CHANDRA Detection of the AM CVn binary ES Cet (KUV

arXiv:astro-ph/0405203v223Jun2004CHANDRADetectionoftheAMCVnbinaryESCet(KUV01584-0939)TodE.StrohmayerLaboratoryforHighEnergyAstrophysics,NASA’sGoddardSpaceFlightCenter,Greenbelt,MD20771;[e-mail]ndraACISobservationsoftheultracompactAMCVnbinaryESCet.Thisobjecthasa10.3minutebinaryperiodandisthemostcompactoftheconfirmedAMCVnsystems.Wehave,forthefirsttime,unambiguouslydetectedtheX-raycounterparttoESCet.Ina20ksecACIS-Simageapoint-likeX-raysourceisfoundwithin1”ofthecataloguedopticalposition.ThemeancountrateinACIS-Sis0.013s−1,andthereisnostrongevidenceforvariability.WefoldedtheX-raydatausingtheopticalephemerisofWarner&Woudt,butdidnotdetectanysignificantmodulation.Ifan≈100%modulationsimilartothoseseenintheultracompactcandidatesV407VulandRXJ0806.3+1527werepresentthenwewouldhavedetectedit.Theupperlimit(3σ)toanymodulationattheputativeorbitalperiodis≈15%(rms).WeextractthefirstX-rayspectrumfromESCet,andfindthatitisnotwelldescribedbysimplecontinuummodels.Wefindsuggestiveevidencefordiscretespectralcomponentsat≈470and890eV,thatcanbemodelledasgaussianemissionlines.IncomparisonwithrecentX-raydetectionsofnitrogenandneoninanotherAMCVnsystem(GPCom),itappearspossiblethatthesefeaturesmayrepresentemissionlinesfromthesesameelements;however,deeperspectroscopywillberequiredtoconfirmthis.OurbestspectralmodelincludesablackbodycontinuumwithkT=0.8keValongwiththegaussianlines.Theunabsorbed0.2-5keVX-rayfluxwas≈7×10−14ergscm−2s−1.Theobservedluminosityinthehardcomponentisasmallfractionofthetotalexpectedaccretionluminosity,mostofwhichshouldberadiatedbelow0.1keVattheexpectedmassaccretionrateforthisorbitalperiod.WediscusstheimplicationsofourresultsforthenatureofESCet.Subjectheadings:Binaries:close-Stars:individual(KUV01584-0939,ESCet,Cet3)-Stars:whitedwarfs–X-rays:binaries–GravitationalWaves–2–1.IntroductionTheAMCVnstarsareaclassofultracompact,semidetachedbinarieswhicharetransfer-ringheliumfromadegenerate(orsemi-degenerate)dwarfontoacompanionwhitedwarf(seeWarner1995).Therearepresentlytenconfirmedsystemsofthistype(Woudt&Warner2003),andaneleventhhasrecentlybeenproposed(SDSSJ1240-01;Roelofsetal.2004)basedonopticalspectroscopy.ESCet(alsoknownasKUV01584-0939,andCet3)wasdiscoveredintheKosisurveyforUV-excessobjects(Kondo,Noguchi&Maehara1983),anditisalsolistedintheCVcatologofDownes,Webbink,&Shara(1997).ItsopticalspectrumwasstudiedbyWegner,McMahan&Boley(1987)andshowsstrongHeIIemis-sionlines.Astrongbluecontinuum,likelyduetoaccretion,isalsopresentandprovidesadditionalevidencethatthesystemisacompactbinary.Recently,Warner&Woudt(2002)strengthenedthisconclusionwiththereportofa10.3minutemodulationofitsVbandflux.Themodulationisverylikelytheorbitalperiodora“superhump”periodofanAMCVnbinary.ThesuperhumpperiodsinAMCVn’sarethoughttobecausedbydiskwarpingandprecession,andaretypicallyfoundjustlongwardoftheorbitalperiods(seePattersonetal.2002;Warner1995).The10.3minuteorbitalperiodmakesESCetthemostcompactofthebonafideAMCVnstars.TwoadditionalAMCVncandidateswithshorterperiodshaverecentlybeenproposed;V407Vul(Cropperetal.1998;Ramsayetal.2000;Marsh&Steeghs2002;Wuetal.2002;Strohmayer2002;Strohmayer2004a)andRXJ0806.4+1527(Israeletal.2002;Ramsayetal.2002;Hakalaetal.2003;Strohmayer2003),however,itisstillpossiblethattheobservedperiodsinthesesystemsarenottheorbitalperiodsofultracompactsystems(seeCropperetal.2003;Warner2003;Norton,Haswell&Wynn2004;Strohmayer2004a).IfESCetisindeedanultracompactbinary,thengravitationalradiationcandrivemasstransferatarateashighas1×10−8M⊙yr−1,andtheobjectshouldbeabrightsoftX-raysource.SeveralAMCVnsystemshavebeendetectedintheX-rayband,includingAMCVn,CRBooandGPCom(vanTeeseling&Verbunt1994;Ulla1995;Eracleous,Halpern&Patterson;andStrohmayer2004b).Indeed,recentXMM/NewtonobservationsofGPComhaveprovided,forthefirsttime,bothhighsignaltonoisespectraaswellasdetectionsofX-rayemissionlinesfromanAMCVnsystem(Strohmayer2004b).TheseresultsindicatethatX-rayspectralstudiescanprovidedetailedinformationabouttheaccretionprocessaswellasthecompositionoftheaccretedmatterinsuchsystems.ItisthereforeimportanttoexpandtheX-raysampleandidentifyadditionaltargetssuitableformorecomprehensivefollow-up.Moreover,asthemostcompactoftheconfirmedAMCVnstars,ESCetshouldbeastronggravitationalradiationsource,andchangesintheorbitalperiodshouldbemeasureable(seeWoudt&Warner2003;Strohmayer2004a).–3–TheROSATallskysurvey(RASS)faintsourcecatalogshowsa0.0166countss−1source,1RXSJ020052.9-092435,within≈10′′ofESCet,however,thesourceistoofainttoprovideanyusefulX-raycolors,andROSAT’spositionalaccuracyisnotsufficienttounambiguouslyconfirmitastheX-raycounterparttoESCet(Vogesetal2000).HerewepresentnewChandraobservationswhichhaveunambiguouslyidentifiedtheX-raycounterparttoESCet,furthersolidifyingitsaccreting,ultracompactcredentials.In§2wegiveabriefoverviewofourChandraobservationsandtheimaginganalysis.Wediscussourspectralandtiminganalysesin§3.Weclosein§4withadiscussionoftheimplicationsofourfindingsforthenatureofESCet.2.DataExtractionandAnalysisChandraobservedtheregionaroundESCetfor≈20kseconOct