The transient hard X-ray tail of GX 17+2: new BeppoSAX results

0.31 MB·English

by  R. Farinelli

Astronomy&Astrophysicsmanuscriptno.ms1567 February5,2008
(DOI:willbeinsertedbyhandlater)
The transient hard X-ray tail of GX 17+2: new BeppoSAX results
R.Farinelli1,F.Frontera1,2,A.A.Zdziarski3,L.Stella4,S.N.Zhang5,6,M.vanderKlis7,N.Masetti2,andL.
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Amati
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1DipartimentodiFisica,Universita`diFerrara,viaParadiso12,44100Ferrara,Italy

n 2IstitutodiAstrofisicaSpazialeeFisicaCosmica,SezionediBologna,CNR,viaGobetti101,40129Bologna,Italy
a 3N.CopernicusAstronomicalCenter,Bartycka18,00-716Warsaw,Poland
J 4OsservatorioAstronomicodiRoma,viaFrascati33,00040MonteporzioCatone,Italy

2 5PhysicsDepartment,UniversityofAlabamainHuntsville,AL35899Huntsville,USA
1 6SpaceSciencesLaboratory,NASAMarshallSpaceFlightCenter,SD50,AL35812Huntsville,USA
7AstronomicalInstitute“AntonPannekoek”,UniversityofAmsterdam,Kruislaan403,1098SJAmsterdam,TheNetherlands
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0 Received2July2004 /Accepted17December2004
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1 Abstract.Wereport onresultsof twoBeppoSAX observations oftheZsourceGX17+2.Inbothcasesthesourceisinthe
0 horizontalbranchofthecolour-intensitydiagram.Thepersistentcontinuumcanbefitbytwo-componentmodelsconsistingof
5 ablackbodyplusaComptonizationspectrum.Withoneofthesemodels,twosolutionsfortheblackbodytemperatureofboth
0
theobservedandseedphotonsforComptonizationareequallyacceptedbythedata.Inthefirstobservation,whenthesourceis
/
h ontheleftpartofthehorizontalbranch,weobserveahardtailextendingupto120keV,whileinthesecondobservation,when
p thesourcemovestowardsrightinthesamebranch,thetailisnolonger detected.Thehard(>∼ 30keV) X-rayemissioncan
- bemodeledeitherbyasimplepower-lawwithphotonindexΓ ∼2.7,orassumingComptonizationof∼1keVsoftphotons
o
off ahybridthermalplusnon-thermal electronplasma. Thespectralindexof thenon-thermal injectedelectronsisp ∼ 1.7.
r
t TheobservationofhardX-rayemissiononlyintheleftpartofthehorizontalbranchcouldbeindicativeofthepresenceofa
as thresholdintheaccretionrateabovewhichthehardtaildisappears.Anemissionlineat6.7keVwithequivalentwidth∼ 30
: eVisalsofoundinbothobservations.Wediscusstheseresultsandtheirphysicalimplications.
v
i
X Keywords.stars:individual:GX17+2—stars:neutron—X-rays:binaries—accretion,accretiondisks
r
a
1. Introduction (BB) plus a multicolour disk blackbody (DBB, the “Eastern
Model”,Mitsuda et al. 1984,1989),a BB plusan unsaturated
ZsourcesformthebrightestclassofgalacticLowMassX-ray Comptonization spectrum (F(E) ∝ E−Γexp(−E/E ), the
c
Binaries (LMXBs), with X-ray luminosities typically in the
“WesternModel”,Whiteetal.1986,1988)and,morerecently,
range∼0.5–1timestheEddingtonluminosity(whereL ∼
Edd aDBBplustheComptonizationmodelworkedoutbyTitarchuk
1.5 M/M⊙ ×1038 erg s−1). Their name arises from the pat- (1994, COMPTT in XSPEC, Pirainoetal. 2002;DiSalvo etal.
tern they trace in the colour-colour or hardness-intensity di-
2002).Inrecentyears,transienthardX-raytailsextendingup
agram (CD and HID, respectively) on typical time scales of
to100keVhavebeendiscoveredfromtheclassicalZsources
days. The three regions which characterize the Z-track, from
(GX 5-1, Asai et al. 1994; Cyg X-2, Frontera et al. 1998; Di
thetop,arethehorizontalbranch(HB),thenormalbranch(NB)
Salvo et al. 2002; GX17+2, Di Salvo et al. 2000, hereafter
and the flaring branch (FB). Multifrequency observations of
DS2000;ScoX-1,D’Amicoetal.2001;GX349+2,DiSalvo
ScoX-1 (Vrtilek et al. 1991a; Hertz et al. 1992; Augusteijn
et al. 2001) and from the peculiar Z source Cir X-1 (Iaria et
et al. 1992) and Cyg X-2 (van Paradaijs et al. 1990; Vrtilek
al. 2001). All these hard tails have been fit with power-laws
et al 1990; Hasinger et al. 1990) have shown that the ac- (PL,F(E) ∝ E−Γ) with photonindexΓ rangingfrom∼ −1
cretion rate M˙ increasing as the sources move in the direc-
(ScoX-1,whenthesourcewasintheFB)to∼ 3.3(CirX-1).
tion HB → NB → FB is consistent with UV observations
Thetimebehaviourofthehardtailiscomplexandstillnotwell
and some QPO models. Several two-componentmodels have
understood. DS2000 observed a strong hard tail in GX17+2
been proposed to fit the low energy (<∼ 20 keV) X-ray spec- when the sourcewas onthe leftpartof the HB; asthe source
tra of Z sources, such as a simple or Comptonizedblackbody
movedontherightacrossthebranch,anindependentfitofthe
photonindexand PL normalizationwas notpossible, and fix-
Sendoffprintrequeststo:R.Farinelli ing thephotonindexat2.7yieldedan intensityof the PL that
e-mail:[email protected]
2 R.Farinellietal.:ThetransienthardX-raytailofGX17+2
lowered (chance probability of F-test ∼ 10−4) until it disap- GX17+2 were observed with the Einstein (Kahn & Grindlay
pearedattheHB-NBapex.InthecaseofCygX-2(DiSalvoet 1984),HAKUCHO(Tawaraetal.1984)andEXOSAT(Sztajno
al.2002)thehardtailwasobservedwhenthesourcewasinthe etal.1986)satellites.
HB, while in a later observation, when the source was in the Despite the X-ray brightness, the optical counterpart of
NB,noevidenceofhardX-rayemissionwasfound.Thehard GX17+2hasnotyetbeenunambiguouslyidentified.Thedif-
tailofGX5-1(Asaietal.1994)wasdetectedwhenthesource ficultiesarisemainlybecauseofthepositionofthesourcenear
wasin the NB, anddecreasedasthesourcemovedto theFB. theGalacticcenter(l = 16◦.4,b = 1◦.3),wherethechanceof
IfM˙ actuallyincreasesfromHBtoFB,theseresultscouldbe superpositiontounrelatedfieldobjectsisveryhigh.
indicative of the presence of a anti-correlation between hard In this paper we report on the results obtained from two
X-rayemissionandaccretionrate M˙ . However,theresults of BeppoSAX observations of GX17+2, performed before the
D’Amicoetal.(2001)contrastwiththisscenario,asinScoX-1 October 1999observationreportedby DS2000.In Sect. 2 we
thehardtailwasobservedinallbrancheswithoutanyapparent describe the observations and data analysis, in Sect. 3 we
correlationwiththesourcepositionontheCD. presenttheresultsofthetimingandspectralanalysis,inSect.4
wediscusstheresultsandinSect.5wedrawourconclusions.
1.1. GX 17+2
2. Observationsanddataanalysis
GX17+2 is one of strongest Galactic X-ray sources, and Weobservedthesourcetwice,on1997April3and21,withthe
was detected in the sixties (e.g., Friedman et al. 1967). NarrowFieldInstruments(NFIs)onboardBeppoSAX(Boella
Simultaneous X-ray and radio observations, performed by et al. 1997a). The NFIs include a Low-Energy Concentrator
White et al. (1978), did not show the presence of signifi- Spectrometer (LECS, 0.1–10 keV; Parmar et al. 1997), three
cant radio emission, but revealed correlated X-ray intensity Medium-Energy Concentrator Spectrometers (MECS, 1.5–
andspectralvariationssimilarto thosealreadyobservedfrom 10keV;Boellaetal.1997b),aHighPressureGasScintillation
ScoX-1. Observationof the sourcewith the Gas Scintillation Proportional Counter (HPGSPC, 4–120 keV; Manzo et al.
ProportionalCounter(2–20keV)on-boardEXOSAT(Whiteet 1997),and a Phoswich Detection System (PDS, 15–300keV;
al.1986)providedanaccuratedeterminationofthesourcecon- Frontera et al. 1997).Both observationswere performedwith
tinuum spectrum, which was fit in the contextof the Western all three MECS units (MECS unit 1 failed on 1997 May 6).
model with best fit parameters kT ∼ 1 keV, radius of the Table 1 reportsthe observationslog along with the on-source
bb
BBemittingsphereRbb ∼15km,Γ∼0.6andEc ∼4.5keV. exposuretimesofeachNFIandthemean1.8–10keVand13–
Alsoanemissionlineat∼6.5keVwithequivalentwidth(EW) 200keVcountratemeasuredbyallthethreeMECSunitsand
of ∼110 eV was detected. The data analysis of the same ob- by the four PDS units, respectively. During all pointings the
servation, but with the Medium Energy ProportionalCounter NFIsworkednominallyandthe sourcewasdetectedbyallof
Array, showed that the 1–30 keV source spectrum could be them. Good data were selected from intervals when the NFIs
fit with a BB (kTbb ∼ 1.2 keV) plus the Comptonization elevationanglewasabovetheEarthlimbbyatleast5◦and,for
model proposed by Sunyaev & Titarchuk (1980), with elec- theLECS,duringthedarkearth.TheSAXDAS2.0.0dataanal-
tron temperature kT ∼ 3 keV and optical depth of the elec- ysis package was used for processing the LECS, MECS and
e
tron cloud of τ ∼ 13 (White et al. 1988). The column den- HPGSPC data, while the PDS data reduction was performed
sity along the source direction was determined by Vrtilek et using theXAS packagev2.1(Chiappetti& Dal Fiume1997).
al. (1991b)with Einstein (0.2–20keV),whofounda value of TheLECSandMECSspectrawereextractedfromaregionof
NH ∼ 1.5×1022 cm−2, assuminga simple BB (kTbb ∼ 1.5 4′radiuscenteredonthesourceposition.Thebackgroundspec-
keV)asbestfitspectrum.Hoshi&Asaoka(1993)observedthe trum for the LECS and the MECS was determined using the
source with Ginga in both the HB and the NB. By fitting the standardblankfieldspectraasreportedbyFioreetal.(1999),
1–20keVX-rayspectrumwiththefirstversionoftheEastern whilefortheHPGSPCandthePDSthebackgroundwasesti-
model(BB+DBB),theseauthorsfoundthattheinnerdisktem- matedwiththerockingcollimatortechnique.Thespectraofall
perature and the projected radius of the DBB did not change NFIs were rebinnedto reduce the oversamplingin the energy
significantly from the NB to the HB, while the BB tempera- resolution and to have a minimum of 20 counts in each bin,
ture graduallyincreased from to 2.1 keV to 2.7 keV. DS2000 inordertoreliablyusetheχ2 statisticsforspectralfitting.For
analyzed the 0.1–100 keV X-ray spectrum of GX17+2 with eachinstrumentweperformedthespectralandtemporalanal-
BeppoSAX,discoveringa transienthard X-ray tail in addition ysis in the energy range where the response function is well
to the persistent continuum. The latter was well described by determined;on the basis of this extension,theenergyrange is
a BB plus COMPTT model, with best fit parameters kTbb ∼ 0.4–4.0keV forLECS, 1.8–10keV for MECS and8–30keV
0.6keV,R ∼ 40km,temperatureoftheseedphotonstobe for HPGSPC, while for PDS the range is 13–120 keV in the
bb
ComptonizedkT ∼ 1keV,electrontemperaturekT ∼3keV firstobservationand13-40keVinthesecondoneasthesource
s e
andopticaldepthof the Comptonizingcloudτ ∼ 10,assum- wasnotdetectedbeyond40keV.Weperformedspectralanaly-
ingasphericalgeometry.Additionally,theyfoundevidencefor sisoverfourdifferentregions,namedI,II,IIIandIV,according
an emission line with energycentroidEl ∼ 6.7 keV and EW tothe1.8–10keVMECScountratebeing<∼180s−1,between
∼ 35 eV, and an absorption edge with Eedge ∼ 8.6 keV and ∼ 180and ∼ 185s−1, between∼ 185and ∼ 195s−1 and>∼
optical depth τ ∼ 3 × 10−2. Type I X-ray bursts from 195s−1 (seeFig.1).We notethatregionIisentirelycovered
aedge
R.Farinellietal.:ThetransienthardX-raytailofGX17+2 3
Table1.LogofthetwoobservationsofGX17+2withtheon-sourceexposuretimeforeachNFI.
Obs. StartTime(UT) EndTime(UT) LECS MECS HPGSPC PDS 1.8–10keVMECS 13-200keVPDS
ks ks ks ks countrate(s−1) countrate(s−1)
1 1997Apr0300:27:52 1997Apr0306:00:06 2.3 10.7 4.1 3.4 176 35
2 1997Apr2102:29:00 1997Apr2106:13:55 1.7 6.5 2.4 2.3 190 24
by the points of the first observation while the other regions energy ranges adopted by us. From Fig. 2 it is possible to
arecoveredbythesecondobservation.Onlyfewpointsofthe see that GX17+2, during our observations, traced almost the
secondobservationfallinregionI,sowecouldnotextractsta- entire HB albeit in a slightly shifted position with respect to
tistically significant spectralinformationin this regionduring that observed by DS2000. Secular shifts of the Z pattern in
the second observation. For the latter we then extracted both the HID of GX17+2 have been previously observed to oc-
the averagespectrumand the separatedspectra for regionsII, cur on time scales of several months by RXTE (Homan et al.
IIIandIV. 2002)andEXOSAT(Kuulkersetal.1997).Ontheothershand,
We used the XSPEC v11.2.0 package to fit the multi- bothRXTEandEXOSATobservationsrevealedstrongstability
instrument energy spectra. In the broad band fits, normaliza- (within ∼ 2%) of the hard colour, and this confirms the HB
tion factors were applied to LECS, HPGSPC and PDS spec- natureofthesourceduringourobservations.
tra followingthe cross-calibrationtests betweenthem andthe We also investigated the source erratic time variability in
MECS, constrainingthemwithinthe allowedranges(Fioreet the 1.8–10 keV energy band (MECS data), by deriving the
al. 1999).Interstellarphotoelectricabsorption,modeledusing powerspectraldensity(PSD)inthe8×10−4–50Hzfrequency
thecrosssectionsimplementedinXSPECandwithsolarabun- range, obtained by averaging data intervals of 131072 points
dancesgivenbyAnders&Ebihara(1982),wasincludedinall with a binningtime of 10−2 s, for the first and second obser-
spectral models used. Uncertainties in the parameters values vationrespectively.Unfortunately,becauseofthe lowcollect-
obtained from the spectral fits are single parameter errors at ingareaofthetelescopes,thestatisticalqualityofthederived
90% confidence level, while upper limits are reported at 2σ PSD did not allow us to resolve the broad band noise com-
level.Forthe evaluationof thesourceluminositywe haveas- ponentstypicalofZ sourcesortodetectQPOsobservedwith
sumedadistanceof7.5kpc(Penninxetal.1988) othersatellites(EXOSAT,Hasinger&vanderKlis1990;RXTE,
Homan et al. 2002). We thus only derived the integrated root
meansquarefractionalvariationofthe1.8–10keVfluxwhich
3. Results is7.1%±1.3%inthefirstobservationand6.3%±2.4%inthe
secondone.
3.1. Temporalproperties
The source 1.8–10 keV light curve of the two observations
3.2. Spectralproperties
is shown in Fig. 1. A flux variability on time scales of hours
is apparent and is confirmed by the fit with a constant func- Following the results obtained on GX17+2 by DS2000, we
tion, which gives χ2/dof = 436/104. Unfortunately, the pres- firstattemptedtofitthesourcespectrawiththeBB + COMPTT
ence of several gaps in the light curve prevents us to investi- continuum model plus a fluorescence Fe line and an absorp-
gatewhetherthesourceshowsperiodicfluxmodulations.The tionFeK-edge.ThebestfitresultsareshowninFig.3,where
HIDofthetwoobservations,obtaineddefiningasintensitythe thetime-averagedcountratespectraoftheobservations1and
1.8–10keVMECScountrateandashardcolourthecountrate 2areshownalongwiththeresidualstothismodel.Ascanbe
ratio in the 6.5–10keV and 5–6.5 keV energybands, respec- seen, for the second observation (Fig. 3) the model describes
tively, is shown in Fig. 2. As it can be seen, the data points quitewellthedata,withnoapparentsystematicresiduals,even
of the first observation are all clustered in the left side of the though the χ2 is not satisfactory (χ2/dof = 192/141). This
diagram,while those of the secondobservation,reflectingthe couldbeduetosomespectralevolutionofthesourcewithtime
source intensity decrease (from∼ 200 s−1 to ∼ 180 s−1, see (seeSect.3.3.2).Inaddition,nosignificantK-edgeisrequired
Fig. 1), trace an extended path (regions II, III and IV) that, by the data (see Table 2). Unlike the second observation, the
on the left side, smoothly merges with the data points of the above model does not provide a good description of the data
first observation.Howeverfrom the shown HID, it is difficult of the first observation, with a χ2/dof = 222/144, mainly
to establish which branch of the Z-track our data correspond due to the residuals to the best fit model above 30 keV (see
to. To solve this issue, we plot in the lower panel of Fig. 2 Fig. 3). It is sufficient to add, as also done by DS2000, a PL
both our data points and those obtained with BeppoSAX by componentto the above spectral model to get a very good fit
DS2000, when the source clearly traced the HB and NB. As (χ2/dof = 143/142).The best fit results are shown in Table
DS2000 adopted differentenergy ranges to produce the HID, 2.Afeatureofthedataobtainedfrombothobservationswhen
we used the MECS event files of their observations from the they are fit with the above model (with or without the PL) is
BeppoSAXpublicdataarchivetoderiveanewHIDinthesame that two equally acceptablesolutionsfor the valuesof the BB
4 R.Farinellietal.:ThetransienthardX-raytailofGX17+2
Fig.1.MECS1.8–10keVlightcurvesofthefirst(upperpanel)andsecond(lowerpanel)observationofGX17+2.Thebinsize
inbothcasesis100s.Timeisgiveninsecondsfromthebeginningofeachobservation(seeTable1).
temperaturekTbb andtemperatureoftheseedphotonskTsare PAIR in XSPEC), which has the advantage of being general
found(see Table 2):onewith kT < kT andthe otherwith and can be used to fit the source spectra of both observa-
bb s
kT >kT ,whiletheotherparametervaluesremainsubstan- tions. In this model, non-thermalelectrons with energy spec-
bb s
tiallyunchangedwithinerrors. trum ∝ γ−p (where γ is the Lorentz factor) are injected in
Finally,totestthephysicalhypothesisofthethree-layered theplasmacloudatarate∝ γ−p.Theseelectronsloseenergy
atmosphericstructureinaccretiondiskssuggestedbyZhanget because of Compton,Coulomband bremsstrahlungprocesses
al.(2000)fortheblackholecandidates(BHC)GROJ1655–40 andthusestablishasteady-statedistribution.Athighenergies
andGRS1915+105,inthefirstobservationwereplacedthePL (for γ > γmin up to a certain value of γmax) the distribution
componentwithafurtherCOMPTTmodelwithhigherelectron is non-thermal (power-law like), but at low energies it joins
temperature,whichwe labelas kTh. Approximatingthe seed theMaxwelliandistributionandthefinalresultofthisprocess
e
photonstemperatureofthishotterelectronstothetemperature isahybridthermalplusnon-thermalplasma.Thetemperature
of the electronsof the underlyingwarm layer (an assumption of thethermaldistribution,kTe, is self-consistentlycomputed
justifiedbythefactthattheComptonparameteryofthewarm bymeansofenergybalanceandisnotafreeparameterofthe
electronsis∼3,sothattheiremergingspectrumisnotfarfrom model.Theplasmapropertiesarecharacterizedbythepowers
a Wien-like), namely keeping kTh = kT , does not give any Lsuppliedtothedifferentcomponents.Acompactnessparam-
s e
stable solution. A good fit is obtained by keeping free all the eterisintroducedanddefinedas:
modelparameters(χ2/dof= 149/140),butin this case, unlike
σ L
T
the results by Zhang et al. (2000), the seed photons tempera- ℓ≡ , (1)
m c2R
tureofthehotterCOMPTTcomponent(kTh ∼1.3keV,witha e
s
correspondingkTeh ∼75keVandτh ∼0.05)isquitedifferent where R is the characteristic size of the region and σT the
fromtheelectrontemperature(∼8keV)ofthewarmelectrons. Thomsoncross-sectionforscattering.Thecompactnessofthe
input soft photons is parametrizedby ℓ . The power supplied
s
totheComptonizingelectronsisparametrizedasℓ =ℓ +ℓ ,
3.3. A physical modelforthe hardX-ray tail h th nth
whereℓ isthepowersuppliedtoheatthermalparticles,while
th
In order to investigate the physical origin of the hard X-ray ℓ isthepowergoingintoacceleratingnon-thermalparticles.
nth
taildetectedduringthefirstobservationofGX17+2,weused Ifℓ =0,thenonehasapurelythermal(Maxwellian)distribu-
nth
the Comptonization model worked out by Coppi (1999, EQ- tion. EQPAIR canalsotestthe presenceofComptonreflection
R.Farinellietal.:ThetransienthardX-raytailofGX17+2 5
Fig.2. Left panel: HID of the two BeppoSAX observations of GX17+2. Diamonds: first observation. Open squares: second
observation.Thefourcountrateregionsarealsoshown.Eachpointcorrespondstoabinsizeof100s.Rightpanel:sourceHID
ofbothourobservations(samesymbolsasintheleftpanel)andthatperformedbyDS2000(crosses).Thebinsizeofthe1997
Octoberdatapointsis400s.Atypicalerrorbarisshownonthetoprightofeachpanel.
Fig.3.Countrate spectraof the firstobservation(leftpanel)andof thesecondobservation(rightpanel)of GX17+2andbest
fit folded model WABS(BB + COMPTT + GAUSSIAN) with kTbb < kTs. In the first observation, an absorption edge was also
included.Beloweachpanelareshowntheresidualstothemodelinunitsofσ.ThelasttwoPDSpointsofthesecondobservation
haveS/N<3,sotheywerenotincludedinthefitandarereportedas2σupperlimits.Similarresultsareobtainedforthesame
modelbutwithkT >kT .
bb s
of the primary photon spectrum from a neutral or a ionized alsoadirectBBcomponentplusaFeKfluorescencelineinthe
medium(Magdziarz& Zdziarski1995)allowing for different source model, as discussed in Sect. 3.2. This model well de-
metalabundanceswithrespecttothesolarones. scribesthedatabelow30keVbut,asinthecaseofCOMPTT,a
systematicexcessisagainobservedintheresidualsabovethis
threshold(seeFig.4).Wethusleftfreetovarytheℓ param-
nth
3.3.1. First observation eter:sincethestatisticsofthedatadonotallowustosimulta-
neouslyconstrainγ ,γ andpinthefits,γ wasfrozen
min max max
Wefirstassumedthatallthepowergoesinheatingthethermal atvalueof12(the χ2 is poorlysensitive toγ in the range
max
particles (ℓnth/ℓh= 0, purely thermal Comptonization) with a from2to50).Thechosenvalueofγmaxtakesintoaccountthe
simpleBBspectrumoftheseedphotons.WeaddedtoEQPAIR
6 R.Farinellietal.:ThetransienthardX-raytailofGX17+2
Fig.4.ResidualsinunitsofσtothemodelWABS(BB+EQPAIR+GAUSSIAN),assumingpurelythermalComptonization,forthe
first(leftpanel)andforsecond(rightpanel)observationofGX17+2.
Table 2. Best fit parameters of the multi-component model WABS(BB + COMPTT + GAUSSIAN + K-edge + PL) for the first
and second observation of GX17+2. The two solutions of the model are reported. The F-test gives the probability of chance
improvementoftheχ2 fortheadditionofthe PL.TheratiosLbb/Ltot andLpl/Ltot arecomputedinthe 0.4–200keVenergy
range.
Obs.1 Obs.2 Obs.1 Obs.2
I II+III+IV I II+III+IV
Parameter Solution1 Solution2
Na 2.5+0.2 2.2+0.1 2.5+0.2 2.3+0.1
H −0.3 −0.2 −0.3 −0.1
kTbb(keV) 0.66+−00..0043 0.61+−00..0033 1.79+−00..1106 1.38+−00..0052
Rbb(km) 35+−44 40+−44 3.7+−00..66 7.4+−00..55
kTs(keV) 1.24+−00..0098 1.09+−00..0064 0.58+−00..0022 0.55+−00..0013
kTe(keV) 3.4+−00..11 3.3+−00..11 3.5+−00..11 3.15+−00..0064
τ 11.0+0.4 10.5+0.4 12.0+0.6 13.0+0.3
−0.5 −0.4 −0.5 −0.5
Γ 2.8+−00..12 [2.8] 2.8+−00..12 [2.8]
Nb 2.1+1.2 0.7+0.6 2.3+1.2 1.1+0.2
pl −1.2 −0.6 −1.3 −0.6
F-test 3×10−14 0.11 5×10−12 0.03
E (keV) 6.7+0.1 6.7+0.1 6.7+0.1 6.7+0.1
l −0.1 −0.1 −0.1 −0.1
σ (keV) 0.1+0.1 0.2+0.1 0.1+0.1 0.2+0.1
l −0.1 −0.1 −0.1 −0.1
Ic 3.6+1.2 4.6+1.2 3.8+1.2 4.5+1.6
l −0.8 −1.4 −0.8 −1.0
EW (eV) 26+9 32+9 27+8 31+11
l −6 −9 −6 −7
E (keV) 8.2+0.4 [8.2] 8.3+0.3 [8.3]
edge −0.3 −0.2
τ (10−2) 2.5+1.4 <2.0 3.3+1.1 <1.8
edge −0.7 −1.5
Lbb/Ltot 0.16 0.17 0.10 0.15
Lpl/Ltot 0.30 0.13 0.32 0.18
Ld0.4−30keV 1.87 1.62 1.91 1.69
Ld30−200keV 0.02 0.01 0.02 0.01
χ2/dof 143/142 188/140 140/142 189/140
aInunitsof1022cm−2.
bPhotonskeV−1cm−2s−1at1keV.
cTotalphotonsinthelineinunitsof10−3cm−2s−1.
dUnabsorbedluminosityinunitsof1038ergs−1.
energyrangeinwhichtheexcessoverthermalComptonization wefindisξ ∼800ergcms−1.Animportantissuetopointout
islocatedandthetemperatureoftheseedphotonsfoundfrom isthat,unlikeCOMPTT,with EQPAIRthereisnotadoubleso-
thefit.AscanbeseenfromTable3,theagreementbetweenthe lutionasconcernsthedirectBB andseedphotonstemperature
modelandthedataisexcellent.TheprobabilitygivenbytheF- values.
testthattheχ2reductionobtainedfromtheinclusionofanon-
thermalcomponentintheComptonizingcloudisduetochance
isverylow(seeTable3).ThepresenceofaComptonreflection
component,assuming solar abundancesand a low inclination ReplacingthedirectBBcomponentwithaDBB,westillob-
anglei = 30◦ (GX17+2isaSco-likesource,Kuulkers&van tainanacceptable,albeitabitworse,result(χ2/dof=156/142).
derKlis1996),isonlymarginallydetected(∆χ2∼7,probabil- If,ontheotherhand,weassumea DBB seedphotonspectrum
ityofchanceimprovementof∼3%)withbestfitvalueofthe inEQPAIRplusadirectBBmodel,thefitisevenworse(χ2/dof
coveringfactorΩ/2π ∼ 0.04.Theionizationparametervalue =163/142).
R.Farinellietal.:ThetransienthardX-raytailofGX17+2 7
Table3.Bestfitparametersofthemulti-componentmodelWABS(BB+EQPAIR+GAUSSIAN)forthefirstandsecondobservation
ofGX17+2.TheF-testgivestheprobabilityofchanceimprovementoftheχ2switchingfromthermaltohybridComptonization.
Thecompactnessparametersℓ ,ℓ andℓ aredefinedinSect.3.3.TheratioL /L iscomputedintheenergyrange0.4–200
s h nth bb tot
keV.
Parameter Obs.1 Obs.2
I II+III+IV
Na 2.00+0.05 1.97+0.07
H −0.05 −0.06
kTbb(keV) 0.58+−00..0021 0.60+−00..0012
Rbb(km) 33+−22 31+−33
kTs(keV) 0.88+−00..1048 0.95+−00..0035
kTe(keV) 3.5 3.5
ℓs 7600+−73040000 [7600]
ℓh/ℓs 0.87+−00..0022 0.63+−00..0013
ℓnth/ℓh 0.11+−00..0022 <0.04
τ 8.4+1.5 8.4+0.1
−0.7 −0.2
p 1.7+1.1 [1.7]
−0.6
γmin 1.15+−10..5145 [1.15]
γmax [12] [12]
F-test 4×10−11 0.07
E (keV) 6.7+0.1 6.7+0.1
l −0.1 −0.1
σ (keV) 0.2+0.1 0.2+0.1
l −0.1 −0.1
Ib 4.4+1.3 4.8+2.2
l −0.8 −0.8
EW (eV) 32+9 33+15
l −6 −6
Lbb/Ltot 0.11 0.11
Lc0.4−30keV 1.44 1.46
Lc30−200keV 0.019 0.006
χ2/dof 143/142 185/141
aInunitsof1022cm−2.
bTotalphotonsinthelineinunitsof10−3cm−2s−1.
cUnabsorbedluminosityinunitsof1038ergs−1.
Table4.SameasinTable3butforregionsII,IIIandIVofthesecondobservationasreportedinFig.2andassumingthermal
Comptonization.
Parameter II III IV
Na 2.03+0.11 2.05+0.10 1.93+0.18
H −0.05 −0.13 −0.14
kTbb(keV) 0.61+−00..0013 0.53+−00..0096 0.55+−00..0086
Rbb(km) 37+−33 32+−1100 29+−99
kTs(keV) 1.17+−00..0027 0.85+−00..0058 0.90+−00..1004
kTe(keV) 4.0 3.5 3.1
ℓs [7600] [7600] [7600]
ℓh/ℓs 0.47+−00..0013 0.71+−00..0043 0.59+−00..0022
τ 6.7+0.2 8.6+0.4 8.9+0.3
−0.5 −0.3 −0.2
El(keV) 6.6+−00..11 6.7+−00..22 6.8+−00..11
σl(keV) 0.2+−00..42 0.3+−00..32 0.1+−00..21
Ib 5.0+3.6 6.5+4.8 5.3+2.4
l −2.3 −3.1 −1.8
EW (eV) 35+25 44+33 36+16
l −16 −21 −12
Lbb/Ltot 0.17 0.07 0.07
Lc0.4−30keV 1.47 1.50 1.48
Lc30−200keV 0.01 0.007 0.004
χ2/dof 145/142 133/142 145/142
aInunitsof1022cm−2.
bTotalphotonsinthelineinunitsof10−3cm−2s−1.
cUnabsorbedluminosityinunitsof1038ergs−1.
8 R.Farinellietal.:ThetransienthardX-raytailofGX17+2
Fig.5.UnabsorbedEF(E)spectrumofthefirst(leftpanel)andofthesecond(rightpanel)observationofGX17+2alongwith
the best fit model WABS(BB + EQPAIR + GAUSSIAN). A hybrid spectrum of the Comptonizing electrons is assumed. The 2σ
upperlimitofthe lasttwo PDSpointsofthe secondobservationisalso reported.Beloweachpanel,the residualsbetweenthe
data andthe modelin unitsof σ areshown.Differentline stylesrepresentthe singlecomponents.Dotted: BB. Dotted-dashed:
EQPAIR.Dashed:Gaussianemissionline.
3.3.2. Secondobservation a BB, we findthattwosolutionsarepossible,whichareremi-
niscentofthewellknownEastern(Mitsudaetal.1984,1989)
As reported in Sect. 3.2, during the second observation there
andWestern(Whiteetal.1986,1988)models.Indeed,looking
wasslightevidenceofa spectralevolutionofthesource,sug-
attheresultsofTable2,forsolution1,wecaninterpretthe∼
gested by the somewhat worse fit of the COMPTT model to
0.6keVBBasthedirectemissionfromtheaccretiondisk,with
the data, in spite of the fact that no systematic residuals to
Comptonizationmainlyoccurringinthehotterboundarylayer
thebestfitmodelareapparent(seeFig.3).Thisresultiscon-
(kT ∼ 1.2keV,EasternModelscenario).Butthesamespec-
s
firmedalsobyusingtheEQPAIRmodel,assumingpurelyther-
trum can be described (solution 2) by a ∼ 1.5 keV BB plus a
mal Comptonization, to fit the whole observation (χ2/dof =
Comptonizationregionofcoolerphotons(kT ∼ 0.6keV).In
186/142). In this case we also find that the soft compactness s
this case the direct BB emission can be interpretedas coming
ℓ isunconstrainedso,duringthefit, wefixeditatthebestfit
s fromtheboundarylayer,whileComptonizationtakesplacein
valueofthefirstobservation.Inordertoputanupperlimiton
the disk (Western modelscenario). From a statistical pointof
the power supplied in accelerating non-thermal electrons, we
view,wedonotfindstrongreasonsforfavouringonesolution
allowed the ℓ /ℓ parameterto vary,by keepingγ and p
nth h min overtheother;however,theformersolutionistheonlyonewe
frozenatthebestfitvaluesofthefirstobservation.Theresults
findwhenCOMPTTisreplacedbyEQPAIR.
arereportedinTable3.Asitcanbeseen,thepowergoinginac-
celeratingnon-thermalelectronsisconsistentwithzeroandits The soft compactness estimated by EQPAIR (see Table 3)
2σ upperlimitislowerthanthevalueofthefirstobservation. is much higher than that reported in the case of BHCs (e.g.,
It is worth pointing out that this upper limit value on ℓnth/ℓh Frontera et al. 2001; Gierlin´ski & Done 2003); this is how-
is obtained even including in the fit the last two PDS points ever not surprising given that in neutron star systems, unlike
(see Fig. 3),in order to have the same energy coverage of the BHCs, the source environment in the inner disk region has
firstobservation.The2σupperlimitonthe40-120keVsource an additionalsource of soft luminosity coming from the neu-
flux,assumingasbestfitmodeltheonereportedinTable3,is tron star. Apartfromthis crucialdifference,related to the na-
∼7×10−11ergcm−2s−1(seeFig.5)avaluebelowthatfound tureofthe compactobject,therearesomephysicalcharacter-
inthefirstobservation(∼1.0×10−10ergcm−2s−1).Wesub- istics which link in some way GX17+2 to BHCs. First, the
sequently performedspectral analysis separately for the three spectralindexofthe non-thermalinjectedelectrons(p ∼1.7)
regionsoftheHIDlabeledinFig.2;theresultsarereportedin is consistent, within errors, with that found, e.g., in Cyg X-
Table4. 1 (Frontera et al. 2001) and GRS1915+105 (Zdziarski et al.
2001).Moreover,thespectralshapeofthesourceintheEF(E)
diagram (see Fig. 5) strongly resembles that of BHCs in the
4. Discussion
so-called ultrasoft state, when L ∼ L (e.g., GX339–4,
X Edd
Some relevant results have been derived from our observa- GRS1915+105, XTEJ1550-564, see the review by Zdziarski
tions of GX17+2 with BeppoSAX. The persistent continuum &Gierlin´ski2004),wherethepeakoftheenergyemissionoc-
X-rayemission of GX17+2canbe describedby severaltwo- cursbelow10keV.ThesimilaritywithGRS1915+105,inpar-
componentmodels. When we adopt the COMPTT model plus ticular,resultsevenmoreintriguinglookingattheX-rayspec-
R.Farinellietal.:ThetransienthardX-raytailofGX17+2 9
tral parameters foundby Zdziarskiet al. (2001)by fitting the reviewbyCoppi1999).Inthiscaseparticleaccelerationcannot
sourceRXTE/OSSE3–600keV X-rayspectrumwith EQPAIR: be100%efficientintheproductionofnon-thermalhighenergy
the authors indeed found kT ∼ 3.6 keV, τ ∼ 4.4, kT ∼ particles(e.g.,Zdziarskietal.1993).Thiscanoccuronlyabove
e s
1.3 keV and ℓ /ℓ ∼ 0.1. Only the ratio ℓ /ℓ was lower (∼ acertainmomentumthresholdwhentheaccelerationtimescale
nth th h s
0.3) than that found in GX17+2 by us. As Z sources stably foraparticlebecomessignificantlyshorterthanitscorrespond-
accrete at near-Eddington rate, these similarities lead to sug- ingthermalization(orenergy-exchange)timescale.Belowthis
gest that Comptonization/acceleration processes in X-ray bi- threshold particles are tightly coupled and their acceleration
narysystemsinthisphasedonotstronglydependonthepres- goes mainly in a bulk heating of the thermal plasma compo-
enceor notofa solid surfaceatthe innerboundaryof theac- nent.
cretiondisk. We have also explored the physical scenario proposed by
Animportantresultofourstudyconcernsthetransienthigh Zhangetal.(2000)forGROJ1655–40andGRS1915+105,in
energyX-raytail.We confirmitspresencewhenthesourceis which a much hotter, optically thin thermal corona is located
intheupperleftHBoftheZdiagram,asalreadyobservedby abovea warm layer with lower temperatureand highopacity.
DS2000. But, unlike DS2000, who found that the hard X-ray This scenarioseems notsuitable for GX17+2. In fact, in this
excessgraduallydecreasedasthesourcemovedtowardsright hypothesis,onewouldexpectthattheseedphotonsofthehot-
intheHB, completelydisappearingwhenthesourceachieved ter layer are those coming from the underlyingone, and thus
theNB,wefindthatthisdisappearanceoccurswhenthesource shouldhaveatemperaturesimilartothatofthecolderelectrons
is still in the HB, in particular during its transition from re- (kTw ∼ 3–5 keV). This is indeed what Zhang et al. (2000)
e
gion I to region II of the HID (Fig. 2). The hard X-ray ex- foundwithASCA/RXTEdatainthecaseofGROJ1655–40and
cessisdescribedbyaPLwiththesamephotonindexfoundby GRS1915+105.However,inourcasekThsignificantlydiffers
s
DS2000(Γ∼2.7,seeTable2).Inordertogainsomeinsighton fromkTw (∼ 1.3keVand∼8keV,respectively),thusruling
e
thephysicaloriginofthe hardX-raycomponent,wereplaced outthethree-layeredaccretionhypothesis.
theCOMPTTmodelwiththeComptonizationmodelEQPAIRof In addition to the origin of the hard X-ray emission, its
Coppi (1999).In this model, in addition to a thermal popula- correlation with the position of Z sources in the CD/HID
tion of electrons, a non-thermalelectron tail can be included. is still unclear. As discussed in Sect. 1, the results obtained
AssumingapurelyMaxwelliandistributionoftheelectronen- fromGX17+2, CygX-2 and GX5-1(butnot, however,from
ergies,thehardX-rayexcessisstillapparentabovetheBBplus ScoX-1), could suggest the possibility of an anti-correlation
EQPAIRmodel(seeFig.4).Onlyassumingathermalplusnon- between presence of hard X-ray emission and accretion rate
thermalplasmathe EQPAIR modelgivesagooddescriptionof M˙ inZsources,inasimilarfashiontowhatobservedbothin
thehighenergyexcess.Toourknowledge,thisisthefirsttime atollsourcesandBHCs,wherethehardspectraareusuallyob-
thatthe transienthardX-raytailofa Zsource(see Sect.1)is served duringthe low inferredM˙ states (e.g., Barret & Olive
describedintermsofhybridComptonization. 2002;McClintock&Remillard2004).OurresultsonGX17+2
An important question is whether the two electron pop- do notcontrastwith this possibility but, when comparedwith
ulations (thermal and non-thermal) are physically connected. thosefoundbyDS2000,theypointtothepresenceofathresh-
Unfortunatelythepresentdatadonotallowtoanswerthisques- old, that we thus tentatively identify with a threshold in M˙ ,
tion.Weconsidertwocasesseparately.Ifthetwoelectronpop- abovewhichthehardtaildisappears.Adefiniteanswertothis
ulations are not physically related, a likely origin of the non- suggestion could come from very sensitive monitoring of the
thermalelectronsisarelativisticjetescapingthesystem.Inre- sourceathighenergiesinregionIofFig.2duringthesecond
centyears,relativisticjetshaverevealedtobeaquitecommon observation, but this was not possible because of the too few
property of LMXBs, providing a link between these systems pointsfallinginthatregion.
and AGNs (for a review see, e.g., Fender 2002). In this ther- Animportantissue iswhatwe meanbyM˙ :itappearsev-
mal plus non-thermal scenario, the Comptonized spectrum is identthatthisM˙ isnottheonethatdrivestheX-rayluminos-
producedby a thermal corona (likely located above the inner ity,sincethelatterdoesnotchangealongthetrack(seeTables
diskclosetotheboundarylayer)plusarelativisticnon-thermal 3 and 4). Thus, if some M˙ increases along the Z-track, it is
electronjet,withthesoftseedphotonsmainlycomingfromthe not correlated with the X-ray luminosity: an intriguing way
innerdisk.Theenergyoftheseedphotons(∼1keV,seeTable out to this problem was proposed by van der Klis (2001) to
3) is indeed consistent with the value expected for the inner explain the “parallel tracks” phenomenon. He suggested that
disk temperature of LMXBs hosting a neutron star (see, e.g., what does change along the track is not the prompt disk ac-
thereviewbyBarret2001).Thuswecouldtentativelyidentify cretion rate M˙ (t) but rather the ratio η = M˙ (t)/hM˙ (t)i,
d d d
the decrease of the power supplied to non-thermal electrons wherehM˙ (t)iisthelong-termM˙ (t)average.Theluminos-
d d
(ℓ /ℓ dropsfrom∼0.1tolessthan∼0.04fromthefirstto ityissupposedtoberelatedtothesumofM˙ (t)plushM˙ (t)i
nth h d d
the second observation),as a switching-offof the mechanism (L ∝ M˙ (t)+ αhM˙ (t)i) while the curve length S across
X d d
producingthejet. thetrackisameasureofη.Thethresholdabovewhichwesee
Ontheotherhand,ifthetwoelectronpopulationsarephysi- thedisappearanceofthehardtailcouldthusbea thresholdin
callyrelated,apossibleoriginofthenon-thermalcomponentis M˙ (t)/hM˙ (t)i. It is quite difficult to establish whether this
d d
inprocessesofmagnetohydrodynamicturbulenceoccurringin hypothesisis correct. In the positive, one has to explain what
thecorona(e.g.,magneticreconnection),inanalogywithwhat producesthehardtail:thephysicalprocessshouldbestrongly
observedin the solar corona(Crosby et al. 1998, see also the dependentonwhathappensin the disk. Whenthe hardX-ray
10 R.Farinellietal.:ThetransienthardX-raytailofGX17+2
componentisnolongerdetected(regionsII,IIIandIVofthe ingtheoriginoftheFeemissionlineandtodetermineunam-
HID), we observe a slight decrease of the temperature of the biguouslythepresence/absenceofanabsorptionedge.Onthe
scatteringcloud,asthesourcemovesfromthelefttotheright other hand, simultaneousX-ray/radio observationswould test
in theHB. Inthe vanderKlis (2001)model,thisis relatedto the possibility of a correlation between hard X-ray and radio
the variation of M˙ : a higherpromptdisk accretionrate is ex- emissioninthissource,withpresenceornotofrelativisticex-
pectedtopushtheinnerradiustowardstheneutronstarwitha pandinglobes.
consequentlargerproductionofseedandhotterphotonsforthe
Acknowledgements. This research is supported by the Italian Space
corona, which is thus more efficiently Compton-cooled (kT
e Agency(ASI)andMinistryofUniversityandScientificResearchof
decreases fromregionsII to IV, see Table 4). We confirm the
Italy(PRINN.2002027145).AAZhasbeensupportedbyKBNgrants
presence of the Fe K emission line at ∼ 6.7 keV, while the
PBZ-KBN-054/P03/2001and1P03D01827.
edgedetection(firstreportedbyDS2000)isnotsoevidentand
should be treated carefully. Indeed we find that the EQPAIR
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