The evolution of black hole states(9)

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

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itispossiblethatsuchQPOsappearforintervalsshorterthanforothersources,whichwouldmakethemdi culttoobserve.

Itisduringthesoftintermediatestatethatmosthigh-frequencyQPOshavebeendetected,indicatingthattheremightbearelationbetweenthoseandtypeA/BQPOs;indeed,thefrequenciesofbothfeaturesarenotobservedtovarybyalargeamountbetweendi erentobservations.ItispossiblehoweverthatthehighfrequencyQPOsinsoftintermediatestateevolvefrombroaderfeaturesinthehardin-termediatestate,asissuggestedbyobservationsofXTEJ1650–500(Homanetal.,2003).Noticethatwhilethesoftintermediatestateisoftenobservedathigh uxbetweenthehardintermediateandthesoftstates,thesametimingpropertiesarenotobservedatlow ux,whenthereversetransitiontakesplace.Moreover,inXTEJ1550–564(Homanetal.,2001)typeAandBQPOsappearedatseveralwellseparatedluminositylevels,stronglysuggestingtheyarenotstrictlyrelatedtohard softtransitions.InGX339-4astrong1HzQPOwithsomepropertiessimilartothoseofthetypeBQPOsisobservedatlow ux,indicatingthatitispossiblethatasimilarstateexistalsoatmuchloweraccretionrate,althoughwithdi erentcharacteristicfrequencies(Bellonietal.,2004).

4.3.Thesoftstate

Duringthesoftstate,whichinGX339-4tookplaceafterthesoftintermediatestate,thespectralandtimingpropertiesareratherwellde ned,althoughasinglecleartransitionfromthesoftintermediatestateishardtoidentify.Theenergyspectrumofthesoftstateisdom-inatedbyastrongthermalcomponent,withthepresenceofaweaksteeppower-lawcomponent,whichwasnotobservedtoshowahigh-energycuto (seeGroveetal.,1998).Variabilityinthesoftstateisweakcomparedtotheotherstates,withtypicalrmsamplitudesofatmostafew%rms.Unfortunately,bythetimemosttransientsreachthesoftstatetheyareusuallyinthedecayphaseandobservationshavebecomeshorterandlessfrequent,sodetailedstudiesofitsvariabilitypropertiesarerare.Nevertheless,afewweakQPOshavebeendetectedinthesoftstatesofGROJ1655–40,XTEJ1550–564,andH1743–322.TheseQPOsallhadfrequenciesthatwerehigherthantheotherlowfrequencyQPOsdetectedinthosesourcesandthenoisecontinuacouldallbe ttedwithbrokenpowerlaw.Surprisingly,theQPOandbreakfrequenciesofthesepowerspectrafallontopoftheWijnands-vanderKlisrelation(Wijnands&vanderKlis1999)forblackholesinthehardandhardintermediatestates.ThissuggestsnotonlythatthesemightbetypeCQPOs,butmoreimportantly,thatvariability

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