INTEGRAL observation of hard X-ray variability of the TeV binary LS5039 / RX J1826.2-1450

0.22 MB·English

by  A. D. Hoffmann

Astronomy&Astrophysicsmanuscriptno.hoffmann0812.0766 (cid:13)c ESO2009
January14,2009
LettertotheEditor
INTEGRAL observation of hard X-ray variability of the TeV binary
⋆
LS 5039/RX J1826.2-1450
A.D.Hoffmann1,D.Klochkov1,A.Santangelo1,D.Horns2,A.Segreto3,R.Staubert1,G.Pu¨hlhofer1
1 KeplerZentrumfu¨rAstro-undTeilchenphysik,Institutfu¨rAstronomieundAstrophysik,UniversityofTu¨bingen,Sand1,72076
9
Tu¨bingen,Germany
0
2 Universita¨tHamburg,Institutfu¨rExperimentalphysik,Hamburg,Germany
0
3 INAFIFC-Pa,viaU.LaMalfa153,90146Palermo,Italy
2

n
a ABSTRACT
J
Context.LS 5039/RX J1826.2-1450 is one of the few High Mass X-ray binary systems from which radio and high energy TeV
4
emissionhasbeenobserved.Moreover,variabilityoftheTeVemissionwithorbitalperiodwasdetected.
1
Aims.WeinvestigatethehardX-ray(25−200keV) spectralandtimingpropertiesofthesourcewiththemonitoringIBIS/ISGRI

instrumenton-boardtheINTEGRALsatellite.
]
h Methods.WepresenttheanalysisofINTEGRALobservationsforatotalofabout3Msecexposuretime,includingbothpublicdata
p anddatafromtheKeyProgramme.Wesearchforfluxandspectralvariabilityrelatedtotheorbitalphase.
- Results.Thesourceisobservedtoemitfrom25upto200keVandtheemissionisconcentratedaroundinferiorconjunction.Orbital
o variabilityinthehardX-raybandisdetectedandestablishedtobeinphasewiththeorbitallymodulatedTeVemissionobservedwith
r H.E.S.S.Forthisenergyrangewedetermineanaveragefluxfortheinferiorconjunctionphaseintervalof(3.54±2.30)×10−11 erg
t
s cm−2s−1,andafluxupperlimitforthesuperiorconjunctionphaseintervalof1.45×10−11ergcm−2s−1 (90%conf.levelrespectively).
a ThespectrumfortheinferiorconjunctionphaseintervalfollowsapowerlawwithanindexΓ=2.0+0.2(90%conf.level).
[ −0.2
Keywords.X-raybinaries;TeVemission;Microquasars
2
v
6
6 1. Introduction thesourcetobethecounterpartoftheunidentifiedhighenergy
7 EGRETsource3EGJ1824-1514(Hartmanetal.,1999)andalso
LS 5039/RX J1826.2-1450 is a High Mass X-ray Binary
0 acomponentoftheMeVemitterGROJ1823-12,whichisasu-
(HMXRB) system, which was discovered in the ROSAT all-
. perpositionofthreeEGRETsources(Collmar,2003).
2 sky survey and identified with optical follow-up observations
1 by Motchetal. (1997). The compact companion is estimated ThesourcehaslikewisebeendiscoveredtoemitatTeVen-
8
to have a mass of M ∼ 3.7M , orbiting a bright, V = 11.2, ergies with the High Energy Stereoscopic System (H.E.S.S.)
0 ⊙
O6.5V((f))starwithanorbitalperiodP ∼3.9060±0.00017d, (Aharonianetal., 2005). This demonstrated that X-ray bina-
: orb
v an eccentricity e = 0.35 ± 0.04 and a distance of d = 2.5 ± ries are capable of accelerating particles to TeV energies. The
i 0.1kpc(Casaresetal., 2005).Basedontheobservationofper- H.E.S.S. measurements have revealed that the flux and energy
X
sistentasymmetricmilliarcsecondradiooutflows(extendingup spectrum of the source are indeed modulated with the ∼ 3.9 d
r to∼1000AU),Mart´ıetal.(1998)andParedesetal.(2000)sug- orbital period of the binary system (Aharonianetal., 2006).
a
gested the presence of a mildly relativistic (v ∼ 0.2 c) jet. The VHE γ-ray emission is largely confined to half of the or-
Paredesetal. (2002) and Ribo´ etal. (2008) have thereforepro- bit and peaks around the inferior conjunction(INFC) epoch of
posedthesourcetobeamicroquasar. the compact companion,where a hardening of the spectrum is
LS 5039 has been observed by several X-ray satellites. A observed.According to the authorsthese findings indicate that
summaryofallobservationscanbefoundinBosch-Ramonetal. γ−ray absorption with pair production occurs in the system.
(2005). Flux variations on timescales of days as well as mini- Interestingly,orbitalvariabilityat TeV γ−ray energieshas also
flaresonshortertimescaleshavebeenobserved.RXTEobserva- beendiscoveredinthesimilarsystemLSI+61◦303withMAGIC
tions,coveringthewholeorbitalperiod,showvariabilitywitha (Albertetal.,2006)andVERITAS(Acciarietal.,2008).
maximumofemissionatorbitalphaseφ=0.8,interpreteddueto
ThenatureofthecompactobjectinthesystemRXJ1826.2-
periastronpassage.Moreover,thepower-lawshapedX-rayspec-
1450/LS 5039 as well as the orgin of the TeV emis-
trumappearsharderin the highflux state (Bosch-Ramonetal.,
sion is still under debate. A microquasar scenario (see
2005).
e.g. Paredesetal., 2000; Bosch-Ramon&Paredes, 2004;
Because inhomogeneousjetscouldemitγ−raysvia inverse
Torres&Sierpowska-Bartosik, 2008) as well as a pulsar sce-
Compton scattering, Paredesetal. (2000, 2002) have proposed
nario (see e.g Dubus, 2006; Sierpowska-Bartosik&Torres,
2007; Bosch-Ramonetal., 2008) have been discussed. In this
⋆ BasedonobservationswithINTEGRAL,anESAprojectwithin-
Letter, based mainly on archival data fromINTEGRALwhich
strumentsandsciencedatacentrefundedbyESAmemberstates(espe-
ciallythePIcountries:Denmark,France,Germany,Italy,Switzerland, span 4.6 years between March 2003 and October 2007, we
Spain),CzechRepublicandPoland,andwiththeparticipationofRussia confirmthedetectionofthesourcereportedbyBirdetal.(2007)
andtheUSA. in the energyrangefrom25 keV up to 200keV. We reportthe
2 Hoffmann,A.I.D.etal.:LS5039observedbyINTEGRAL
Table 1. Summary of selected and analyzedINTEGRAL data
forLS5039.Analyzeddataweretakenfrom69revolutions,i.e.
from revolution 0050 (03-11-2003)to revolution 0614 (10-25-
2007).
NumberofallextractedScWs 1648
NumberofScWsafterselection 1393
NumberofScWsinphaseSUPC 721
NumberofScWsinphaseINFC 672
finaltotalanalyzedexposuretime ≈ 2.99Msec
splittedinSUPCphase ≈ 1.56Msec
andinINFCphase ≈ 1.43Msec
into10smallerpartsaccordingtotheorbitalphasetoanalysethe
fluxdependenceonorbitalphase(seeSect.2.2).Table1summa-
Fig.1. A sketch of the system. The orbital values are listed in
rizestheobservationaldataofthesourceLS5039takenbythe
Casaresetal.(2005)andthedefinitionofphaseintervalsisfrom
INTEGRAL satellite and the selection made for this analysis.
Aharonianetal.(2006).
Only in less than 10% of the selected ScWs was LS 5039 in
thefullycodedfieldofviewofJEM-X.Basedonthesensitivity
of JEM-X1 and the amount of available data, a JEM-X analy-
discoveryofthe modulationofthehardX-rayfluxwith orbital
sis for a faintsource like LS5039is notsuitable. Inthis paper
phase.
we thereforefocusouranalysisonthe data of theIntegralSoft
Gamma-RayImager(IBIS/ISGRI).
2. Observationsanddataanalysis
2.2.Analysis
2.1.Observations
For the analysis of ISGRI data the Off-line Science Analysis
The International Gamma-Ray Astrophysics Laboratory
(OSA) software (version 7.0) providedby the ISDC was used.
(INTEGRAL, Winkleretal., 2003) carries on boardtwo main
Inaddition,wecrosscheckedtheresultsandalsogeneratedthe
gamma-rayinstruments,theSpectrometeronINTEGRAL(SPI,
lightcurveinSec.2.2.2withtheanalysispackagedevelopedby
20keV–8MeV, Vedrenneetal.,2003),andtheImageron-Board
INAF-IFCPalermo2(Segreto&Ferrigno,2006).
INTEGRAL Satellite (IBIS, 15keV–10MeV, Ubertinietal.,
A special handling of the OSA was used for this analysis.
2003) as well as two monitoring instruments in the X-ray and
First, we used a user catalogue which includes 115 sources,
optical range, the Joint European X-Ray Monitor (JEM-X,
i.e. all sources listed in the INTEGRAL General Reference
3–35keV, Lundetal., 2003), and the Optical Monitoring
Catalog3,whichareinthefieldofview(FoV)andwhichturned
Camera(OMC, Mas-Hesseetal.,2003).
out to be as bright as or brighter than RX J1826.2-1450 (≡
1648 science windows (ScWs) in the region that in-
LS5039).Thisisrequiredbecausethesourceofinterestisquite
cludes LS 5039 are found in the data archive avail-
faint and the crowdedfield producestoo many ghostimages if
able at the INTEGRAL Scientific Data Center (ISDC,
theknownsourcesarenotsubtracted.Second,thespectrumwas
http://isdc.unige.ch/),includingpublicdataaswellasdatafrom
produced with the help of the OSA tool mosaic spec4 from 13
theKeyProgramme.TheseScWswerepreselectedchoosingan
mosaics,eachcoveringoneenergybininthe13−250keVrange.
off-axisangleofthesourcesmallerthan10degreesandchoos-
Thebinningwaschosenidenticaltothedefaultenergybinning
ing only pointing observations that had a nominal observation
OSA uses for the spectrumcalculation step (SPE). Thisproce-
timeofmorethan1000seconds.1627ScWsweresuccessfully
dure is needed due to the fact that the source is faint and the
processed through the analysis with OSA 7.0 up to the image
FoVistoocrowded,andprovidesmorereliableresultsthanthe
stepIMA.
standardOSApipelinedoes.
In order to reduce statistical effects a further selection of
WealsocheckedourresultsusingthestandardSPEandthe
”good science windows” was necessary. ScWs are included in
lightcurvecreation(LCR)stepsofOSA.Inthiscaseweuseda
theanalysiswhentheyfulfillthefollowingcriteria:
special catalogue including all sources with a detection signif-
icance greater than 6 σ in the energy band 20 − 60 keV. The
1. The effective exposure time after extraction is larger than
catalogue includes 65 sources. According to the User Manual
1000sec.
oftheINTEGRALIBIS/ISGRIdataanalysisthisisbeyondthe
2. The number of good time intervals (GTI) does not exceed
standard method (see the IBIS Analysis Manual1 for details).
10.
3. ThemeanbackgroundintensitylevelofasingleScWinthe
1 The User Manual to the OSA can be found at:
energyrange25−60keVfallswithin3σofthedistribution
http://isdc.unige.ch/?Support+documents. See p.15 of
ofthemeanbackgroundintensitylevelsofallotherScWsin
JEM-XAnalysisUserManualforaJEM-Xsensitivityplot.
thesameenergyrange. 2 http://www.pa.iasf.cnr.it/∼ferrigno/
INTEGRALsoftware.html
The resulting 1393 ScWs cover the time range from 3 The INTEGRAL General Reference Catalogue is available at
03-11-2003 (ScW: 005000080010) to 10-25-2007 (ScW: ISDC:http://isdc.unige.ch/index.cgi?Data+catalogs
061400950010).Firstly,thesamplewassplitintotwopartsac- 4 http://isdc.unige.ch/Soft/download/osa/osa doc/
cordingtothe orbitalphaseforimaginganalysis,andsecondly osa doc-5.0/osa um ibis-5.0/node108.html#mosaspec
Hoffmann,A.I.D.etal.:LS5039observedbyINTEGRAL 3
However,ourresultsshowthata carefultreatmentofthisstan-
dardmethodprovidesacomparableresultinspiteoftherestric-
tions(seeFig.4andSec.2.2.2).
2.2.1. Imaging
Thesourceisclearlyseenatasignificancelevelof7.7σinthe
lowerenergybandof25−60keVinthesignificancemapbased
onthesumofallScWsfulfillingtheselectioncriteria.Inaddi-
tion,wedividedthedatasetintotwophaseintervals:thesuperior
conjunction passage at orbital phase 0.9 < φ ≤ 1.45 (SUPC),
and the inferior conjunction, at orbital phase 0.45 < φ ≤ 0.9
(INFC)(seeFigs.1and4fordetails).Phaseintervalsaredefined
inAharonianetal.(2006)fortheH.E.S.S.data.Thetwoorbital
resolvedsignificancemapsintheenergyrange25−60keVare
showninFig.2.Positionsofsources(openboxes)areprovided
bythelatestINTEGRALGeneralReferenceCatalog3.
At the INFC phase interval, the source was detected with
a significance ≈ 7.8 σ, for an exposure time of ≈ 1.43 Msec
(Fig. 2, lower panel). At the SUPC phase interval the source
was observed for a total exposure time of 1.56 Msec, and was
only marginallydetected: at the source position there is an ex-
cess at the level of 2.8 σ (Fig. 2, upper panel). For the total
sample and the total energy band 25 − 200keV the source is
detected at the position fitted by OSA (RA = 276.517 deg,
DEC=−14.812deg)with7.4σ.Withintheerrors,thisisconsis-
tentwiththesourcepositiondeterminatedbyBirdetal.(2007).
Fig.2.INTEGRALsignificancemapsintheenergyrange25−
2.2.2. Timinganalysis
60keV.Theopenboxesareknownsourcepositions.Comparable
A lightcurve produced with the INAF-IFC data analysis soft- exposure times and a higher significance for LS 5039 in the
ware(Segreto&Ferrigno,2006)wasusedforthetiminganaly- INFCphaseintervalindicatesvariabilityofthehardX-rayemis-
sis.Accordingtotheauthorsthissoftwareissuitabletoproduce sionwithorbitalphase(seealsoFig.4).TheINFCphaseinter-
reliableresultseveninthecaseofthepresenceofalargernum- val includes the maximum flux state of the source. Details can
berofsourcesintheFoV. befoundinthetext.
Searching for an unambiguous period failed due to low
statisticsandlargegapsinthesampleofobservations.Butape-
riodogramproducedby epoch foldingprovidesa peak at a pe-
riodof3.903days,whichiswithinuncertaintiesidenticaltothe ergcm−2 s−1 (90%conf.level).FortheSUPCphaseintervala
orbitalperioddeterminedbyCasaresetal.(2005)intheoptical fluxupperlimitof1.45× 10−12ergcm−2s−1(90%conf.level)
band.Fig.3showstheperiodogram. fortheenergyrange25−200keVcanbedetermined.
In a next step the lightcurve was folded using the known
orbital period of P = 3.90603 days with HJD(T ) =
orb 0
2451943d.09±0d.10 (≡ phase φ = 0.0 and periastron passage) 3. Summaryanddiscussion
determinedbyCasaresetal.(2005),toproducea10binorbital
phaseprofile.Inaddition,10mosaicscorrespondingtoeachof In this paper we report on a 3 Msec INTEGRAL monitoring
the10orbitalphaseintervalswereproducedandthecountrates oftheHMXRBRXJ1826-1450/LS5039.Thefaintnessofthe
at the catalogue3 position were determined. The values of the sourceandthefactthatitliesinacrowdedfieldrequiredavery
fluxes were derived from a 2D-Gaussian fit with fixed width careful,non-standardhandlingofthedata.
(HWHM=6.0arcmin)andfixedsourceposition.Bothmethods Ourresults,obtainedbyapplyingdifferentanalysismethods,
yield (within errors) comparable results: The hard X-ray emis- pointoutthatthesourcesignificantlyemitsathardX-rays(25−
sion is clearly modulated with the orbit, and appears to be in 200 keV) and that this emission varies with the orbit in phase
phasewiththeTeVemission(Fig.4).TheH.E.S.S.datapoints with the very high energy γ-rays detected with H.E.S.S. (Fig.
inthefigurearefromAharonianetal.(2006). 4). The spectrum at the inferior conjunction is well described
by a power law, while at the superior conjunction the hard X-
rayemission,ifany,isbelowthesensitivityofINTEGRAL.Our
2.2.3. Spectralanalysis
result indicates that accretion might not be the mechanism for
AstatisticallysignificantIBIS/ISGRIspectrumofLS5039was theproductionofthehardemission,since,inthiscase,wewould
obtained for the orbital phase interval corresponding to INFC expectarathersharpfluxmaximumnearperiastron.
(notshown here)and was modeledwith a powerlaw with Γ = Moreover,theobservedclosecorrelationofhardX-rayand
2.0+0.2(90%confidencelevel).Thereducedχ2forthefitis1.4(9 VHEγ-rayemissioninphasesuggeststhattheemissionwould
−0.2
dof).Meancountratesforthetwophaseintervalsallowustoes- originate from the same source region, possibly from particles
timateafluxfortheINFCphaseintervalof(3.54±2.30)×10−11 producedinthesameaccelerationprocess.
4 Hoffmann,A.I.D.etal.:LS5039observedbyINTEGRAL
40 This casts doubt on the simple explanation that the TeV
P =3.906 d variability is mainly caused by γ-γ absorption,as suggested in
orb
Aharonianetal.(2006).
30 We note that, from the correlation alone, more compli-
cated scenarios involving γ-γ absorption such as described by
Dubusetal. (2008) cannotbe ruled out. However the cascades
initiated in the γ-γ absorptionprocessproducesecondaryelec-
χ2 20 trons (Bednarek, 1997) also emitting X-ray synchrotron emis-
sion. Therefore, soft and hard X-ray flux levels can be used to
constrain the amount of absorption involved. The detection of
10 TeVemissionatSUPCtogetherwiththeupperlimitinthehard
X-ray domain derived here may pose problems for models in-
vokingonlyoneemissionregionclosetothecompactobject,as
recentlydiscussedinBosch-Ramonetal.(2008).
0
3.8 3.9 4.0 4.1 Radio observations interpreted as jets have led to a micro-
Period [d]
quasar interpretation for this source. In this scenario, particles
Fig.3.AnepochfoldingontheINTEGRALlightcurvesshows are accelerated to ≈ TeV energies (Perucho&Bosch-Ramon,
a significant peak at a period that is nearly identical to the or- 2008)duetotheinteractionbetweenthejetandthestellarwind
bitalperiodknownand determinedfromopticalobservationof ofthecompanion.Alternatively,Dubus(2006)hassuggestedthe
thesystem(Casaresetal.,2005).Allotherpeaksarelikelybeat sourcetobeapulsarbinarysystem,whereVHEemissionarises
frequenciesduetothelargegapsinthesampleofobservations. fromthepulsarwindmaterialshockedbytheinteractionwiththe
stellarcompanionwindandafterwardsflowingawayinacomet-
shapetail.Unfortunately,theobservationspresentedheredonot
enable us to discriminate between the two scenarios, although
thepredictedX-rayvariabilitybyParedesetal.(2006)seemsto
benotinaccordancewithourobservations.Finally,weremark
that a hadronic scenario (Romeroetal., 2003; Romero&Vila,
2008) is not ruled out by our findings. A longer and more de-
tailed interpretation of our results is beyond the scope of this
Letter,andwillbethesubjectoffurtherinvestigations.
Acknowledgements. WeacknowledgethesupportoftheDeutschesZentrumfu¨r
Luft-undRaumfahrt(DLR)undergrantnumber50OR0302.Thisworkisbased
onobservations withINTEGRAL,anEuropean Space Agency (ESA)project
withinstrumentsandsciencedatacentrefundedbyESAmemberstates(espe-
ciallythePIcountries:Denmark,France,Germany,Italy,Switzerland,Spain),
CzechRepublicandPoland,andwiththeparticipationofRussiaandtheUSA.
Wealsothanktheanonymousrefereeforhis/hervalueablecomments.andsug-
gestions.
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