AstudyofspectraofCygX-3observedby
BeppoSAX
A.Szostek∗andA.A.Zdziarski∗
arXiv:astro-ph/0412595v2 10 Jan 2005∗
CentrumAstronomiczneim.M.Kopernika,Bartycka18,Warszawa,Poland
Abstract.Wemodelthe∼1–200keVspectraofCygnusX-3observedbyBeppoSAX.Thecontin-uum,modeledbyComptonizationinahybridplasma,ismodifiedbythestronglyionizedplasmaofthestellarwindoftheWolf-Rayetcompanionstar.Discreteabsorptionandemissionspectralfea-turesaremodeledwithXSTAR.Themodelhasbeenappliedtophase-resolvedspectrainthehardandsoftspectralstates.
INTRODUCTION
CygnusX-3,ahigh-massX-raybinary,containsacompactobject(eitherablackholeoraneutronstar)andaWolf-Rayetstar.Theorbitalperiodisveryshort,4.8h.Astrong,dense,stellarwindfromthecompanionstronglymodifiestheintrinsiccontinuumbyabsorptionandemission.
High-resolutionspectroscopyprovidesvaluablediagnostictoolstoexaminethedis-creteabsorptionandemissionfeaturesofthewind.However,therelativelynarrowen-ergybandofChandradoesnotallowustomodeltheintrinsicbroad-bandcontinuum.Ontheotherhand,thelowspectralresolutionandthelackofsoftX-raycoverageofRXTEdonotallowustostudythediscreteemissionandabsorptionfeaturesimprintedbythewind.Thus,herewetakeadvantageofthedatafromBeppoSAX,featuringbroad-bandcoverageandmediumenergyresolution.
OBSERVATIONSANDMODEL
Weanalyze9outof10observationsofCygX-3performedbyBeppoSAX,usingtheMECS2(1.7–10.5keV)andPDS(15–120keV)data.Eachobservationcontainedmorethanfourorbits.Weusetheparabolicephemeris[6]todividethedataintointervalsof1/6oftheperiod,andthenaddtogetherthespectrafromagivenobservationcorre-spondingtothesamephaseinterval.Theminimaandmaximacorrespondtothephasesof−0.08–0.08and0.42–0.58,respectively.
Allthespectrahavebeenfittedwithaphysically-motivatedmodelincludingComp-tonizationofblackbodyemissionbyhybrid(thermalandnonthermal)electrons[1],[3],Comptonreflectionfromanionizedmedium[5],emissionandabsorptioninthepho-toionizedwindcalculatedwiththeXSTARphotoionizationcode[4],neutralabsorption,andarelativisticallybroadenedFeKαline[2].
RESULTSANDCONCLUSIONS
ThesoftestandthehardestspectraatthemaximumoftheorbitalmodulationareplottedinFig.1.Thesoft-statespectrumisdominatedbyasoftblackbody-likecomponentandahigh-energytailduetoemissionofnonthermalelectrons.ThehardestspectrumobservedbyBeppoSAXiswellmodeledbythermalComptonization,withnononthermalelectronsrequiredbythefit,andweakComptonreflection.ThisspectrumisstillsofterthanthehardestspectraobservedbyRXTE,similartotheaveragespectrum2intheclassificationof[7].Bothsoftandhardstatespectraaresignificantlyabsorbedandcontainnumerousdiscretespectralfeatures.
ThemodelsfittedtoallanalyzedspectrainthemaximaandminimaareshowninFig.2(a,b).Weclearlyseethatwhereasthespectrabelow∼10keVshowacontinuityoffluxlevels,thespectraabove∼20keVshowabimodaldistribution.Thus,webaseourstateclassificationhereonthelatter,withthesoftandhardstatecorrespondingtothelowandhigh,respectively,levelofthehigh-energyflux.(ThisclassificationmaydifferfromsomeotheronesusedforX-raybinaries.)Interestingly,allthe(soft-state)spectrawiththeweaktailcorrespondtoanalmostuniquespectrumbelow10keV,formingathicklineinFig.2(a).Thestatetransition(i.e.,thedeclineofthetailamplitude)takesplaceonlywhenthesoftX-rayfluxreachesitsverymaximum.Thesoftandhardstatespectraintersectatthepivotenergyof∼10keV.
Theamplitudeoftheorbitalmodulation,seeFig.2(c,d),below∼10keViscompa-rablewiththatoftheaperiodicvariability,seeFig.2(a,b).Ontheotherhand,X-raysabove∼20keVchangemuchless.WefindthatthefluxoftheFeKαlinedoesnotvarysignificantlyduringeithertheorbitalmodulationorstatetransition.Thisissuggestiveoftheoriginofthisemissionfromanextendedregion.Naturally,theequivalentwidthisthehighestattheminimumofthecontinuumflux,whichcorrespondstothephaseminimumofthehardstate.
AlthoughtheenergyresolutionofBeppoSAXdoesnotallowforpreciseseparationofadjacentspectralfeaturesinsoftX-raysorprecisemeasurementoftheirequivalentwidth,westillhavebeenabletoidentifyseverallinesandrecombinationcontinuasuch
FIGURE1.Resultsofspectralfits.(a)Thesoftestand(b)thehardestobservedspectraatthemaximumoftheorbitalmodulationplottedtogetherwiththemodel(solidcurves,green)consistingofComptoniza-tioncontinuum(dot-dashedcurves,blue)softX-raylineemission(dottedcurves,magenta)andtheFeKline(dashedcurves,red).
FIGURE2.Themodelsfittedtothephase(a)maximaand(b)minimainthesoft(solidcurves)andhard(dottedcurves)states,andthemodelsfittedto(c)thesoftestand(d)thehardestspectrainthephaseminimum(dot-dashedcurves,blue)andthemaximum(solidcurves,red).
as:SiXII,SiXIVLyα,RRCSiRRCSXVI,CaXIX.
XIII,S
XVI
Lyα,SiXIV,ArXVII,ArxviiiLyα,
ACKNOWLEDGMENTS
WethankL.Hjalmarsdotterforvaluablecommentsonthispaper.ThisresearchhasbeensupportedbyKBNgrants1P03D01827,4T12E04727andPBZ-KBN-054/P03/2001.
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