INTEGRAL spectral variability study of the atoll 4U 1820-30 first detection of hard X-ray e
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arXiv:astro-ph/0608599v1 28 Aug 2006INTEGRAL⋆spectralvariabilitystudyoftheatoll4U1820–30:
firstdetectionofhardX-rayemission
A.Tarana,1,2A.Bazzano,1P.Ubertini1andA.A.Zdziarski3
antonella.tarana@iasf-roma.inaf.it
ABSTRACT
Westudythe4–200keVspectralandtemporalbehaviourofthelowmass
X-raybinary4U1820–30withINTEGRALduring2003–2005.Thissourceas
beenobservedinboththesoft(banana)andhard(island)spectralstates.A
highenergytail,above50keV,inthehardstatehasbeenobservedforthefirst
time.ThisplacesthesourceinthecategoryofX-rayburstersshowinghigh-
energyemission.Thetailcanbemodeledasasoftpowerlawcomponent,with
thephotonindexof≃2.4,ontopofthermalComptonizationemissionfroma
plasmawiththeelectrontemperatureofkTe≃6keVandopticaldepthofτ≃4.
Alternatively,butatalowergoodnessofthefit,thehard-statebroadband
spectrumcanbeaccountedforbyemissionfromahybrid,thermal-nonthermal,
plasma.Duringthismonitoringthesourcespentmostofthetimeinthesoft
state,usualforthissource,andthe4keVspectraarerepresentedbythermal
ComptonizationwithkTe≃3keVandτ≃6–7.
Subjectheadings:X-rays:binaries–X-rays:stars–stars:individual:4U1820–30
–stars:neutron
1.Introduction
4U1820–30isalowmassX-raybinaryseenat0.66′′fromthecentreoftheglobularclus-
terNGC6624.Itwasthefirstidentifiedsourceoftype-IX-raybursts(Grindlayetal.1976).–2–
Distancehasbeenestimatedasd≃5.8–8.0kpc(e.g.,Shaposhikov&Titarchuk2004;Ku-
ulkersetal.2003).Kuulkersetal.(2003)foundtheirdeterminationofd=7.6±0.4kpc
compatiblewiththepeakX-rayburstluminositybeingastandardcandleintheirsam-
pleofsources,andequaltotheEddingtonlimitforHe.Thebinarysystemconsistsof
aHewhitedwarfwithmassof0.06–0.08M⊙(Rappaportetal.1987)andaneutronstar,
withthemassestimatedbyShaposhnikov&Titarchukas∼1.3M⊙,orbitingatshortpe-
riodof11.4minutes(Stellaetal.1987).InX-rays,4U1820–30isclassifiedasanatoll
(Hasinger&vanderKlis1989).However,itsfluxvariationbetweenthesoft(banana)to
thehard(island)statearequasiperiodicat∼170d(Priedhorsky&Terrell1984;ˇSimon
2003;Wenetal.2006),whichquasiperiodicityhasbeenproposedtobeduetotidaleffects
ofamoreremotethirdstar(Chou&Grindlay2001;Zdziarski,Wen&Gierli´nski2006).
X-rayburstsoccuronlyatlowfluxlevels(Cornelisseetal.2003)andthekHzQPOsare
correlatedwiththeflux(Zhangetal.1998),provingthatthevariabilityisduetoanintrinsic
changeofthesourceluminosity(andtheaccretionrate)ratherthanbeingduetovariable
eitherobscurationorviewingangleofananisotropicemitter.X-rayspectraofthesource
fromRXTEandBeppoSAXwerefittedbythermalComptonizationwithorwithoutan
additionalblackbody(Bloseretal.2000;Sidolietal.2001;Migliarietal.2004).
Inthispaper,wereportthefirstdetectionofX-rayemissionabove50keVfrom4U
1820–30inahardspectralstate.Westudiedthesourceinthe4–200keVenergyrange
withtheJEM-X(Lundetal.2003)andIBIS(Ubertinietal.2003)instrumentsonboard
theINTEGRALsatellite(Winkleretal.2003a),collectedduringtheGalacticCentreDeep
Exposure(GCDE)program(Winkleretal.2003b).Wecombinedthreemethodsofanalysis:
Temporal,usingthelightcurvesindifferentenergybands;Photometric,withtheHardness–
InstensityDiagram;andSpectral,byspectralfitting.
2.ObservationsandDataAnalysis
ThecodedmaskimagerIBISonboardINTEGRALhasafieldofview(FOV)of19◦×19◦
or9◦×9◦whenpartiallyorfullycoded,respectively.TheX-raymonitorJEM-X(4–35keV)
hasthefullycodedFOVof4.8◦×4.8◦.AnINTEGRALrevolutionlasts3days,andconsists
ofadjacentpointingsof∼2000s,calledScienceWindows(SCW;Winkleretal.1999).
WehaveanalysedalltheavailabledataduringwhichthesourcewaswithintheIBIS/ISGRI
andJEM-XfullycodedFOV,sothatthefluxevaluationisnotaffectedbycalibrationun-
certaintiesintheoff-axisresponse.Thisyields308and51SCWsforIBISandJEM-X
respectively,duringrevolutions50–363.DatawereextractedwiththeOff-LineScientific
Analysis(OSA)(Goldwurmetal.2003)v.5.1softwarereleasedbytheINTEGRALScience–3–
DataCentre(Courvoisieretal.2003).A2%systematicerrorstobothJEM-XandISGRI
datasetshasbeenadded1.FluxeshavebeennormalizedtoISGRIvalue.Thespectral
analysiswasperformedwiththeXSPECpackagev.11.3.
3.Lightcurvesandhardness–intensityDiagrams
Fig.1showsthelightcurvesofthemonitoringperiod(2003March12–2005October
5)fromsofttohardX-rays,inthe4–10,10–20keVbandsfromJEM-Xand20–30,30–60,
60–120keVwithIBIS/ISGRI,aswellasthecorrespondingRXTE/ASM2lightcurve.The
∼170-dquasiperiodicvariabilitycanbeseenintheASMdata.TheINTEGRALlightcurves
aremarkedwithverticallinestoidentifyfiveepochs(A,B,C,D,E)duringwhichwehave
performeddetailedspectralanalysis(seeTable1).EpochAcorrespondstothejointIBIS
andJEM-Xcountratemaximum(∼530mCrabat4–10keV).EpochCcorrespondstothe
minimumofthe4–10keVcountrate(∼100mCrab),buttohighcountratesfrom20to
60keV,implyingaspectralhardeningacrossthe4–60keVband.Wealsostudiedepochs
before(B),andafterthatevent(D,E).
Fig.2showshardness–intensitydiagramswithdatafromJEM-X(a)andIBIS/JEM-X
