3002 tcO 51 1v3040130/hp-ortsa:viXraYoungNeutronStarsandTheirEnvironmentsIAUSymposium,Vol.218,2004F.CamiloandB.M.Gaensler,eds.
AccretionontotheneutronstarinBe/X-raybinaries
KimitakeHayasaki
DevisionofPhysics,graduateschoolofscience,HokkaidoUniversity,Kitaku,Sapporo060-0810,Japan
Atsuo.T.Okazaki
FacultyofEngineering,Hokkai-GakuenUniversity,Toyohira-ku,Sapporo062-8605,Japan
Abstract.
WestudyaccretionontotheneutronstarinBe/X-raybinaries,usinga3DSPHcodeandthedataimportedfromahighresolutionsimula-tionbyOkazakietal.(2002)foracoplanarsystemwithashortperiod(Porb=24.3d)andmoderateeccentricity(e=0.34).Wefindthatatime-dependentaccretiondiskisformedaroundtheneutronstarinBe/X-raybinaries.ThediskshrinksaftertheperiastronpassageoftheBestarandrestoresitsradiusafterwards.OursimulationsshowthatthetruncatedBediskmodelforBe/X-raybinariesisconsistentwiththeobservedX-raybehavior.
1.Introduction
TheBe/X-raybinariesrepresentthelargestsubclassofhigh-massX-raybinaries.ThesesystemsconsistofaneutronstarandaBestarwithacoolequatorialdisk.Theorbitiswideandusuallyeccentric.MostoftheBe/X-raybinariesshowonlytransientactivityintheX-rayemission.TheseoutburstsresultfromthetransientaccretionontotheneutronstarfromthecircumstellermatteroftheBestar.Inthispaper,wesimulatetheaccretionflowaroundtheneutronstarinBe/X-raybinaries,usinga3DSPHcode(Bateetal.1995)andthemass-transferratefromtheBe-stardiskobtainedbyOkazakietal.(2002).2.
Non-steadyaccretiondiskaroundtheneutronstar
InBe/X-raybinaries,themass-transferratehasstrongphasedependence(Okazakietal.2002).Therefore,thestructureofanaccretiondiskformedaroundtheneutronstarinthesesystemsisalsolikelytobestronglyphasedependent.
Figure1givestheradialdiskstructuresandthesnapshotsofadevelopedaccretiondiskaroundtheneutronstarfor7≤t/Porb<8.Fromthefigure,wenotethatthematerialtransferredfromtheBe-stardisktotheneutronstarformsanon-steadyaccretiondiskaroundtheneutronstar.ItisnotedfromtheupperpanelsthatthediskisnearlyKeplerianatanyphaseandtheaccretion
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2KimitakeHayasaki&Atsuo.T.Okazaki
Figure1.Phase-dependentradialstructures(upperpanels)andsnapshots(lowerpanels)ofadevelopedaccretiondiskaroundtheneu-tronstarfor7≤t/Porb<8.Intheupperpanels,thesolid,thedot-dashedandthedashedlinedenotethelogarithmofthesurfacedensity,theradialMachnumberandtheazimuthalvelocitynormalizedbytheKeplerianvelocityatrin=3.0×10−3a,wherea=6.6×1012cmisthesemi-majoraxis,respectively.Thesurfacedensityismeasuredinunitsofρ−11gcm−2,whereρ−11ishighestlocaldensityintheBe-stardisknormalizedby10−11gcm−3.Thelowerpanelsshowthelogarithmofthesurfacedensity.ThedashedcircledenotestheeffectiveRochelobeoftheneutronstar.Theshortline-segmentindicatesthedirectionoftheBestar.Annotatedatthetop-leftcornerofeachpanelarethenumberofSPHparticles,NSPH,andthenumberofparticlesinsidetheeffectiveRochelobeoftheneutronstar,NRoche.
flowishighlysubsonicexceptintheoutermostpartforashortperiodoftimewhenthematerialoftheBe-stardiskistransferredtotheneutronstar.
ThediskshrinksatperiastronpassagebyanegativetorqueexertedbytheBestar.Inaddition,therampressureofthesupersonicinfallofmattercapturedbytheneutronstararoundperiastronenhancesthedensityintheoutermostpartoftheaccretiondisk,makingthediskouteredgesharp.Afterwards,thediskrestoresitsradiusbytheviscousdiffusion.Thus,oursimulationconfirmsthatthestrucutreoftheaccretiondiskinBe/X-raybinarieshasastrongdependenceontheorbitalphase.Acknowledgments.Foundation.References
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