INTEGRAL and Swift observations of EXO 2030+375 during a giant outburst

0.18 MB·English

by  D. Klochkov

Astronomy&Astrophysicsmanuscriptno.6801 c ESO2008
(cid:13)
February5,2008
LE
INTEGRAL and Swift observations of EXO 2030+375 during a giant
⋆
outburst
D.Klochkov1,D.Horns1,A.Santangelo1,R.Staubert1,A.Segreto3,C.Ferrigno3,P.Kretschmar4,I.Kreykenbohm1,5,
A.LaBarbera3,N.Masetti8,M.McCollough2,K.Pottschmidt6,G.Scho¨nherr1,J.Wilms7
1 Institutfu¨rAstronomieundAstrophysik,UniversityofTu¨bingen,Sand1,72076Tu¨bingen,Germany
7
2 SmithsonianAstrophysicalObservatory,60GardenStreet,Cambridge,MA02138,U.S.A
0
3 INAFIFC-Pa,viaU.LaMalfa153,90146Palermo,Italy
0
4 IntegralSOCESA,Madrid,Spain
2
5 INTEGRALScienceDataCentre,Chemind’Ecogia,16,1290,Versoix,Switzerland

n 6 CenterforAstrophysicsandSpaceSciences,UniversityofCalifornia,SanDiego,LaJolla,CA92093-0424,USA
a 7 Dr.KarlRemeis-Sternwarte,AstronomischesInstitut,Universita¨tErlangen-Nu¨rnberg,Sternwartstr.7,96049Bamberg,Germany
J 8 INAFIASF-Bo,viaGobetti101,40129Bologna,Italy

7
2
ABSTRACT

1
v Aims.Weinvestigate theX-ray spectral and timingproperties of thehigh mass X-raybinary EXO 2030+375 observed during its
1 June–September2006giant(typeII)outburst.
9 Methods.ThedataanalyzedinthisworkarefrompartlysimultaneousobservationswithINTEGRALandSwift.ThepulseperiodP
7 anditstemporalderivativeP˙ aremeasured.X-raypulseprofilesindifferentenergyrangesandtimeintervalsareconstructed.Pulse
1 averagedX-rayspectrafordifferenttimeintervalsarestudied.
0 Results.Wereportastrongspin-upofthesourceduringtheoutburst,comparabletothatobservedin1985duringthepreviousgiant
7 outburstwhenthesourcewasdiscovered.ThevalueofP˙ isfoundtobelinearlyrelatedtotheX-rayluminosityofthesourceduring
0 theoutburst.ForthefirsttimethehardX-ray(>25keV)characteristicsofthesourceduringatypeIIoutburstarestudied.TheX-ray
/ pulseprofilesapparentlychangewithluminosity.TheX-rayspectralcontinuuminthe3–120keVenergyrangeismodeledwithan
h absorbedpowerlawwithanexponential cutoffaround E 26keV.Anironemissionlineat 6–7keVisobserved.Thespectrum
p revealssomefeaturesbetween10and20keVwhichcanbe∼modeledeitherbyabroademission∼lineat 13–15keV(a“bump”)orby
- twoabsorptionlinesat 10and 20keV. ∼
o ∼ ∼
r Keywords.X-raybinaries;neutronstars;accretiondisks
t
s
a
:
v 1. Introduction Table1.Summaryofobservations.
i
X
Discovered by EXOSAT in 1985 during a giant outburst Obs. Instrument Obs.time Mean MeanASM
ar (Parmaretal. 1989), the transient accreting pulsar EXO (ks) MJD flux(mCrab)
2030+375 belongs to a high mass X-ray binary system with OBS1 ISGRI+JEM-X 62 53942.9 500
a B0 Ve star as optical companion (Coeetal. 1988). The or- OBS2 ISGRI+JEM-X 140 53967.6 500
bitalperiodandeccentricityare 46d and 0.42,respectively OBS2 XRT+BAT 6.3 53967.3 500
(Wilsonetal.2005).Accordingto∼thegener∼allyacceptedmodel OBS3 XRT+BAT 6.4 54002.5 160
forBe/X-raybinaries,the X-rayemission in suchsystems dur-
ingoutburstsisproducedwhenthecompactobjectaccretesfrom
aquasi-Kepleriandiskaroundtheequatoroftherapidlyrotating
Duringthe 1985 giantoutburstthe X-ray luminosity of the
Be star. Such a mechanism explains normal (type I) outbursts
sourcereachedavalueof L 2 1038ergs 1 (foradis-
with X-ray luminosity LX . 1037ergs−1, associated with the tance of 7.1 kpc, Wilsonet1−a2l0.k2e0V0∼2). T·he spin p−eriod of the
periastronpassageofthecompactobject.Sometimes,however,
pulsar( 42s)changeddramatically,witha spin-uptimescale
Be/X-ray binaries show giant (or type II) outbursts with X-ray P/P˙ ∼30yr(Parmaretal.1989).
luminosity LX & 1037ergs−1. Such outbursts are thoughtto be − Fro≈m the nearly continuous monitoring of the source with
duetoadramaticexpansionofthedisksurroundingtheBestar,
the Burst and Transient Source Experiment (BATSE) on the
leadingtotheformationofanaccretiondiskaroundthecompact
Compton Gamma Ray Observatory (CGRO) and the All Sky
object(Coe2000).
Monitor (ASM) on the Rossi X-ray Timing Explorer (RXTE)
itisknownthatthepulseperiodremainedroughlyconstantfor
aboutayearafterthebeginningofthemonitoringin1991,fol-
⋆ BasedonobservationswithINTEGRAL,anESAprojectwithin-
lowedby2yearsofrelativelyslowspin-upand6yearsofslow
strumentsandsciencedatacentrefundedbyESAmemberstates(espe-
ciallythePIcountries:Denmark,France,Germany,Italy,Switzerland, spin-down(Wilsonetal.2002,2005).After2002atransitionto
Spain),CzechRepublicandPoland,andwiththeparticipationofRussia aglobalspin-upalongwithanoverallbrighteningofthenormal
andtheUSA. outburstswasreportedbyWilsonetal.(2005).
2 Klochkovetal.:EXO2030+375ingiantoutburst
2006
1985 1990 1995 2000 2005
Feb Mar Apr May Jun Jul Aug Sep Oct
00..88
1 2 3
S S S
OB OB OB 24.10
00..66
)
ab]ab] WIFTT./Swift y (mHz 24.05
x [crx [cr 00..44 SIN uenc 24.00
uu q
flfl re
F
00..22 INT. WIFT 23.95
S
T./
N 23.90
00..00 I
46000 48000 50000 52000 54000
5533775500 5533880000 5533885500 5533990000 5533995500 5544000000 5544005500
Time (MJD)
MMJJDD
Fig.1. The ASM light curve of EXO 2030+375. The times Fig.2.SpinfrequencyofEXO2030+375.Smalldiamondsshow
ofINTEGRAL(INT.) andSwiftobservationsanalyzed in this the spin frequencydevelopmentbeforethe 2006giant outburst
workaremarkedbysolidlines.Thedashedlineshowsanother (they are taken from Wilsonetal. 2002, 2005). The large tri-
simultaneousobservationbyINTEGRALandSwiftfor which anglescorrespondtoourpulsefrequencymeasurementsduring
thedataarenotyetavailable. theoutburst.Theuncertaintiesofmeasurementsanddurationof
eachobservationaresmallerthanthesizeofsymbolsintheplot.
The giant outburst of EXO 2030+375 presented here
Table 2.Pulse perioddevelopment.Theuncertaintiesinparen-
(Corbet&Levine 2006; Krimmetal. 2006; McColloughetal.
theses(68%)refertothelastdigit(s).
2006)isthesecondofitskindsincethediscoveryofthesource.
The X-ray luminosity at the maximum of the outburst reached
Obs. MJD P dP/dt Flux(3–10keV)
LLe1−tt2e0rk,ebVas∼ed1o.n2I·N1T0E38GeRrgAsL−1a(nWdiSlswonift&dFaitna,gewre2i0n0v6es).tigInatethiins [s] [1−0−8s/s] [10−9ergcm−2s−1]
OBS1 53942.7714 41.57958(2) 2.93(12) 9.61(6)
2.1 the pulse period and the spin-up rate of the source and
OBS2 53967.4260 41.51706(5) 3.29(19) 10.41(4)
§
in 2.2 we construct X-ray pulse profiles for different energy OBS3 54002.3877 41.45954(4) 0.67(10) 3.25(3)
§
rangesandtimeintervals.Tocharacterizethebroad-bandspec-
tralbehaviourweperformin 2.3thespectralanalysisofpulse-
§
phase averaged spectra of the source. A detailed study of the aremarkedbysolidlinesandconsistofthreesetsofpointings.
timingandspectralbehaviourasafunctionoftheluminosityis ThefirstsetwasdonebyINTEGRAL.Thesecondsetcontains
beyondthescopeofthisLetterandwillbepresentedinaforth- simultaneous observations bySwift andINTEGRAL. The last
comingpaper. setofobservationswasperformedbySwift.Thesethreesetswill
bereferredtoasOBS1,OBS2,andOBS3throughoutthepaper.
OBS1andOBS2observationsweremadewhenthesourcewas
2. Observationsanddataanalysis
atapproximatelythesameluminositylevel,beforeandafterthe
maximumoftheoutburst.OBS3setwasmadeduringthedecay
EXO 2030+375 was observed by INTEGRAL (Winkleretal.
oftheoutburstwhentheluminosityofthesourcedroppedbya
2003) on 27 July and 19–21 August 2006 and by Swift
factorof 3.
(Gehrelsetal. 2004) on 19–20 August and 23–25 September
∼
2006. Another simultaneous observation by INTEGRAL and
Swiftwasperformedon6–8October2006,butthedatawerenot 2.1.Pulseperiodbehaviour
availableatthetimeofwritingthisLetterandwillbepresented
later.Forouranalysisweusedthedataobtainedwiththeinstru- Forthetiminganalysisalltimesweretranslatedtothesolarsys-
mentsIBIS/ISGRI(20–300keV,Ubertinietal.2003)andJEM- tem barycenter and corrected for orbital motion in the binary.
X(3–30keV,Lundetal.2003)onboardINTEGRALaswellas The pulse periods and associated derivatives were determined
XRT(0.2–10keV, Burrowsetal. 2005) andBAT(15–150keV, foreachofthethreeobservationperiodsindividually(Table2).
Barthelmyetal. 2005) ofSwift. Forthe analysis ofISGRIand Thesevalueswerefoundbyemployinginitialepoch-foldingand
JEM-XdatatheOff-lineScienceAnalysis(OSA)software(ver- asubsequentphaseconnectionanalysissimilartoFerrignoetal.
sion 5.1) was used (Courvoisieretal. 2003). Analysis of XRT (2006)usingwelldefinedpulseprofilesfromasufficientlylarge
data was done with the standard HEADAS package (version numberofpulses.Variationinpulseshapeinsideeachobserva-
6.0.4). tionismarginalanddoesnotaffectourmethod.
Table 1 containsthe summaryof the observationsanalyzed Fig. 2 displaysour pointstogetherwith the historicalpulse
in this work. A part of theASMlight curve1 of the source in- perioddevelopmentaspresentedbyWilsonetal.(2002,2005).
cluding three normal outbursts and the 2006 giant outburst is The three datasets are consistent with a global solution (valid
presented in Figure 1. The observations analyzed in this paper between MJD 53943 and MJD 54003) with the following pa-
rameters: P = 41.47268(6) s, P˙ = 1.627(5) 10 8 s/s and
−
1 WeusedtheresultsprovidedbytheASM/RXTEteamsatMITand P¨ = 4.058(25) 10−15s−1 for the r−eference M·JD 53992.25
·
attheRXTESOFandGOFatNASA’sGSFC. (TDB). Theuncertaintiesin parentheses(68%) referto the last
Klochkovetal.:EXO2030+375ingiantoutburst 3
OBS2
0 model):
0
3
ISGRI
200 20−150 keV ddNE ∝EE−−ΓΓ,·expE−EEfocludtoff, iiffEE ≤> EEccuuttooffff,
where dN/dE is the differential count rate, E is the energy of
0
ec 10 thephotons;Γ, Ecutoff, and Efold aremodelparameters.Aniron
s
ount/ JEM−X ethmeislasriognesltintoetaatl∼n6u.m5bkeerVowfacsouonbtsse)rvaendddmuroidnegleOdBbSy2a(wGhaiuchsshiaans
C 0 3−20 keV emissionline.JEM-Xdatarevealedanadditionalfeaturein10–
5
1 20keVrange,whichcouldbeexplainedbyaGaussianemission
modelat 13–15keV (a “bump”).Alternatively,followingthe
100 suggestion∼of a cyclotron line at 10 keV by Wilson&Finger
∼
(2006), we equally well fitted the spectra addingtwo Gaussian
0 0.5 1.0 1.5 absorptionlinesat 10and 20keV tothe broadbandcontin-
Pulse phase ∼ ∼
uum.To accountforlargesystematic uncertaintiesin the abso-
Fig.3.X-raypulseprofilesobtainedwithISGRI(toppanel)and lutefluxmeasuredbytheinstrumentsweintroducedinourmod-
JEM-X(bottom panel) during OBS2 (MJD 53966), near the elsafreemultiplicativefactorforeachinstrument:FISGRI, FXRT,
maximum of the outburst. Phase 0.0 was ar∼bitrarily chosen to and FBAT (forJEM-Xthe factor was fixed to 1.0). The best fit
bedefinedbythedip-likefeaturebeforethemainpeak.Typical spectralparametersforOBS1andOBS2arelistedinTable3(for
errorbarsareindicated. themodelwitha“bump”)andTable4(forthemodelwithtwo
absorptionlines).ThebroadbandspectrumofOBS2fittedwith
highecutisshowninFig.5a.Figures5b, 5cand 5dshowthe
digit(s). The characteristic spin-up time scale, P/P˙, is 40 residualsafterfittingthespectrumbythehighecutmodelwith-
− ∼
years,somewhatlongerthanduringthefirst giantoutburst.We out additionalfeaturesbetween 10 and 20 keV, with a “bump”
also note that the P˙ appears to be linearly correlated with the around15keV,and,alternatively,withtwoGaussianabsorption
X-rayluminosity(seelasttwocolumnsinTable2).Theformal lines.
correlation coefficient is 1.0 and the slope from a linear fit is
36 2(ss 1)/(ergcm 2s 1), although it is based on three data
− − −
±
points. 3. SummaryandDiscussion
InJune–September2006EXO2030+375hasundergonethesec-
2.2.Pulseprofiles ondgiant(typeII)outburstsince its discoveryin 1985.During
theoutburstthesourcehasbeenobservedwithINTEGRALand
UsingthevaluesofPandP˙ determinedhere(Table2)wecon-
Swift.Themainresultsoftheseobservationscanbesummarized
structedpulseprofilesforeachobservation.Pulseprofilesfrom
asfollows:
OBS1 and OBS2 are foundto be very similar. This can be ex-
1. The spin frequency of the pulsar has dramatically in-
pectedconsideringthattheluminosityofthesourceduringthese
creased,indicatingthepresenceofanaccretiondiskaroundthe
observationswasapproximatelythesame.Theresultsareshown
neutronstar.Themaximumvalueof P˙ isfoundtobecompara-
inFigs.3and4.WedonotshowpulseprofilesfromOBS1since
ble to that observed during the first outburst during which the
therearesimilartoOBS2.
sourcewasdiscovered.ThevaluesofP˙ determinedforthethree
Fig. 4 shows the 0.2–10 keV pulse profiles obtained with
XRT.OnecanseethatthepulseprofileobtainedinOBS3,dur-
ing the decay of the outburst, appears different from that ob- Table 3. Best fit spectral parameters of EXO 2030+375 using
tainedinOBS2whentheluminositywashigher. the model with a “bump” around 15 keV (see text). 1σ(68%)-
Theanalysisofthepulseprofiles,includingthecomparison uncertaintiesforoneparameterofinterestareshown.
withthoseobtainedduringthe1985outburstandduringnormal
outbursts,ison-going.
53943(OBS1) 53967(OBS2)
Timeofobservation ISGRI, ISGRI,JEM-X
2.3.Spectralanalysis (MJD)/Instruments JEM-X XRT,BAT
Γ 1.93 0.01 1.93+0.01
For OBS1 pulse averaged X-ray spectra ofISGRIandJEM-X ± 0.02
E [keV] 25.9+0.2 26.4−+0.2
were analyzed. For OBS2 spectra from ISGRI,JEM-X,XRT cutoff 0.3 0.5
and BAT were fitted simultaneously. In order to account for Efold[keV] 26.1±−0.2 26.9±−0.2
smallscaleuncertaintiesintheresponsematricesoftherespec- EFe[keV] — 6.6±0.1
tiveinstrumentssystematicuncertaintieswereaddedatalevelof σ [keV] — 1.2 0.1
Fe ±
1%forISGRIand3%forJEM-XandXRT.ForBATweused E [keV] 15.3 0.2 13.4+0.4
bump ± 0.5
energy-dependentsystematic errors provided by the HEADAS σ [keV] 2.7 0.2 4.1+−0.3
package.Wenotethatthereareadditionalglobalsystematicun- Nbum[1p022cm 2] 3.1±0.2 3.4 −00.5.1
certaintieswiththeinstrumentsonINTEGRAL(e.g.thecanon- H − ± ±
F 1.0(fixed) 1.0(fixed)
JEM-X
icalspectrumof theCrab isnotreproduced).Thestated uncer-
F 1.20 0.01 1.20 0.01
tainties(e.g.inTables3and4)arethereforetobetakenaslower ISGRI ± ±
F — 0.926 0.004
limits. XRT ±
F — 0.91 0.01
Tofitthebroadbandcontinuumofthesourceweusedanab- BAT ±
sorbedpowerlawwithanexponentialcutoff(XSPEChighecut χ2red/d.o.f. 1.1/259 1.3/937
4 Klochkovetal.:EXO2030+375ingiantoutburst
XRT (0.2−10 keV)
a)
0 OBS2
0 JEM−X
2
ISGRI
50 XRT
1
c
e0
s0
ount/01 OBS3 BAT+ 103
C6
0
5 b)
0
4
0
3
0 0.5 1.0 1.5
Pulse phase c)
Fig.4.X-raypulseprofileobtainedwithXRTduringOBS2(top
panel)and duringOBS3 (bottompanel).Typicalerrorbarsare
indicated. d)
observationperiodsarelinearlyrelatedtotheX-rayluminosity,
in line with previousobservations (Parmaretal. 1989) and the
predictionofaccretiontorquetheory(Ghosh&Lamb1979).
Sincetheluminosityofthesourceanditsspin-upratecould
Fig.5. The broad band spectrum of EXO 2030+375 from si-
be measured, we can estimate the value of the magnetic field
multaneous fits of INTEGRAL and Swift data of OBS2 with
strengthonthe neutronstar usingthe classical modelofan ac-
highecut(a)andresidualplotsafterfittingitwithoutadditional
cretingX-raypulsar(Pringle&Rees1972).Theequationdeter-
features(b),addinga“bump”around15keV(c),oralternatively
miningthechangeoftheangularmomentumoftheneutronstar
has the form Iω˙ = M˙ √GMRA, where I is the moment of in- includingtwoabsorptionlinesat∼10and∼20keV(d).
ertia of the neutron star, ω is the angular frequency, R is the
A
Alfve´n radius, M is the mass of the neutron star, and M˙ is the
accretion rate. R depends on the magnetic field strength and strength B 1 4 1012Gauss. Thiswouldbe consistentwith
A
∼ − ·
mthaegancectriectdioipnorlaetemaosmReAnt≃of[tµh2e/n(2eMu˙tr√on2GstMar.)]U2/s7i,nwghtheereobµsiesrvthede (aEfund=am11e.n6tal(Bcy/1cl0o1t2roGn)lin[e1i+nzt]he1rkaenVg,ewEhceyrce∼zis10th−eg5r0avkietaV-
cyc −
· ×
valuesoftheluminosityL(whichcanbetranslatedintotheac- tionalredshiftwhichwasassumedtobe0.2inourcalculations).
cretionrateusingtherelationL 0.1M˙c2),P˙ andstandardneu- 2. For the first time the broad band X-ray spectrum of the
∼
tronstarparameters,weobtainanestimateofthemagneticfield source(includingenergiesabove 25keV)duringatypeIIout-
∼
burst is analyzed. The continuum is modeled by a power law
withanexponentialcutoff.Anironemissionlineisobservedat
Table4.BestfitspectralparametersofEXO2030+375usingthe 6–7 keV.Thespectrumshowsadditionalfeaturesbetween10
∼
modelwithtwoabsorptionlinesat 10and 20keV(seetext). and 20 keV which can be equally well modeled with a broad
1σ(68%)-uncertaintiesforonepara∼meterof∼interestareshown. emission line at 13 15keV or two absorption lines at 10
∼ − ∼
and 20keV.Onthebasisofphaseaveragedanalysiswecannot
∼
distinguishbetweenthetwooptionsandwecannotconfirmthe
53943(OBS1) 53967(OBS2)
presenceofacyclotronlineat 36keVreportedbyReig&Coe
Timeofobservation ISGRI, ISGRI,JEM-X ∼
(1999).Apreliminarypulsephaseresolvedanalysisshowsthat
(MJD)/Instruments JEM-X XRT,BAT
the spectrum is highly variable with pulse phase. This means
Γ 1.79 0.01 1.78 0.01
± ± thatanyinterpretationofthephaseaveragedspectrumshouldbe
E [keV] 19.8 0.4 19.5 0.3
cutoff ± ± made with caution. Moreover,residualfeaturesaround25 keV
Efold[keV] 24.2±0.2 24.7+00..21 mayhintatnotyetwellunderstoodcalibrationuncertainties.
EFe[keV] — 6.51±−0.03 3.Theshapeoftheprofilesisfoundtovaryduringtheout-
σFe[keV] — 0.32+00..0034 burst,probablyrelatedtotheluminosity.
E1stline[keV] 10.0+00..32 10.6−+00..21 Pulse-phase-resolvedanalysisalongwiththeanalysisofthe
σ [keV] 1.6+−0.4 0.7 −0.2 spectralandtimingbehaviourofthesourceatdifferentluminos-
1stline 0.3 ±
E [keV] 20.7− 0.3 20.6 0.3 itystates,ison-going.
2ndline ± ±
σ [keV] 1.3 0.3 2.0+0.3
2ndline ± 0.2
N [1022cm 2] 2.2 0.2 3.1 −0.1 Acknowledgements. D.K.thanksKonstantinPostnovforusefulsuggestions.We
χ2H/d.o.f. − 1.1±/256 1.3±/934 thank the referee Dr. Lucien Kuiper forhis comments and corrections which
red improvedclearnessandreadibilityofthepaper.WewishtothanktheSwiftPI,
FJEM-X 1.0(fixed) 1.0(fixed) NeilGehrels,andtheStaffoftheSwiftMissionsOperationCenteraswellas
F 1.25 0.01 1.20 0.01 theINTEGRALMissionScientist,C.Winkler,andtheESAISOCpersonnelfor
FISGRI —± 0.926±0.004 theirpatienthelpinschedulingthesimultaneousobservationsofthisTargetof
XRT ± OpportunityProgram.ThisresearchissupportedbytheGermanSpaceAgency
F — 0.90 0.01
BAT ± (DLR)undercontractsnos.50OG9601and50OG0501.
Klochkovetal.:EXO2030+375ingiantoutburst 5
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‘EXO2030+375’onpage1