The evolution of black hole states(2)

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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˙-drivenpicture,withtransitionsonlybeingtriggeredbyasimpleM˙,isprobablynotabletoexplaintheobservedtransitionchangesinMbetweenthestates.Homanetal.(2001)suggestedthatanadditionalparametermayplayaroleinthosetransitions.Theyalsosuggestedthatthevery-highstateandtheintermediatestate(s)areoneandthesamestateandrepresenttransitionsbetweenthelow/hardandhigh/softstatesatdi erentluminosities.Thefactthatmostofthestateswereobservedoverwideandoverlappingrangesinluminosityalsomeantthatattributeslike‘low’,‘high’and‘veryhigh’hadlostmostoftheirsigni cance.

McClintockandRemillard(2004)introducedanewclassi cationschemethatispartlybasedontheold ve-statesscheme,butnolongerusesluminosityasaselectioncriterionsinceitappearedthatanyoftheactivestatesmayoccuratanyluminosity.Theystillrecognizeaquiescentstate,hardstate,softstate(renamingthelatterthe‘thermaldominantstate’),andvery-highstate(renamingitthesteeppower-lawstate),butdroptheintermediatestateasabona destate.Thethreeactivestatesarede nedonthebasisofthefractionalcontributionofthedisk- ux(orpower-law ux)tothe2–20keVspectrum,spectralpower-lawindex,andthestrengthofthepowercontinuum.Allobser-vationsthatcannotbeclassi edaccordingtotheirstatede nitions,arecombinedintoanintermediatestate,inwhichobservationscanshowpropertiesofanyofthethreemainstates.

Someofthestatetransitionsdiscussedinthispaperinvolve(andarede nedonthebasisof)suddenchangesinthepropertiesofthequasi-periodicoscillations(QPOs)inthe～1–10Hzrange.Currentlythreetypesoftheselow-frequencyQPOsarerecognizedinblackholebinaries,calledtypeA,BandC(Wijnandsetal.,1999,Remillardetal.,2002).Thethreetypescanbedistinguishedonthebasisofstrength,coherence,phaselags,energydependence,harmoniccontent,andfrequencystabilityonatimescaleofdays.TypeCQPOsarethemostcommonones;theyareobservedinthespectrallyhardstatesoverawiderangeinfrequency(～0.1–10Hz),andarestrongerandmorecoherentthantheothertwotypes.TypeAandBQPOshaveonlybeenobservedinafewsources;theyareonlyseeninthevery-high/steeppower-lawstate,inanarrowfrequencyrange(～4–8Hz).RecentobservationsofH1743–322(Homanetal.,2004b)suggestthatthesetwotypesmightbemoreintimatelyrelatedthanwaspreviouslybelieved.TransitionsfromonetypeofQPOtoanotheralwaysseemtoinvolvetypeB(Casellaetal.,2004),withtransitionsbetweentypeCandBresultinginaclearchangeintheshapeandstrengthofnoisecontinuum.ThemainreasonforincludingQPOtypeinourdiscussionofblackholestatesisthattheyprovideanadditionalindicationfor

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