Discovery of the accretion-powered millisecond pulsar SWIFT

We report on the discovery by the Swift Gamma-Ray Burst Explorer of the eighth known transient accretion-powered millisecond pulsar, SWIFT J1756.9-2508, as part of routine observations with the Swift Burst Alert Telescope hard X-ray transient monitor. The

7

2

ep

S

1

1

]h

p

-

o

r

t

s

a

[

1

v

3

9

6

1

.

9

7

:0v

i

X

r

aAcceptedbyApJLettersDiscoveryoftheAccretion-PoweredMillisecondPulsarSWIFTJ1756.9 2508withaLow-MassCompanionH.A.Krimm1,2,C.B.Markwardt1,3,C.J.Deloye4,P.Romano5,6,D.Chakrabarty7,S.Campana5,J.R.Cummings1,8,D.K.Galloway9,N.Gehrels10,J.M.Hartman7,P.Kaaret11,E.H.Morgan7,J.Tueller10February1,2008ABSTRACTWereportonthediscoverybytheSwiftGamma-RayBurstExploreroftheeighthknowntransientaccretion-poweredmillisecondpulsar,SWIFTJ1756.9 2508,aspartofroutineobservationswiththeSwiftBurstAlertTele-scopehardX-raytransientmonitor.ThepulsarwassubsequentlyobservedbyboththeX-RayTelescopeonSwiftandtheRossiX-RayTimingExplorerPro-portionalCounterArray.Ithasaspinfrequencyof182Hz(5.5ms)andanorbitalperiodof54.7minutes.Theminimumcompanionmassisbetween0.0067

We report on the discovery by the Swift Gamma-Ray Burst Explorer of the eighth known transient accretion-powered millisecond pulsar, SWIFT J1756.9-2508, as part of routine observations with the Swift Burst Alert Telescope hard X-ray transient monitor. The

and0.0086M⊙,dependingonthemassoftheneutronstar,andtheupperlimit

onthemassis0.030M⊙(95%con dencelevel).Suchalowmassisinconsistent

withbrowndwarfmodels,andcomparisonwithwhitedwarfmodelssuggeststhat

thecompanionisaHe-dominateddonorwhosethermalcoolinghasbeenatleast

modestlyslowedbyirradiationfromtheaccretion ux.NoX-raybursts,dips,

eclipsesorquasi-periodicoscillationsweredetected.Thecurrentoutburstlasted

≈13daysandnoearlieroutburstswerefoundinarchivaldata.

Subjectheadings:binaries:close—pulsars:general—stars:neutron—white

dwarfs—X-rays:binaries

1.Introduction

Itisnowwellestablishedthatasubsetofneutronstarsinlow-massX-raybinarysystems

aremillisecondpulsarspoweredbyaccretionfromeitherabrowndwarfordegeneratecom-panioninaveryclosebinarycon guration.Suchsystemsarebelievedtobeprogenitorsofmillisecondradiopulsars(Alparetal.1982).BeforethediscoveryofSWIFTJ1756.9 2508,thereweresevenknownaccretion-poweredmillisecondpulsars(seeKaaretetal.2006,forafulllisting).Thesampleisnowbecominglargeenoughtostarttostudyclassesoftheseob-jectswhichcaninturnleadtoabetterunderstandingofthepropertiesofneutronstarX-raybinariesandtheirrelationshiptomillisecondradiopulsars.Theknownaccretion-poweredmillisecondpulsarsallappeartobeinverycloseorbits(withorbitalperiodsrangingbetween41.1and256.5min).Forallbutonesuchsystem,XTEJ1814 338(Bhattacharyyaetal.2005),orbitalconstraintsruleoutahydrogen-burningmainsequencecompanion,andforfoursystems,evenabrowndwarfcompanionisnotconsistentwiththeorbitalparame-ters.Theapplicationofrecentmodelsofwhitedwarfdonorsinultracompactbinaries(e.g.Deloye&Bildsten2003;Deloyeetal.2007),showsthat,withincertaintemperatureranges,whitedwarfsareaviablemodelforthecompanioninthemostcompactoftheX-raymil-lisecondpulsarsystems.

InthisLetter,wereportthediscoveryofSWIFTJ1756.9 2508(hereafterreferredto

asJ1756),withafrequencyof182.07Hz.ItwasdiscoveredbytheSwiftGamma-RayBurstExplorerBurstAlertTelescope(BAT)aspartofthehardX-raytransientmonitorprogram.In§2wediscusstheobservationsmadewithSwiftandtheRossiX-rayTimingExplorer(RXTE).In§3and§4wediscussthetimingandspectralanalysisandin§5wederivelimitsonthecompanionmassandcomparethissourcetotheotherknownaccretingX-raymillisecondpulsars.Allerrorsareatthe90%-con dencelevelunlessotherwisestated.

We report on the discovery by the Swift Gamma-Ray Burst Explorer of the eighth known transient accretion-powered millisecond pulsar, SWIFT J1756.9-2508, as part of routine observations with the Swift Burst Alert Telescope hard X-ray transient monitor. The

2.Observationsanddatareduction

2.1.Swift

SWIFTJ1756.9 2508wasdiscoveredusingtheSwiftBAThardX-raytransientmonitor

(Krimmetal.2006).TheBATisalarge eldofviewinstrumentthatcontinuallymonitorstheskytoprovidethegamma-raybursttriggerforSwift.Onaveragemorethan70%oftheskyisobservedonadailybasis.Skyimagesareprocessedinnearreal-timetodetectastrophysicalsourcesinthe15-50keVenergyband12.

J1756wasdetectedinthedailyfull-skymosaicsthatareproducedinthetransient

monitor.It rstrosetodetectability(Fig.1)on2007June7(MJD54258)withanaver-agerateinSwiftBATof0.0083±0.0015cts 1cm 2,correspondingtoapproximately37mCrab.ThediscoverywasannouncedonJune13(Krimmetal.2007).TheBATdataforallpointingswhenthesourcewasfullycoded(visibletoallBATdetectors)wereprocessedusingtheSwiftanalysistools,whichincludedcleaningoutthedi usebackgroundand uxfromotherbrightsourcesinthe eldofview.

ThediscoveryofJ1756triggeredatargetofopportunity(ToO)observationintheSwift

X-rayTelescope(XRT)beginningat15:25:32UT,2007June13.Itwasintermittentlyob-servedover13daysforatotalexposureof60ks.TheXRTdatawereprocessedwithstandardprocedures(xrtpipelinev0.10.6withinFTOOLSintheHeasoftpackagev.6.1.2), ltering,screening,andgradeselectioncriteria(Burrowsetal.2005).Thephotoncounting(PC)datacollectedduringthe rstthreesegmentswerecorrectedforpile-upandforthepresenceofsingle-re ectionringsduetothenearbybrightsourceGX5 1.13Forthewindowedtimingdataweextractedsourceeventsinasquareregionwithasideof20pixels.Ancillaryresponse lesweregeneratedwithxrtmkarfandaccountfordi erentextractionregions,vignettingandPoint-SpreadFunction(PSF)corrections.Thelightcurve(Fig.1)wascorrectedforPSFlossesandforvignetting.Thebest tXRTpositionisshowninTable1.

2.2.RXTE

ThediscoveryofJ1756alsotriggeredaToOobservationintheRossiX-rayTimi …… 此处隐藏：20544字，全部文档内容请下载后查看。喜欢就下载吧 ……