Astronomy&Astrophysicsmanuscriptno.bjzv06 February5,2008 (DOI:willbeinsertedbyhandlater) Two new candidate ultra-compact X-ray binaries C.G.Bassa1,P.G.Jonker2,3,1,J.J.M.in’tZand2,1,andF.Verbunt1 6 0 1 AstronomicalInstitute,UtrechtUniversity,POBox80000,3508TAUtrecht,TheNetherlands 0 2 e-mail:[email protected] 2 SRONNetherlandsInstituteforSpaceResearch,Sorbonnelaan2,3584CAUtrecht,TheNetherlands n 3 Harvard-SmithsonianCenterforAstrophysics,60GardenStreet,MS83,Cambridge,Massachusetts,USA a J Received/Accepted 3 1 Abstract. We present the identification of the optical counterparts to the low-mass X-ray binaries 1A 1246−588 and v 4U 1812−12. Wedetermine the X-ray position of 1A 1246−588 from ROSAT/PSPCobservations and find within the error 5 circleabluestarwithV = 19.45, B−V = 0.22andR−I = 0.22whichweidentifyasthecounterpart.WithintheChandra 4 errorcircleof4U1812−12,asinglestarispresentwhichappearsbluewithrespecttothestarsinthevicinity.IthasR=22.15, 0 R−I=1.53.Distanceestimatesforbothsystemsindicatethattheopticalcounterpartsareintrinsicallyfaint,suggestingthatthey 1 areultra-compactX-raybinaries.Theseidentificationswouldincreasethenumberofcandidateultra-compactX-raybinaries 0 from2to4,whereasorbitalperiodsaremeasuredforonly7systemsintheGalacticdisk. 6 0 Keywords.X-rays:binaries–X-rays:individual:1A1246−588;4U1812−12 / h p - o 1. Introduction 2. X-rayObservations r st The canonical low-mass X-ray binary consists of a neutron 4U1812−12(l = 18◦.03,b = 2◦.40)hasbeenobservedbyvari- :a starorblackholeandalow-massmain-sequenceor(sub)giant ousX-raysatellites,buttheobservationswithBeppoSAX/WFC v donorstar, and hasan orbitalperiodlongerthan onehour,up and Chandra are most relevant for this paper. Type I X-ray Xi toseveralhundreddays. burstswithphotosphericradiusexpansionhavebeenobserved with the former, providing an unabsorbed bolometric peak r a Recently,ithasbeenfoundthattheclassofultra-compact flux of (1.5 ± 0.3) × 10−7ergcm−2s−1 (Cocchietal. 2000). low-massX-raybinariesmakesupabouthalf(5outof12)of A 1ks observationwith the back-illuminatedS3 CCD aboard the low-mass X-ray binaries in globular clusters (e.g., review ChandrawasanalyzedbyWilsonetal.(2003),yieldinganac- byVerbunt&Lewin2004),whereasagrowingnumberofsuch curatepositionof4U1812−12;α =18h15m06s.18,δ = J2000 J2000 systems is also discovered in the Galactic disk (7 with mea- −12◦05′47′.′1,withanuncertaintylimitedbytheChandrabore- sured periods,and2 candidates,see e.g.,Juett&Chakrabarty sight (0′.′6, 90% confidence; Aldcroftetal. 2000). A flux of 2003; Nelemansetal. 2004; Wang&Chakrabarty 2004). In 4.4 × 10−10ergcm−2s−1 (1–10keV) and absorption of N = H addition, observations with the Wide Field Cameras (WFCs) (1.1±0.2)×1022cm−2weredeterminedfromspectralfits. of BeppoSAX have found a new class of low-mass X-ray 1A 1246−588(l = 302◦.70, b = 3◦.78) has received much binaries, bursters with (very) low persistent X-ray emission less attention,thoughit has been observedserendipitouslyby (Cocchietal.2001).Thedistributionofthesourcesinthisclass several X-ray observatories. A type I X-ray burst was ob- is more concentrated towards the Galactic center than that of served with BeppoSAX/WFC (Piroetal. 1997) and a short thecanonicallow-massX-raybinaries(Cornelisseetal.2002). (0.85ks) follow-up observation with ROSAT/PSPC linked it To elucidate the evolutionary status and history of to 1A 1246−588 (Bolleretal. 1997). In this observation N H these systems, observations at longer wavelengths, in was measured to be (2.9 ± 0.9) × 1021cm−2 and the 0.1– particular optical/infrared, are crucial. Such observations 2.4keV flux was 1.7×10−10ergcm−2s−1 (Bolleretal. 1997). may reveal the orbital period, directly or indirectly (e.g., WehavereanalyzedtheROSAT/PSPCobservationsofthefield vanParadijs&McClintock 1994), or provide informa- of 1A 1246−588 using standard routines from the EXSAS tion about the donor and its chemical composition (e.g. distribution (Zimmermannetal. 1996). In the 0.85ks expo- Nelemansetal.2004).Thefirststeptosuchstudiesistheop- sure obtained in February 1997, the X-ray binary is only 7′ ticalidentificationof theX-raybinary.In thisletter wereport off-axis, compared to 39′ in the much longer PSPC obser- two new optical identifications, which may be ultra-compact vation from 1993, and the shorter observation provides the X-raybinaries. best source position. In this observationthe X-ray binary has 2 C.G.Bassaetal.:Twonewcandidateultra-compactX-raybinaries Fig.1.(left)A1′×1′ subsectionofthecombined2minR-bandWFIimageof1A1246−588.The95%confidenceerrorcircle on the ROSAT positionhas a radiusof 14′.′6 and is depictedwith the large circle. The proposedopticalcounterpart,star X, is indicated with the tick marks, while another blue star, star A, which is located outside the error circle, is encircled. (right) A 30′′×30′′ subsectionofthe5minR-bandIMACSimageof4U1812−12.Thehorizontalblackbarinthetoppartoftheimage isareadoutstreakofabrightstar,whereasthegreystreakinthebottompartisadeadcolumnoftheCCD.TheChandraerror circle (95%confidence,1′.′25in radius)is depictedwith the circle.The proposedopticalcounterpartis indicatedwith the tick marks. α = 12h49m39s.61,δ = −59◦05′13′.′3,withaninternal taining the X-ray binaries (chip 8 for 4U 1812−12 and chip J2000 J2000 uncertaintyof0′.′3oneachcoordinate.Theexternaluncertainty 2 for 1A 1246−588). The WFI images have a pixel scale of on this position,the uncertaintyin the bore-sightof the satel- 0′.′24pix−1, while IMACSobservationsweretaken with 2×2 lite, is about6′′ (Ayres2004).Due to theshortexposureonly binning,yieldinga pixelscale of 0′.′22pix−1. The science im- a fewX-raysourcesarepresentinthe PSPC observation,and ageswerecorrectedforbiasandflatfieldedwithdomeflatsus- none of them are coincidentwith brightstars. Thus, no bore- ingstandardroutinesrunningwithinMIDAS. sightcorrectionispossibleandtheuncertaintyontheposition Large-scalevariationsinthebackgroundoftheI-bandim- isdominatedbythepointinguncertaintyofROSAT. ages,knownasfringing,werepresentintheWFIobservations. 1A1246−588and4U1812−12havebeenpersistentlyde- We corrected for this with a fringe frame. This fringe frame tected in X-rays by the All Sky Monitor (ASM) onboard the was constructedby median combininga set of 20 I-band im- RossiX-rayTimingExplorer(RXTE).Bothduring,afewdays agesobtainedearlierthatnight,suchthatitcontainedonlythe beforeandafewdaysafterthetimeoftheopticalobservations contributionsoftheskyandthefringevariations.Thelevelof describedbelow bothsourcesweredetectedat dailyaveraged theskywasestimatedandsubtractedfromthisimage,leaving countratesofatleast1ASMcountpersecond. onlythe fringevariations.Thisresultwasscaled to the fringe variationsintheI-bandimagesoftheX-raybinaryandsubse- quentlysubtractedfromtheseimages. 3. OpticalObservations For the WFI observations, the images taken through the We retrieved archival observations of 1A 1246−588 obtained samefilterwerealignedusingintegerpixeloffsetsandmedian with the Wide Field Imager (WFI) at the ESO 2.2metre tele- combined to remove image artifacts and increase the overall scopeon La Silla onMarch 26/27,2000.A series of dithered signal-to-noise ratio. Finally, a 4′ × 4′ subsection of the av- 4 B,6 V, 5 R and5 I-bandimageswere taken,allwith expo- eragedimages, centeredon the nominalpositionof the X-ray sure times of 2min under clear conditionswith 0′.′8–1′.′0 see- binary,wasextractedandusedfortheastrometryandphotom- ing. The field of 4U 1812−12 was imaged with the 6.5metre etry.For4U1812−12,a3.′8×3.′8subsectionoftheIMACSR Inamori Magellan Areal Camera and Spectrograph (IMACS) andI-bandimageswasextracted. attheMagellanBaadetelescopeonLasCampanasonJuly6/7, For the astrometry of the WFI observations of 2005.Asingle5minimagewasobtainedinbothRandIunder 1A 1246−588 we measured the centroids of all 31 astro- 0′.′6seeing.BothIMACSandWFIaremosaicsofeight4k×2k metric standards from the second version of the USNO CCD detectors and we analyzed the images from the detector con- Astrograph Catalog (UCAC2; Zachariasetal. 2004) that C.G.Bassaetal.:Twonewcandidateultra-compactX-raybinaries 3 overlappedwiththe4′×4′subsectionofthecombinedR-band image and that were not saturated and appeared stellar and unblended. We removed one outlier that had a total residual of 0′.′74, and the remaining stars were used to compute an astrometric solution, fitting for zero-point position, scale and positionangle.Theastrometricsolutionhasroot-mean-square (rms) residuals of 0′.′054 in right ascension and 0′.′047 in declination. We used a similar approach for the IMACS observations of 4U 1812−12, but had to use the USNO-B1 catalog (Monetetal. 2003) as very few UCAC2 standards overlapped with the IMACS images. About 70 USNO-B1 standardswereused to calibratea 3.′8×3.′8 subsectionofthe R-bandimage,givingasolutionwithrmsresidualsof0′.′19in rightascensionand0′.′21indeclination. The DAOPHOT II package (Stetson 1987), running in- side MIDAS, was used to determineinstrumentalmagnitudes throughpointspreadfunction(PSF)fitting.Aperturephotom- etryofseveralbrightstarswasusedtodetermineaperturecor- rections.ForthecalibrationoftheWFIobservationswedeter- minedinstrumentalmagnitudesofsome100photometricstan- dards in the standard field SA98 and calibrated these against the calibrated values by Stetson (2000), fitting for zero-point andcolourcoefficients.We assumedextinctioncoefficientsof 0.22, 0.19, 0.14 and 0.11mag per airmass for B, V, R and I- band, respectively, taken from the WFI webpage1. The rms residuals of the calibration were 0.04mag in B and R, 0.03 in V and 0.05mag in I. The IMACS observations were cali- bratedusing11standardsintheTPhefield,againusingvalues from Stetson (2000) and fitting for zero-point and colour co- efficients.Thestandardfieldwasimagedatsimilarairmassas the4U1812−12fieldandnoextinctioncoefficientswereused. The rms residuals of the calibration were 0.05mag in R and Fig.2. (top) A colour-magnitude and colour-colour diagram 0.07maginI. (bottom)ofthe BVRIphotometryofthe1A1246−588region. In Fig. 1 we present finding charts for the regions of StarsinsidetheROSAT errorcircle(Fig.1)areencircled.Also 1A1246−588and4U1812−12.Wesearchedforopticalcoun- shown in the CMD are two solar metallicity isochrones from terpartstotheX-raysourcesin95%confidenceerrorcircleson Girardietal.(2000),placedatadistanceof4.0kpcandanab- the ROSAT and Chandra position. For 4U 1812−12,a single sorption of A = 1.0. The left is for an age of 0.1Gyr, while V starislocatedinthe1′.′25errorcircle,whileseveralstarsliein- the right has an age of 10Gyr. These isochrones are merely sidethe14′.′6errorcircleonthepositionof1A1246−588.We to guide the eye, as there will be a spread in distance, age, notethatasbothX-raybinariesareexpectedtohaveanaccre- metallicity and absorption. The arrows indicate the effects of tiondisk,theopticalcoloursshoulddisplayanexcessofemis- absorption, where the length of the arrow is for an extra ab- sionatbluewavelengthsandthusappearblue,typicallyhaving sorptionof∆A = 1.0.Ofthestarspresentintheerrorcircle, V (B−V) ≈0.0(vanParadijs,J.&McClintock,J.E.1995). star X is exceptionallyblue and the likely optical counterpart 0 InFigure2weshowthecolour-magnitudediagram(CMD) to1A1246−588.StarAhassimilarcoloursandmagnitudesas and a colour-colour diagram of the BVRI photometry of all starX,butislocatedoutsidethe95%confidenceerrorcircle. stars on the 4′ × 4′ image of 1A 1246−588. To illustrate the structure seen in the CMD, we have overplotted isochrones fromGirardietal.(2000).Thereare3starsthatareexception- allyblue(having B−V < 0.25andR−I < 0.25)andtwo of bluer than this. Furthermore, at the observed R-band magni- tude and R − I colour, both stars are about half a magnitude them, stars A and X, are near the error circle (Fig. 1). Star X hasV =19.45±0.02,B−V =0.22±0.03,V−R=0.14±0.03 bluerthanthebulkofthestarsatthesameR-bandmagnitude. andR−I = 0.22±0.06,while star A hasV = 18.99±0.01, TheopticalpositionofstarXisαJ2000 =12h49m39s.364±0′.′06, B − V = 0.15 ± 0.02, V − R = −0.01± 0.02 and R − I = δJ2000 = −59◦05′14′.′68 ± 0′.′05, which is only 2′.′4 from the 0.16 ± 0.04. The mean colour and standard deviation of all ROSAT position and well within the 1σ uncertainty of 6′′. stars in the 1A 1246−588regionis B−V = 1.03±0.22 and Star A on the other hand has αJ2000 = 12h49m35s.660±0′.′06, R− I = 0.73±0.28, and both star X and A are significantly δJ2000 = −59◦05′12′.′94±0′.′05,whichis30′′ (about5σ)from theROSATposition.Weestimatethattheprobabilityoffinding 1 http://www.ls.eso.org/lasilla/sciops/2p2/E2p2M/WFI/zeropoints/ astarasblueasstarXwithinthe95%confidenceerrorcircle 4 C.G.Bassaetal.:Twonewcandidateultra-compactX-raybinaries of1A1246−588isabout2%.Hence,weidentifystarXasthe vanParadijs&McClintock(1994),thissuggeststhatthesesys- opticalcounterpartto1A1246−588. temsareultra-compactX-raybinaries(UCXBs;havinganor- A single star is present within the Chandra error circle bital period below an hour). Here, the faintness of the coun- of 4U 1812−12 at α = 18h15m06s.155 ± 0′.′19, δ = terpart is due to the reprocession of X-rays in a physically J2000 J2000 −12◦05′46′.′70±0′.′21.Thispositionisonly0′.′5fromtheX-ray small accretion disk. However, these systems remain candi- position of 4U 1812−12 (Wilsonetal. 2003). From the pho- date UCXBs until the orbital period is determined, i.e. either tometryweobtainR=22.15±0.02andR−I =1.53±0.03.This throughoptical/IRorX-rayobservations. starisnotasblueasthecounterpartof1A1246−588,however, Ifthese systemsindeedturnoutto haveultra-compactor- inlightofthelargerabsorbingcolumnfor4U1812−12,thisis bits, it is interesting to note that for the observed X-ray lu- notsurprising.Still,thecounterpartisbluerthanthebulkofthe minosities of L ≈ 0.9 × 1036ergcm−2s−1 (1–10keV) for X starsinthe4U1812−12region,whichhaveR−I =1.88±0.23. 4U 1812−12 and L <∼ 1036ergcm−2s−1 (0.1–2.4keV) for X Furthermore, there are 9 stars within a radius of 5′′ from the 1A 1246−588, these systems satisfy the notion presented by Chandra position of 4U 1812−12, and all, except the candi- in’tZandetal. (2005); that LMXBs with persistent luminosi- datecounterpart,areredderthanR−I =1.73.Finally,wenote ties with L <∼ 1036ergcm−2s−1 may be ultra-compactX-ray X that the probability of a chance coincidence of a star within binaries. the 95%confidenceerror circle is about0.15%.We conclude Acknowledgements. The Munich Image Data Analysis System thatthestarinsidetheerrorcircleistheopticalcounterpartto (MIDAS) is developed and maintained by the European Southern 4U1812−12. Observatory. This research made use of results provided by the ASM/RXTEteamsatMITandattheRXTESOFandGOFatNASA’s 4. Discussion GSFC. CGB and JJMZ acknowledge support by the Netherlands Organization for Scientific Research (NWO). PGJ acknowledges Wehaveidentifiedtheopticalcompanionstothelow-massX- fundingfromNASAgrantGO4-5033X. raybinaries1A1246−588and4U1812−12basedontheirpo- sitionalcoincidencewiththeX-raypositionandtheircolours. References The counterpart to the first has V = 19.45, B − V = 0.22, while that of 4U 1812−12 is somewhat fainter at R = 22.15, Aldcroft, T. L., Karovska, M., Cresitello-Dittmar, M. L., R−I =1.53. Cameron, R. A., & Markevitch,M. L. 2000,in Proc. SPIE Due to its position somewhat out of the Galactic plane, Vol. 4012, p. 650-657, X-Ray Optics, Instruments, and thehydrogenabsorptioncolumn N towards1A 1246−588is Missions III, Joachim E. Truemper; Bernd Aschenbach; H moderate and suggests A = 1.7 (Predehl&Schmitt 1995). Eds.,ed.J.E.Truemper&B.Aschenbach,650–657 V This is smaller than the maximum absorption in this line-of- Ayres,T.R.2004,ApJ,608,957 sight, whichis predictedto reach A = 1.9aroundd = 7kpc Boller, T., Haberl, F., Voges, W., Piro, L., & Heise, J. 1997, V by the model of Drimmeletal. (2003). This limit constrains IAUCirc.,6546 the absolute magnitude of the companion of 1A 1246−588 Cocchi,M.,Bazzano,A.,Natalucci,L.,etal.2001,A&A,378, to M >∼ 3.5. Though this reasoning assumes that both L37 V the model by Predehl&Schmitt (1995) and Drimmeletal. Cocchi,M.,Bazzano,A.,Natalucci,L.,etal.2000,A&A,357, (2003)arecorrect,thisdistanceisinagreementwithestimates 527 fromBeppoSAX/WFCandRXTE/ASMobservationsofphoto- Cornelisse,R.,Verbunt,F.,in’tZand,J.J.M.,Kuulkers,E.,& spheric radius expansion bursts of 1A 1246−588,which sug- Heise,J.2002,A&A,392,931 gestadistanceof5kpc(in’tZandetal.inprep.). Drimmel, R., Cabrera-Lavers, A., & Lo´pez-Corredoira, M. For4U1812−12,thephotosphericradiusexpansionbursts 2003,A&A,409,205 thathavebeenobservedbyCocchietal.(2000)provideanes- Girardi, L., Bressan, A., Bertelli, G., & Chiosi, C. 2000, timateonthedistancetothisLMXB.AssuminganEddington A&AS,141,371 peak luminosity of L = 3.8× 1038ergs−1 for the accretion in’tZand,J.J.M.,Cornelisse,R.,&Me´ndez,M.2005,A&A, X of helium-rich material (Kuulkersetal. 2003), the distance is 440,287 estimated at 4.6kpc. 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