Galactic Black-Hole Candidates Shining at the Eddington Luminosity

[摘要] WediscussdistinctivefeaturesofluminousaccretiondisksshiningattheEddingtonluminosityinthecontextofgalacticblack-holecandidates(GBCs).Wefirstnotethatthestandard-diskpictureisnotapplicable,althoughitisoftenpostulated.Rather,thediskbecomesadvection-dominatedwhileremainingopticallythick(theso-calledslimdisk).Theslimdiskexhibitsseveralnoteworthysignatures:(1)Thediskluminosityisinsensitivetothemass-flowrates,á¹€,andisalwayskeptaroundtheEddingtonluminosity,LE,evenifá¹€greatlyexceedsLE/c2.Thisreflectsthefactthatradiativecoolingisnolongerbalancedbyviscousheatingandexcessenergyiscarriedbyaccretingmattertoblackholes.(2)Thespectraoftheslimdisksaremulti-colorblackbodycharacterizedby(i)ahighmaximumtemperature,kTin∼afewkeV,(ii)asmallsizeofanemittingregion,rinlt;3rg(withrgbeingSchwarzschildradius),duetosubstantialradiationcomingoutfrominside3rg,and(iii)flatterspectrainthesoft-Xbands,vSv∼v0,becauseofaflattereffectivetemperatureprofileoftheslimdisk,Teff∝r−1/2(incontrastwithTeff∝r−3/4inthestandarddisk).Thus,asmallrin(≪3rg)doesnotnecessarilymeanthepresenceofaKerrhole.Furthermore,(3)asá¹€increases,Tinincreases,whilerindecreasesasrin∝(Tin)−1approximately.Thatis,thechangesinrinderivedfromthefittingdonotnecessarilymeanthechangesinthephysicalboundaryoftheopticallythickportionsofthedisk.Observationalimplicationsarediscussedinrelationtobinaryjetsources.

[学科分类] 天文学(综合)

[关键词] "accretion;accretion disks";"black holes";"stars: X-rays"