The evolution of black hole states(7)

We discuss the evolution of black hole transients on the basis of a few systems that were intensively observed with the Rossi X-ray Timing Explorer. We focus on the global evolution and the observed state transitions. Rather than giving a numerical recipe

7

4.2.Thehardstate softstatetransitions

Immediatelyafterleavingandbeforereturningtothehardstate,blackholetransientsareobservedinanintermediatestate,withspectralpower-lawindicesbetweenroughly1.5–2.5.Wearenotawareofanyexceptiontothis.Thisstateisdi erentfromthehardstateinsev-eralaspects.Dependingonwhethertheintermediatestateisobservedduringthehard→softorsoft→hardtransition,sourcesshowaclearsteepening/ atteningofthespectralpowerlawcomponent,astrongincrease/decreaseinthenoiseandQPOfrequencies,andanincreas-ing/decreasingfractionaldisk ux,comparedtothehardstate.Allthesepropertiesevolvesmoothlyfromandtothehardstate.Infact,withouttheHIDandoptical/IRinformation(seeFig.4)itwouldbedi culttoexactlypin-pointthetransitionbetweenthesetwostates.Inthecaseofthehard→softtransitionthemovementtotheleftintheHIDisduetothecombinationofincreaseddisk uxandsteepeningofthepowerlaw.Inthetimingdomain,thebroad-bandvariabilitycomponentsseeninthepowerdensityspectrumincreasetheirchar-acteristicfrequencies,showinganevolutionthatclearlylinksthemtothecorrespondingcomponentsinthehardstate.AcleartypeCQPOappears,alsowithacharacteristicfrequencyincreasingwithtimeanddecreasinghardness(seeBellonietal.(2004)).Atypicalpower-densityspectrumfromtheintermediatestateisshowninFigure3.

Itisimportanttonotethatthehard→intermediatestatetransitiondoesnotalwaysoccuratthesame uxlevel,ascanbeseenfromFigure

4.Duringthe2004outburstofGX339–4thistransitionoccurredata uxlevelthatwasaboutafactorof4lowerthanin2002.Thetransitionin2004wasprecededbyasmallhardstateoutburst,whichsuggeststhatthe uxlevelatwhichthetransitionoccursdependsontherecentaccretionhistory.AfterpassingthroughtheintermediateGX339-4continuedtobrighteninthesoftsateduringits2004outburst.

Anotherimportantpointtonoteisthefactthattheoccurrenceoftheintermediatestatedoesnotseemtodependonthetimederivative˙.InXTEJ1650–500itoccurredduringthedecay,butinGX339–ofM˙wasapparentlystillincreasing,aswitnessedbythesecond4whileM(soft)maximuminthelightcurve.However,inbothcasesthetransitionresultedina(temporary)dropinthecountratebyafactorof～2.Takingoncemorethe2002/2003outburstofGX339–4asatem-plate,weseethatwhenthehardnessgoesbelowawellde nedthresh-old,thetimingpropertieschangesharply:acleartypeBQPOappearsinthepowerdensityspectrum(Figure3,seealsoCasellaetal.,2004forsimilarbehaviorinXTEJ1859+226).InmostsourcesthechangefromtypeCtotypeBQPOsseemstotakeplacewhenthepower-law

搜索“diyifanwen.net”或“第一范文网”即可找到本站免费阅读全部范文。收藏本站方便下次阅读，第一范文网，提供最新资格考试认证The evolution of black hole states(7)全文阅读和word下载服务。