Systematic spectral analysis of GX 339-4: influence of Galactic background and reflection models

1.1 MB·

by  M. Clavel

Astron.Nachr./AN—,No.—,1–6(2016)/DOI—
Systematic spectral analysis of GX339–4: influence of Galactic back-
ground and reflection models
M.Clavel1,⋆,J.Rodriguez1,S.Corbel1,andM.Coriat2,3
1 LaboratoireAIM,UMR7158,CEA/CNRS/Univ.ParisDiderot,CEADSM/IRFU/SAp,F-91191Gif-sur-Yvette,France
2 Universite´deToulouse,UPS-OMP,IRAP,Toulouse,France
6 3 CNRS,IRAP,9av.ColonelRoche,BP44346,F-31028Toulousecedex4,France
1
0
2 Received7Sep2015,accepted21Oct2015
Publishedonlinelater
n
a
J Keywords accretion,accretiondiscs;blackholephysics;X-rays:binaries;Stars:individuals:GX339–4.

2 BlackholeX-raybinariesdisplaylargeoutbursts,duringwhichtheirpropertiesarestronglyvariable.Wedevelopasys-
2 tematicspectralanalysisofthe3–40keVRXTE/PCAdatainordertostudytheevolutionofthesesystemsandapplyitto
GX339–4.Usingthelowcountrateobservations,weprovideaprecisemodeloftheGalacticbackgroundatGX339–4’s
] locationanddiscussitspossibleimpactonthesourcespectralparameters.Athigherfluxes,theuseofaGaussianlineto
E
modelthereflectioncomponentcanleadtothedetectionofahigh-temperaturedisk,inparticularinthehigh-hardstate.
H
Wedemonstratethatthiscomponentisanartifactarisingfromanincompletemodelingofthereflectionspectrum.
.
h
(cid:13)c 2016WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
p
-
o
r 1 Introduction validity of our models we also extend the spectral energy
t
s band of our study to lower X-ray energies, making use of
a Alongthecourseoftheirlargeoutbursts,thespectralshape Swift data (0.6–8keV) obtained in quasi-simultaneity with
[
ofblackholeX-raybinaries(BHXB)varies,tracingtheevo- agivenRXTEobservation.

1 lutionofajetoracorona,ofanaccretiondiskandoftheas-
v
sociatedreflectionprocesses.Howthesephysicalstructures 2.1 RXTE/PCA
7
form and evolve overtime is still under investigation(e.g.
6
8 Remillard&McClintock2006).Tobetterconstrainthethe- The RXTE mission operated from December 1995 to Jan-
5 oretical models, one needs to compare the simulation out- uary 2012, providing a quasi-systematic follow-up of the
0 putswiththerealobservations,applyingthesamemethods X-ray binary outbursts over 16 years. We reduced all
1. to both data sets. Therefore, we are developing a system- the 1389 Proportional Counter Array (PCA) observations
0 aticproceduretoreducetheRXTE/PCAdata(Sec.2)andto available for GX339–41. We used the HEASOFT software
6 performthecorrespondingspectralanalysis(Sec.3).Inor- suite v6.16 to reduce the corresponding data in a stan-
1 dertotestourmethodandtoobtainaninitialsetofgeneric dard way, restricting it to the top layer of Proportional
v: spectralpropertiesforBHXB,weuseGX339–4whichun- Counter Unit 2 (PCU2)2. We time-filtered the data, us-
i dergoesfrequentoutbursts,asprimeexample(e.g.Dunnet ing maketime and xtefilt, to remove PCU2 breakdowns
X
al.2008).Wepresentafirstoverviewoftheresults(Sec.4) and to restrict it to the times when the elevation an-
ar aswellastwofurtherstudiesaimingtoimproveoursystem- gle above the Earth is greater than 10◦, and when the
atic procedure: a detailed modeling of the Galactic back- satellite pointing offset is less than 0.02◦. The PCA re-
groundatGX339–4’sposition(Sec.5)andacomparisonof sponsefilewascomputedusingpcarspandtheinstrumental
thespectralparametersobtainedusingdifferentmodelsfor noisewasestimatedusingpcabackestwiththebackground
thereflectioncomponent(Sec.6). model pca bkgd cmbrightvle eMv20051128.mdl. All ob-
servation/backgroundspectraandaveragecountrateswere
then extracted using saextrct. For PCA observations hav-
2 Observations anddata reduction ing a net average count rate lower than 40ctss−1 over
the full energy range, the instrumental background esti-
GX339–4’s recurrent outbursts have been monitored with mation was replaced by the one calculated from model
a large numberof observatoriesat all wavelengths. In this
1 ThiscorrespondstoallRXTE/PCAobservationspointingwithinone
work we primarily analyze the 3–40keV RXTE/PCA data
degreefromGX339–4,ignoringslewandrasterobservations,aswellas
in order to perform a systematic analysis of the spectral observationscontainingdatagapspossiblyaffectingthedata(obsIDsend-
evolution of the source over several outbursts. To test the ingwithG,TorU).
2 PCU2istheonlyunitthatwasactiveduringallRXTEobservations,
⋆ Correspondingauthor:e-mail:[email protected] and we tested that, for ourstudy, the spectra were not significantly im-
provedwhenaddingthephotoncountsfromitssecondandthirdlayers.
(cid:13)c 2016WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
2 M.Clavel,etal.:SpectralanalysisofGX339–4
pca bkgd cmfaintl7 eMv20051128.mdl. All spectra were fixedatE =6.4keV,onlymodelingthestrongironfluo-
line
binned in order to have at least 200 counts per resul- rescentline.Fromouranalysis,itappearsthatasmearedab-
tants channel and a systematic error of 0.6% was added sorptionedgeatEedge =7.1keV,correspondingtothepho-
toelectricabsorptionbyironatoms,isalsosignificantinpart
to account for instrumental uncertainties. In this work we
of the observations (Sec.6). Therefore, we perform a sec-
present 3–10keV lightcurves and spectral fits obtained in
ondsystematicfitting,includingthemultiplicativeSMEDGE
the 3–40keV energy range. The astrophysical background
model when the Gaussian line is present. Finally, we run
includedinthesedataisinvestigatedinSection5.
a third round of fits testing the self consistent model XIL-
LVER to accountfor the reflection component.This model
includesthephotoelectricabsorption,thefluorescentemis-
2.2 Swift/XRT
sionlinesandtheComptonscatteringcontinuumemission
(Garc´ıaetal.2013).
We reducedandanalyzedoneobservationfromSwift, cor-
The three sets of XSPEC models that we test are sum-
respondingto MJD55260.2,usingthe HEASOFT software
marizedbythefollowingthreeequations:
suitev6.16.We consideredonlythedataobtainedwiththe
X-ray Telescope (XRT) that covers the soft X-rays (0.5– phabs×(powerlaw+ezdiskbb+gauss) (1)
10keV). This observation was obtained in window tim-
phabs×(powerlaw+ezdiskbb+gauss)×smedge (2)
ing (wt) mode. The Swift/XRT level 2 cleaned event file
wasobtainedwithxrtpipeline,andfurtherprocessedwithin phabs×(powerlaw+ezdiskbb+xillver) (3)
XSELECTtoobtainthesourceandbackgroundspectra.As
where the disk component (in italic) and of the reflection
recommended in Evans et al. (2009) for the wt data, the
component (in bold) are included only if they are statisti-
sourcespectrumwasextractedfroma30pixelradiuscircu-
callysignificant(seeSec.3.2forthemodelselectionproce-
larregioncenteredonthebestsourceposition.Theresultant
dure).Thecolumndensitycorrespondingtothephotoelec-
dataisnotcontaminatedbypile-upeffects.
The background spectrum was estimated from larger tric absorption PHABS is set to NH = 0.4 × 1022cm−2.
The Gaussian line energy is fixed to E = 6.4keV, the
regions at off-axis positions. The ancillary response file line
(arf) was estimated with xrtmkarf, and the last version SMEDGEthresholdenergytoEedge =7.1keV,itssmearing
widthtoW = 15keVandtheindexforphotoelectriccross
(swxwt0to2s6 20090101v015.rmf)oftheredistributionma-
section to a = −2.67. The XILLVER incident radiation is
trixfiles(rmf)usedinthespectralfits.Thespectralchannels
weregroupedinordertohaveatleast100ctss−1perresul- fixedtothe POWERLAW parameters,theironabundanceto
the solar value,and the system inclinationto i = 40◦. All
tant channels, and only the data between 0.6–8keV were
the other parameters (power law photon index Γ and nor-
consideredforthespectralfits.
malization I , disk maximal temperature T and nor-
pwl max
malization I , Gaussian normalization I , edge maxi-
disk line
3 Spectral analysis mumabsorptionfactoratenergythresholdτmax, XILLVER
ionizationparameterξ andnormalizationI )areleftfree
refl
tovarywithinphysicalrangesandareinitializedattypical
Followingthe methodproposedbyDunn etal. (2010),we
valuesinordertoincreasethechanceofconvergingrapidly
performasystematicanalysisofthe1385spectra3.Foreach
toaconsistentfit.
observation, we test the presence of several spectral com-
ponents using XSPEC software v12.8.2 models and chi-
squaredfittingroutines,aswellasparameterconstraintsdis- 3.2 Proceduretoselectthebestfitmodel
cussedbyDunnetal.(2008)andPlantetal.(2014).
For a given reflection model, our systematic analysis
perform the spectral fit of each observation with four
3.1 Choiceofmodelsets models (POWERLAW, POWERLAW+REFLECTION, POW-
ERLAW+DISK and POWERLAW+DISK+REFLECTION). We
In addition to the non-thermal component modeled by an thenidentifythemodelhavingthe bestchi-squareanduse
absorbed power law, PHABS×POWERLAW4, we test the theXSPECF-statistic testtocheckthestatisticalrelevance
presence of a thermalemission coming from the accretion
of any optional components(ie. any disk and/or reflection
diskmodeledbyamulti-temperatureblackbodyEZDISKBB, components), based on a comparison of chi-squares (χ2)
andofareflectioncomponent.
anddegreesoffreedom(dof).
ThemodelchosenbyDunnetal.(2010)toaccountfor
We are aware that the F-test we use is limited to data
thereflectioncomponentisaGaussianemissionlineGAUSS
sets having a Gaussian statistic (which is the case consid-
3 Four RXTE/PCA observations (MJD52095.999, 52128.566, eringthedatalargecountratesandtheconsequentbinning
54327.758 and 54327.823) out of the 1389 we reduced had to be applied to the spectra, see Sec.2.1) and cannot be used to
excludedduetoatoolownumberofcountsand/ordatareductionissues testthepresenceofa Gaussianline(Protassovetal. 2002)
preventingafullspectralanalysis.
nor of a multiplicative component(Orlandini et al. 2012).
4 Amodelincludingabrokenpowerlawand/orahighenergycut-off,as
Therefore,similarly to whatis doneby Dunnet al. (2010)
testedbyDunnetal.(2010),arenotstatisticallyneededwithintheenergy
rangeofouranalysis(3–40keV). weaddedatestontheironlineparameters:itisconsidered
(cid:13)c 2016WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org
Astron.Nachr./AN(2016) 3
statistically relevantonlyifitsnormalizationisatleast2σ 103
above zero. When the emission line is detected, the edge
shouldalsobepresentinthespectrum.However,thismulti-
plicativecomponentmaynotbestatisticallyneededforthe
bfiet.stTfihtismiosdwelhoynwlyeifdethceidfeoltloowininclgudtweothceonSdMitEioDnGsEarienftuhle- cts/s]102
filled:(i) the Gaussianline is required,(ii) theedgeis sig- e [
at
nificantlyimprovingtheoverallfit(loweringthereducedχ2 nt r
u
frommorethan2.5downtoabout1). co
2
The phenomenologicalmodelswe are testing are quite U
PC101
simple.Therefore,formostoftheobservations,thebestfit V
e
k
model selection can easily be done by eye. When this is 10
3-
the case, the automatic procedure gives results that are in No disk detection
Average trend
good agreement with what one would expect (except for
Fake disk detection
model set1, which is further discussed in Sec.6). For this 100 Disk detection
Galactic background subtracted
reason, we decide not to investigate further the statistical Background sub. average trend
tests.Apartfromfewisolatedobservations(lessthan1%of 10-2 10-1 100
our sample), all best fit models have reduced χ2 around 1 Hardness ratio [6-10keV / 3-6keV]
and the uncertainties we provide correspondto the 1σ er-
Fig.1:HardnessintensitydiagramforallRXTEobservationshav-
rorbars.
ing a net average count rate above 0.77ctss−1. The intensity is
givenasanumberofcountsdetectedinPCU2toplayerbetween
4 Standard spectral evolutionofGX339–4 3and10keV,whilethehardnessratioisdefinedastheratioofthe
6–10keV flux over the 3–6keV one. The different colors repre-
sent thespectral shapeofthedatawithadiscriminationwhether
Our data set is made of 1385observationsspread overthe
adiskcomponentisstatisticallyneeded(redsquares)ornot(blue
entire life time of the RXTE mission, sampling five major
circles).Thepurplediamondshighlightaregionwhereaspurious
outburstsofGX339–4(seethesourcelightcurveinFig.2).
diskcanbedetected, depending onthemodel chosentoaccount
TheseeventsallfollowastandardcycleintheHardnessIn-
forthereflectioncomponent(seealsoFig.2andSec.6).Theblack
tensity Diagram (HID, Fig.1) that can be summarized as emptycirclesarethesameobservationscorrectedfromtheGalac-
follow: (i) an intensity rise in the hard state, best modeled tic background component (Sec.5). The softening visible at low
byanabsorbedpowerlaw,(ii)atransitiontothesoftstate, count rates (average trend shown by the blue line) is partly re-
withthedetectionofanadditionaldiskcomponent,(iii)an movedwhenexcludingtheGalacticbackground(blackline).
intensity decrease in the soft state, where the disk compo-
nentisdominatingthespectraandthepowerlawcomponent
the16yearsofRXTEobservations,weprovideaprecisees-
isoftenpoorlyconstrained,(iv)atransitionbacktothehard
timationofthiscomponentatthepositionofGX339–4and
state, with the disappearanceof the disk, and (v) an inten-
investigatetheconsequencesofneglectingit,regardingthe
sitydecreasewithasourceremaininginthehardstate.
bestfitmodelparameters.
If a sufficientmodel is used for the reflection compo-
nent(i.e.modelset2or3,seefurtherdiscussioninSec.6),
we can identify typical parameters following the spectral 5.1 GalacticbackgroundestimationfromRXTEdata
evolution of the source (Fig.2, middle panels). The power
Our set of data includes a large number of low count rate
law photon index goes from about 1.6 in the hard state to
observations,allowing for a precise estimation of the con-
about2.3duringthetransition,anditispoorlyconstrained
tribution of the Galactic background detected by PCA at
both during the softest observations and the lowest count
thepositionofGX339–4.Indeed,thehistogramgivingthe
rateones(dominatedbytheGalacticbackgroundemission,
number of observations as a function of their average net
Sec.5). The disk temperature varies from about 1keV for
countrateinthe3–10keVrangecanbedescribedasaGaus-
the brightest down to about 0.5keV for the faintest obser-
siandistributioncenteredon0.304ctss−1 (withastandard
vationsinthesoftstate.Theseresultsareinagreementwith
deviation σ = 0.076ctss−1), plus individualobservations
previousworks(Dunnetal.2008,Plantetal.2014).
rangingfrom0.77ctss−1(3σ)uptoabout800ctss−1.Ac-
cordingto previousworks,the distributionatlow fluxesis
5 Galacticbackground dominatedbytheGalacticbackgroundcomponent(e.g.Co-
riat et al. 2009). We select the 125 observations having a
GX339–4 is located within the Norma arm, in the Galac- countratewithin2σoftheGaussiandistributionmeanand
tic plane. Therefore, a diffuse emission from the Galac- add them is orderto get a spectral estimation of the back-
tic background, also called the ridge emission, should be groundcomponentwith a sufficientexposureto performa
presentintheregion(Valinia&Marshall1998).Assuming spectralfit(about140kstotal).
thatthisastrophysicalbackgroundemissionisconstantover Valinia&Marshall(1998)estimatedtheridgeemission
www.an-journal.org (cid:13)c 2016WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
4 M.Clavel,etal.:SpectralanalysisofGX339–4
103 101
s]
cts/ 102 100 o
e [ ati
2 count rat 101 10-1 Hardness r
CU 100 10-2
P
10-1 10-3
d
2.5 n
u
Photon index21..05 detection y Galactic backgro
sk d b
Di ate
n
mi
o
d
2.0 s
Disk temp. [keV]11..05 disk detection Observation
o
N
0.5
n
o
cti
e
103 det
k
m. 102 s ratio ake dis
Disk nor 101 Hardnes F
100
10-1
51000 52000 53000 54000 55000
Time [MJD]
Fig.2:RXTE/PCAlightcurveofGX339–4andcorrespondingspectralcomponents.Fromtoptobottom:(i)3–10keVlightcurvewitha
colorcodinghighlightingwhatobservationsaremodeledwithasimpleabsorbedpowerlaw(bluecircles)andwhichonesneedadisk
component(redsquares).Thepurplediamondshighlightperiodswheremodelset(1)givesabestmodelincludinga1.5keVdiskwhile
model sets(2)and(3)donotstatisticallyrequireanadditionaldiskcomponent (Sec.6).Datapointsinlightbluehavenetcountrates
below0.77ctss−1andareconsideredasdominatedbytheGalacticbackground(Sec.5).Thehardnessratioofeachobservation(black
dots)isdefinedinthecaptionofFig.1;(ii)Powerlawphotonindexofthebestfitobtainedusingmodelset(1,purple)or(3,samecolor
codingastoppanel).Thespuriousdetectionofthe1.5keVdiskisresponsibleforanartificiallylowphotonindexinthecorresponding
periods;(iii)Maximumtemperatureofthediskfoundusingmodelset(1,purple)or(3,red);(iv)Normalizationofthedisk(samecolor
coding).Thefakediskcomponentappearsinpurplewithahightemperatureandaverylownormalization,whileforthetruedetections,
redandpurplepointsareoverlaid(seeSec.6formoredetails).
fromRXTEobservationscoveringthecentralportionofthe I ∼ 0.00391phcm−2s−1keV−1 at 1keV. The overall
pwl
Galactic disk (−45◦ < l < +45◦, −1.5◦ < b < +1.5◦), absorptionisN ∼ 1.8×1022cm−2.The averageGalac-
H
andprovidedanaveragemodelforthislargeregion: tic backgroundestimated by these authors, correspondsto
wabs×(raymond+powerlaw) (4) a flux Fridge ∼ 2.9× 10−3phcm−2s−1 in the 3–40keV
range.
whereRAYMONDisamodelforahotanddiffusegasatred-
Wefitthismodeltotheastrophysicalbackgroundatthe
shiftz = 0,withasolarabundance,atemperatureofabout
kT∼2.9keVandanormalizationI ∼0.021cm−5.The position of GX339–4, fixing all the parameters to Valinia
ray
power law photonindexis Γ ∼ 1.8, and its normalization & Marshall (1998) values except for the normalizations
(cid:13)c 2016WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org
Astron.Nachr./AN(2016) 5
of the plasma and power law components. The fit is sat- morecompletemodelsforthereflectioncomponent(Fig.2,
isfactory (χ2 / dof = 59 / 64) and the parameters found bottompanels).
are I = (4.4± 0.6)×10−3cm−5 and I = (7.8 ±
ray pwl
1.2)×10−4phcm−2s−1keV−1,whicharefivetimeslower 6.1 Spuriousdetectionofa∼1.5keVdisk
than the average values obtained by Valinia & Marshall
(1998), and correspond to a 3–40keV flux F ∼ 5.9× For all GX339–4 outbursts, when using model set (1) we
bkg
10−4phcm−2s−1.Sincetheridgeemissionisnotsupposed significantlydetectadiskcomponentinthehigh-hardstate
tobeuniformalongtheGalacticplane,webelievethatthe observations(seealsoDunnetal.2008,Nandietal.2012).
best fit model we obtained for our average low count rate Thisdiskisbestdescribedbya highmaximaltemperature
spectrum is relevant to model the Galactic background at kTmax ∼ 1.5keVandalownormalizationIdisk < 10,itis
GX339–4’sposition. alsoassociatedtoaharderpowerlawthanthetypicalvalues
found with the other two model sets (Fig.2). We believe
that the high-temperaturedisk is an artifact created by the
5.2 NeglectingtheGalacticbackgroundcomponent
spectralanalysis,forthreemainreasons.
The estimated Galactic backgroundhas a rather soft spec- First,thenormalizationofthediskisdefinedby:
trum (equivalentto a photon index Γ ∼ 2.2, if fitted with
an absorbed power law5) and is likely to affect the low- I = 1 Rin 2 D −2cosi (5)
hardstateobservationsifneglected.Inordertoquantifythis disk f4 (cid:18)1km(cid:19) (cid:18)10kpc(cid:19)
effect, we compare systematic fits performed using model where, for GX339–4,f = 1.7 is the color effective ratio,
set(2)tothesamesetincludinganadditionalGalacticback- D ∼ 8kpcisthedistanceofthesource,i ∼ 40◦ isthesys-
groundcomponent(givenbyequation4andfixedtothepa- teminclinationandR istheinnerradiusofthedisk,which
in
rametersderivedinSec.5.1).Allobservationsdominatedby shouldbelargerthantheSchwarzschildradiusR ∼30km
S
theGalacticbackground(netcountratesbelow0.77ctss−1 for a 10M black hole. Therefore, for GX339–4 we ex-
⊙
inthe3–10keVrange)arepoorlyfitted.Forallobservations pect the disk normalization to be not less than about 120
dominated by GX339–4, the two model sets give consis- (whenR =R )or30tobeconservative.Withsuchalow
in S
tent results concerning the spectral componentsbeing sta- normalization,thehigh-temperaturediskparametersarenot
tisticallysignificant,andthespectralmodelparametersare consistentwiththecompletediskmodelweareusing.
compatiblewithintheerrorbars.Thereforewedecidetore- Second,eveniftheEZDISKBBmodeldoesnotseemrel-
strictthestudyofthespectralevolutionofGX339–4tothe evant,thehigh-temperaturediskcouldfitalow-energyex-
observationsabove0.77ctss−1,andtoneglecttheGalactic cess in the spectra (e.g. thermally emitting disk fragments
backgroundcontributionfortheseobservations. that are embedded in a hot and optically thin disk). How-
However,partofthesofteningtrendvisibleforthelower ever, when available the simultaneous Swift/XRT data do
fluxes (increasing photon index towards low count rates, not highlight any need for an additional thermal compo-
Fig.2)ispartlycorrectedwhenincludingtheGalacticback- nent at low energy.The Swift observationis well fitted by
groundasafixedcomponentinthespectralfit,asshownby an absorbed power law having the following parameters:
thecorrespondingaveragetrendsinFig.1.Fromouranaly- N = (0.37 ± 0.01) × 1022cm−2, Γ = 1.53 ± 0.02
H
sis we believethat the softeningeffectonlybecomestruly and I = 0.39 ± 0.01phcm−2s−1keV−1 (χ2 / dof =
pwl
negligible above about 8ctss−1 (in the 3–10keV range). 147 / 148). When added to the fit, the quasi-simultaneous
Thereforetheaveragespectralparametersobtainedfromthe RXTE/PCA data show discrepancies compared to the sin-
faintestobservationsshouldbeinterpretedwithcaution. gle power law model. The residuals highlight a possible
emissionlineandassociatedabsorptionedgewithinthe5–
20keV range (Fig.3). In order to avoid any strong biased
6 Importance ofthe reflection model
linkedtothereflectioncomponent,weperformasimultane-
ousfitignoringdatapointsbetween5and20keVforboth
We perform the whole systematic fitting procedure three
instruments.Thefitissatisfactoryandtheparametersfound
times,usingthreedifferentmodelstoaccountforthereflec-
arecompatiblewiththeonelistedpreviously,withinthe1σ
tioncomponent(Sec.3).WiththeRXTE/PCAdataalonewe
errorbars(χ2 /dof=166/176),confirmingthatthehigh-
do nothavethespectralresolutiontoextractpreciseinfor-
energy power law is fully compatible with the low-energy
mationaboutthereflectionphenomena,thereforewecannot
one.Therefore,thereisnoneedforanythermalcomponent
provide a precise comparison between the three reflection
atanytemperature.Simultaneousfitsusingmodelset(2)or
models.However,modelset(1),includingonlythefluores-
(3)onthewhole0.6–40keVenergyrangearebothsatisfac-
cent line at 6.4keV, leads to the detection of a high tem-
torywithnodiskcomponentneeded(χ2/dof∼1).
peraturediskthatisnotstatisticallyneededwhenchoosing
Third, to quantify the influence on the best fit param-
5 ThefitisperformedfixingtheabsorptionNHtoGX339–4standard eters of not fitting the absorption edge, we created a fake
valueandleavingthepowerlawphotonindexandnormalizationfree.This spectrum using the following model: PHABS × (POWER-
isnotthebestfitmodelfortheaveragespectrum(χ2 /dof=100/64).
However,thismodelissufficientforindividualobservationshavingalower LAW + GAUSS) × SMEDGE and run the fitting procedure
statistic(Fig.2). using modelset (1). The fit is satisfactory (χ2 / dof = 50 /
www.an-journal.org (cid:13)c 2016WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
6 M.Clavel,etal.:SpectralanalysisofGX339–4
Table1:Models(A)and(B)arestatisticallyequivalentforRXTE/PCAobservationswithin3–40keV(parametersasdefinedinSec.3).
Parameters Γ Ipwl Eline σ Iline Eedge τmax W kTmax Idisk
(phcm−2s−1keV−1) (keV) (keV) (10−4phcm−2s−1) (keV) (keV) (keV)
(A) 1.5 0.20 6.4 0.1 4 7.1 0.8 15 – –
(B) 1.4 0.15 6.4 0.8 0.15 – – – 1.44 0.38
nomenological model set (2) to model set (3), which in-
keV−1 10 cTlhuedseestwaosealnf-acloynsessispternotvimdeodcoelnsfiosrtetnhtererseuflletsctrieognarfdeiantgurtehse.
s−1
unts fit parameters, validating the use of an absorption edge
co whenstatisticallyneeded.
d 1
e
maliz
nor 7 Conclusion
0.1
4 OursystematicanalysisoftheRXTE/PCAdatabetween3–
2 40keVhighlightsimilartrendsforallGX339–4outbursts,
χ
anditis importantto disentanglethe variationstracingthe
0
true evolutionof the sourcefromthe one generatedby the
−2
analysis itself. In this work we demonstrated that neglect-
1 2 5 10 20
Energy(keV) ing the Galactic backgroundis justified but leads to an ar-
tificial softeningof the sourcespectra forthe lowest count
Fig.3: Simultaneous fit of the Swift MJD55260.2 (black) and
RXTE/PCAMJD55260.4 (red)spectraof GX339–4 withanab- rate observations.At higher fluxes, a limited energyrange
sorbed power law. The residuals computed from this poor fit andapossiblyincompletemodelsetcanresultinthedetec-
(χ2/dof=340/213)highlightthepresenceofreflectionfeatures tionof spuriouscomponents,such asthe high-temperature
between5and20keV. diskwhichartificiallysubstitutesformissingfeaturesinthe
reflection model.Therefore,physicalinputson the source,
as well as multi-wavelength observations, are crucial to
59),theinitialandoutputmodelparametersaresummarized
fullytestoursystematicanalysisandtoprovidethegeneric
inTable1.Thebestfithasaharderpowerlawthantheini-
propertiesofGX339–4.Thisextensivestudyisbeyondthe
tial one, better fitting the absorptionedge, andresulting in
scopeofthispaperandwillbepresentedina futurepubli-
an artificial excess of emission at low energy that is fitted
cation(Claveletal.inprep).
by a high-temperaturedisk with a low normalization.This
Acknowledgements. The authors acknowledge funding support
isexactlywhatwearewitnessingwhenusingmodelset(1)
from the French Research National Agency: CHAOS project
ontheRXTE/PCAdata.Therefore,webelievethatthehigh-
ANR-12-BS05-0009(http://www.chaos-project.fr).
temperaturediskcomponentisnotphysicalbutarisesfrom
anincompletemodelingofthereflectionspectrum.
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(cid:13)c 2016WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org