the acceleration at the time of the observations. This contain a neutron star or black hole companion: HE0532-4503, however relies on the assumption that there is no lag between HE0929-0424, TONS183 and PG1232-136. the optical light curve and the orbital motion. We have taken the These candidate sdB - neutron star (sdB-NS) systems were conservative approach to search over the full range of possible identified through spectroscopic observations of sdB binaries accelerations throughout the orbit. As each integration time tint (Geier et al., 2008). In sdB-NS systems, the optical spectrum was shorter than 10% of the binary period, our search in pe- is single lined and the compact companion cannot be detected riod and period-derivative space sufficed (Johnston & Kulkarni, directly optically. Assuming tidal synchronization between the 1991). Using standard PRESTO routines, for each observation orbital period and the sdB rotational period, it is possible to con- the radio-frequency interference was flagged, candidate period strain the binary inclination from measurements of the sdB sur- signals were sifted to remove harmonics, and the top 30 can- face , projected rotational velocity and sdB mass. In their didates we folded and further refined by searching nearby dis- 2010 follow-up paper Geier et al. give a detailed description of persion measures, periods and period derivatives to maximize this method. In binaries with periods shorter than 1.2 days the signal-to-noise ratio. All candidate plots were subsequently vi- orbit and are found to be synchronized. Thus, for sually inspected. such systems the companion mass can be derived from the sdB At the start of each observing session a known pulsar was mass, which is known either from independent mass measure- observed; as listed in Table 2, these were all blindly re-detected ments (e.g. Fontaine et al., 2008) or inferred from binary pop- by the above sdB-NS search pipeline, thus confirming the effec- ulation synthesis (Han et al., 2002, 2003). Our sources all have tiveness of the search method. orbital periods shorter than these 1.2 days. In Table 1 we de- scribe further target parameters such as galactic coordinates, lo- cal background sky temperature, distance and estimated disper- 4. Results sion measure. No new pulsars were detected toward any of the four sdB stars in our sample. 3. Observations and data reduction We next investigate how strongly these non-detections rule out the presence of an MSP. We derive the minimum detectable On 2008 Oct 15, 18 and 20 we observed these four sdB bina- mean flux density S for each observation from the pulsar ra- ries with the Robert C. Byrd Green Bank Telescope (GBT). We min diometer equation (Dewey et al., 1985; Bhattacharya, 1998): used the lowest-frequency receiver PF1, which provided a band- width ∆ f = 50MHz centered around 350MHz. Every 81.92 µs (S/Nmin)Tsys W the Spigot backend (Kaplan et al., 2005) recorded 2048 spectral S = min r channels in 16-bit total intensity. Over the three sessions, three G nptint∆ f P − W test pulsars (Table 2) and four sdB-star binaries (Table 3) were p observed. Due to scheduling and weather constraints, integra- In this equation S/Nmin is the minimum signal-to-noise ratio tion times of the binaries varied, ranging from 12 to 68 minutes at which a pulse profilecan clearly be recognizedas a pulsar pro- (Table 3). file. The system temperature Tsys is defined as Tsys = Tsys,GBT + Thijs Coenen et al.: A search for radio pulsars in four subdwarf B star binaries 3

Object l b Mcomp Porb Tsky d DM (M⊙) (d) (K) (kpc) (pc cm−3) HE 0532-4503 251.01 -32.13 1.4 - 3.6 0.2656 ± 0.0001 17.0 2.8 44 HE 0929-0424 238.52 32.35 0.6 - 2.4 0.4400 ± 0.0002 15.0 1.9 38 PG 1232-136 296.98 48.76 2.0 - 7.0 0.3630 ± 0.0003 21.9 0.57 14 TON S 183 287.22 -83.12 0.6 - 2.4 0.8277 ± 0.0002 19.7 0.54 12 Table 1. Pulsar search targets based on sdB-NS candidate binaries identified by Geier et al. (2008). Companion mass and binary period 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 NE 2001 electron model (Cordes & Lazio, 2002).

T where T = 46K1. For each source, T was extracted Object tint S min L C sky sys,GBT sky 2 from Haslam et al. (1982) and scaled from 408 to 350MHz us- (s) (mJy) (mJy kpc ) (%) −2.6 HE 0532-4503 720 0.41 ± 0.21 3.2 ± 1.8 88 ing the Tsky ∝ ν scaling law from Lawson et al. (1987). The −1 HE 0929-0424 900 0.35 ± 0.18 1.3 ± 0.7 96 gain G for the GBT is 2KJy (Prestage et al., 2006), the num- PG 1232-136 1080 0.37 ± 0.19 0.12 ± 0.07 100 ber of polarizations np, added for these total intensity observa- TON S 183 4080 0.18 ± 0.09 0.05 ± 0.03 100 tions, is 2. Integration time tint and bandwidth ∆ f for each obser- vation are described in Section 3. Finally, W is the pulse width Table 3. Integration time t , flux upper limit S derived from and P is the rotational period. As these are unknown for non- int min radiometer equation, pseudo limit L calculated from / = . ± . detections, we use the average pulse duty cycle W P 0 12 0 10 the flux upper limit, and completeness C compared to the known / determined by averaging over the MSP W50 P duty cycles in the MSPs in the ATNF database. ATNF pulsar database (Manchester et al., 2005). Here and be- low, we define MSPs in the ATNF database as those sources with periods shorter than 50ms (thus, moderately to fully recycled) and magnetic fields lower than 1011 Gauss. For each source, we list the resulting S min value in Table 3. The errors on our values of S min have contributions from the error on the average pulse duty cycle and from the systematic error on the minimum signal-to-noise ratio S/Nmin. The S/Nmin at which a pulsar can be detected depends on the shape of its profile: 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 this systematic error as follows. To set a lower limit to the possi- ble (S/Nmin) range we inspected the pulse profiles of the test pulsars for increasing fractions of the total integration time. We concluded that starting from a peak (S/Nmin) of 7 the pulsar re- detections become unambiguous. As an upper limit to the range of (S/Nmin) values we adopt a value of 10. To estimate the completeness of our search we derived pseudo luminosity L=Sd2 limits for each of the binaries in our sample and compared them with the pseudo of Fig. 1. Cumulative histogram of pseudo luminosities for the known MSPs. We define the completeness of our pulsar search known MSP population in the ATNF database. The left axis is as the percentage of known millisecond pulsars that have pseudo labelled with the cumulative fraction of MSPs; the right axis is luminosities higher than our derived pseudo luminosity upper labelled with survey completeness for each candidate. The er- limits, i.e., as the percentage of known MSPs that would be de- rors on the pseudo luminosity upper limits contain errors on the tected if placed at the distance of the candidate. We searched distance, pulse duty cycle and our estimate of the relevant range the literature for distances d to the binaries in our sample and of (S/Nmin) values used with the pulsar radiometer equation (see used the most recent values. For HE0532-4503 and HE0929- Section 4). 0424 these are from Lisker et al. (2005), while the distances to TONS183 and PG1232-136 are from Altmann et al. (2004), where TONS183 is known as SB410. The former article claims 400MHz fluxes to our central observing frequency of 350MHz, − an error on the distance of 10%. In the latter article distance er- using the 1.6 average spectral index that Kramer et al. (1998) rors are not estimated, so for those sources we also propagatea find for MSPs. We find that our limits exclude with high con- 10% error. All distances were derived from models of sdB atmo- fidence the presence of pulsar emission: in Table 3 we list the spheres combined with the measurement. exact values for this completeness C. In Figure 1 we compare our limits to the pseudo luminosities of the millisecond pulsar population. For this statistical compar- 5. Discussion ison, we have selected all 50 MSPs with known 400 MHz fluxes in the ATNF database (Manchester et al., 2005). We scale these As no radio pulsations were found in any of our observations, we place upper limits on the pulsed radio emission from the putative 1 http://science.nrao.edu/gbt/obsprop/GBTpg.pdf neutron stars in these systems. Assuming the known MSP lumi- 4 Thijs Coenen et al.: A search for radio pulsars in four subdwarf B star binaries nosity distribution, we can exclude the presence of such pulsed 6. Conclusions radio emission with 100% certainty for 2 systems: PG1232-136 and TONS183 (Figure 1). For 2 other systems, HE0532-4503 We have searched for pulsed emission from potential pulsar and HE0929-0424, we were not able to put as strict upper lim- companions of four subdwarf B stars. No pulsed emission was its on the pseudo luminosity due to their larger distance and the found down to luminosities corresponding to, on average over shorter integration time for these pointings. For HE0929-0424, the 4 sources, the 4th percentile of the known millisecond pulsar our survey would have detected 96% of MSPs at the HE0929- population. 0424 distance. For HE0532-4503, this completeness C is 88% Acknowledgements. We thank Jason Hessels for help with the search pipeline. (Table 3, Figure 1). 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 #224838). Computing resources were provided by In relation to these non-detections we now discuss two se- ASTRON. Pulsar research at UBC is supported by an NSERC Discovery Grant. lection effects: the fraction of the sdB lifetime during which an The National Radio Astronomy Observatory is a facility of the National Science MSP is on; and the beamingfraction,which is the fraction of sky Foundation operated under cooperative agreement by Associated Universities, over which an MSP beam sweeps, and thus from which the MSP Inc. is in principle detectable. As outlined in Section 1, the MSP and sdB star are formed simultaneously; as MSPs have ages up of to References 109−10 yearsand sdBs have ages up to 108 years, an MSP formed in a binary with an sdB star will shine for the entire age of the Ag¨ueros, M. A., Camilo, F., Silvestri, N. M., et al. 2009, ApJ, 697, 283 sdB, and longer. We thus conclude there is no age bias against Altmann, M., Edelmann, H., & de Boer, K. S. 2004, A&A, 414, 181 Archibald, A. M., Stairs, I. H., Ransom, S. M., et al. 2009, Science, 324, 1411 detecting MSPs around sdB stars. There is however a non-zero Backer, D. C., Kulkarni, S. R., Heiles, C., Davis, M. M., & Goss, W. M. 1982, chance that for some of our non-detections a bright MSP emis- Nature, 300, 615 sion beam is present, but missing the Earth. The beaming frac- Bailes, M., Harrison, P. A., Lorimer, D. R., et al. 1994, ApJ, 425, L41 tion of MSPs is 0.7±0.2 (Kramer et al., 1998); high compared to Bhattacharya, D. 1998, in NATO ASIC Proc. 515: The Many Faces of Neutron Stars., ed. R. Buccheri, J. van Paradijs, & A. Alpar, 103–+ normal pulsars, but not unity and thus an important factor. Bhattacharya, D. & van den Heuvel, E. P. J. 1991, Phys. Rep., 203, 1 Chakrabarty, D., Morgan, E. H., Muno, M. P., et al. 2003, Nature, 424, 42 Cordes, J. M. & Lazio, T. J. W. 2002, ArXiv Astrophysics e-prints So far we have discussed MSPs, as any regular (non- Demorest, P. B., Pennucci, T., Ransom, S. M., Roberts, M. S. E., & Hessels, recycled) pulsar originally present in these systems will have J. W. T. 2010, Nature, 467, 1081 long shut off before the formation of the sdB began (cf. Section Dewey, R. J., Taylor, J. H., Weisberg, J. M., & Stokes, G. H. 1985, ApJ, 294, 1). For any non-recycled pulsars that are in the beam by chance, L25 the S in Table 3 list the minimum detectable flux for that Edwards, R. T. & Bailes, M. 2001, ApJ, 547, L37 min Ferdman, R. D., Stairs, I. H., Kramer, M., et al. 2010, ApJ, 711, 764 pointing. The pseudo-luminosity distributions of non-recycled Fontaine, G., Brassard, P., Charpinet, S., et al. 2008, in Astronomical Society and millisecond pulsars are similar (Manchester et al., 2005), but of the Pacific Conference Series, Vol. 392, Hot Subdwarf Stars and Related as the distance to any chance-coincident non-recycled pulsar is Objects, ed. U. Heber, C. S. Jeffery, & R. Napiwotzki, 231–+ unknown, their pseudo luminosity is unknown too. Geier, S., Heber, U., Podsiadlowski, P., et al. 2010, A&A, 519, A25+ Geier, S., Karl, C., Edelmann, H., Heber, U., & Napiwotzki, R. 2008, Mem. Soc. Astron. Italiana, 79, 608 We now compare the results of our search, triggered by Green, R. F., Schmidt, M., & Liebert, J. 1986, ApJS, 61, 305 the sample from Geier et al. (2008), with the follow-up re- Han, Z., Podsiadlowski, P., Maxted, P. F. L., & Marsh, T. R. 2003, MNRAS, 341, 669 sults of further spectroscopic investigation recently published in Han, Z., Podsiadlowski, P., Maxted, P. F. L., Marsh, T. R., & Ivanova, N. 2002, Geier et al. (2010). For PG1232-136, where we put a very strict MNRAS, 336, 449 (100% complete) limit on the pseudo luminosity, Geier et al. Haslam, C. G. T., Salter, C. J., Stoffel, H., & Wilson, W. E. 1982, A&AS, 47, 1 Heber, U. 2009, ARA&A, 47, 211 (2010) now report the mass of the unseen companion Mcomp Hu, H., Nelemans, G., Østensen, R., et al. 2007, A&A, 473, 569 to be higher than 6 M⊙. This indicates the compact compan- Jaffe, A. H. & Backer, D. C. 2003, ApJ, 583, 616 ion is a black hole; therefore no pulsed radio emission is ex- Johnston, H. M. & Kulkarni, S. R. 1991, ApJ, 368, 504 pected from that system, in agreement with our non-detection. Kaplan, D. L., Escoffier, R. P., Lacasse, R. J., et al. 2005, PASP, 117, 643 For TONS183 the new spectroscopic results point to a Mcomp Kramer, M., Xilouris, K. M., Lorimer, D. R., et al. 1998, ApJ, 501, 270 of 0.9 M but the error bars allow for a low mass Lawson, K. D., Mayer, C. J., Osborne, J. L., & Parkinson, M. L. 1987, MNRAS, ⊙ 225, 307 (WD). Our non-detection of pulsed radio emission is compat- Lisker, T., Heber, U., Napiwotzki, R., et al. 2005, A&A, 430, 223 ible with a WD companion, and since the constraint we put Manchester, R. N., Hobbs, G. B., Teoh, A., & Hobbs, M. 2005, AJ, 129, 1993 on pulsed radio emission from TONS183 is strict (100% com- Maxted, P. f. L., Heber, U., Marsh, T. R., & North, R. C. 2001, MNRAS, 326, plete) we rule out an MSP beamed towards Earth. For HE0929- 1391 Pfahl, E., Rappaport, S., & Podsiadlowski, P. 2003, ApJ, 597, 1036 0424 a Mcomp slightly above the is reported, Podsiadlowski, P., Rappaport, S., & Pfahl, E. D. 2002, ApJ, 565, 1107 but with error bars that also allow for a high mass WD. Our Prestage, R., Mason, B., & Lockman, J. 2006, 96%-complete non-detection in this case cannot exclude either ftp://ftp.gb.nrao.edu/NRAO-staff/rprestag/prestage.ppt possibility but an MSP is unlikely. The system with the least Ransom, S. M. 2001, PhD thesis, Harvard University strict pseudo luminosity upper limit, HE0532-4503, is reported Ransom, S. M., Hessels, J. W. T., Stairs, I. H., et al. 2005, Science, 307, 892 Ransom, S. M., Ray, P. S., Camilo, F., et al. 2011, ApJ, 727, L16+ to contain a companion to the sdB star of 3 M⊙. Even if one Taylor, J. H. & Weisberg, J. M. 1989, ApJ, 345, 434 allows for an sdB star as light as 0.3 M⊙ the companion re- van Leeuwen, J., Ferdman, R. D., Meyer, S., & Stairs, I. 2007, MNRAS, 374, mains more massive than the Chandrasekhar limit. Given our 1437 non-detection corresponding to the 12th percentile of the known MSP population (88% complete), an MSP is not likely, but this system remains an interesting candidate for deeper radio follow- up. Further observations of this source and of new candidate sdB-NS systems from Geier et al. (2010) are currently ongoing.