XMM-Newton observations of PSr B1259-63 near the 2004 periastron passage

0.38 MB·English

by  M. Chernyakova

Mon.Not.R.Astron.Soc.000,1–9(2005) Printed5February2008 (MNLATEXstylefilev2.2)
XMM-Newton observations of PSR B1259–63 near the 2004
periastron passage
M. Chernyakova1,2⋆†, A. Neronov1,2, A. Lutovinov3, J. Rodriguez4 and S. Johnston5
1INTEGRALScienceDataCenter,Chemind’E´cogia16,1290Versoix,Switzerland
6 2GenevaObservatory,51ch.desMaillettes,CH-1290Sauverny,Switzerland
0 3SpaceResearchInstitute,84/32ProfsoyuznayaStreet,Moscow117997,Russia
0 4CEASaclay,DSM/DAPNIA/Serviced’Astrophysique(CNRSUMR7158AIM),91191GifsurYvette,France
2 5AustraliaTelescopeNationalFacility,CSIRO,P.O.Box76,Epping,NSW1710,Australia.

n
a
J Received<date>;inoriginalform<date>

1
1
ABSTRACT

1 PSR B1259–63isin a highlyeccentric3.4yearorbitwith a Be star andcrossesthe Be star
v disctwiceperorbit,justpriortoandjustafterperiastron.Unpulsedradio,X-rayandgamma-
1 rayemissionobservedfromthebinarysystemisthoughttobeduetothecollisionofpulsar
4
windwiththewindofBestar.WepresentheretheresultsofnewXMM-Newtonobservations
2
ofthePSRB1259–63systemduringthebeginningof2004asthepulsarapproachedthedisc
1
ofBestar.WecombinetheseresultswithearlierunpublishedX-raydatafromBeppoSAXand
0
XMM-NewtonaswellaswithASCAdata.ThedetailedX-raylightcurveofthesystemshows
6
0 thatthepulsarpasses(twiceperorbit)througha well-definedgaussian-profilediskwith the
/ half-openingangle (projectedon the pulsar orbitplane) ∆θdisk ≃ 18.5◦. The intersection of
h thediskmiddleplanewiththepulsarorbitalplaneisinclinedatθ ≃ 70◦ tothemajoraxis
p disk
of the pulsar orbit.Comparingthe X-raylightcurveto the TeV lightcurveof the the system
-
o we find that the increase of the TeV flux some 10–100 days after the periastron passage is
r unambiguously related to the disk passage. At the moment of entrance to the disk the X-
t
s ray photonindexhardensfrom Γ ≃ 1.8 up to Γ ≃ 1.2 beforereturningto the steeper value
a Γ>1.5.SuchbehaviourisnoteasilyaccountedforbythemodelinwhichtheX-rayemission
:
v issynchrotronemissionfromtheshockedpulsarwind.Wearguethattheobservedhardening
i oftheX-rayspectrumisduetotheinverseComptonorbremsstrahlungemissionfrom10-100
X
MeVelectronsresponsiblefortheradiosynchrotronemission.
r
a Key words: pulsars : individual: PSR B1259–63 – X-rays: binaries – X-rays: individual:
PSRB1259–63
1 INTRODUCTION from the system are very different close to and far from the pe-
riastron. Radio observations of the 1994, 1997 (Johnstonetal.
PSR B1259–63 is a ∼48 ms radio pulsar in a highly eccentric
1999), 2000 (Connorsetal. 2002), and the 2004 (Johnstonetal.
(e∼0.87), 3.4 year orbit with a Be star SS 2883 (Johnstonetal.
2005) periastron passages show that when the pulsar is far from
1992). The system’s distance from Earth is d ∼ 2 kpc
periastron the observed radio emission is comprised entirely in
(Johnstonetal.1994),note,however,thatdisuncertainbyafactor
highlylinearlypolarizedpulsedemissionfromthepulsaritselfwith
of∼2basedonradiomeasurements(see,e.g.,Manchester,Taylor,
an intensity practically independent of the pulsar orbital position
&Lyne1993).
(McClure-Griffithsetal. 1998). But, starting about 100 days be-
Be stars are well-known to be sources of strong highly
fore periastron, depolarization of the pulsed emission occurs, the
anisotropicmatteroutflow. Bothadilutepolarwindandadenser
dispersionmeasureandtheabsolutevalueoftherotationmeasure
equatorial disc have been invoked to reconcile models for infra-
increasewhilethefluxdensity decreases. Thepulsed radioemis-
red,ultra-violetandopticalobservations(Watersetal.1988).Tim-
sion then disappears entirely as the pulsar enters the disc and is
ing analysis of the PSR B1259–63 system shows that the disc
hidden behind it (relativetotheobserver) approximately 20days
of Be star is tilted with respect to the orbital plane (Wexetal.
beforetheperiastronpassage.Shortlybeforethedisccrossingun-
1998; Wangetal. 2004). The properties of the radio emission
pulsedradioemissionappearsandwithinseveraldayssharplyrises
toapeakwhichismorethantentimeshigherthantheintensityof
⋆ E-mail:[email protected] thepulsedemissionfarfromtheperiastron.Afterwardtheunpulsed
† M.ChernyakovaisonleavefromAstroSpaceCenteroftheP.N.Lebedev fluxslightlydecreases,asthepulsarpassesthroughperiastronbe-
PhysicalInstitute,Moscow,Russia
2 M. Chernyakova et al.
fore reaching a second peak just after the pulsar crosses the disc
SC X1
forthesecondtime.Theunpulsedradioemissionisdetecteduntil X2 A4
atleast100daysaftertheperiastronpassage.(Johnstonetal.1999, A5
2005;Connorsetal.2002). A3
ThefirstdetectionoftheX-raysfromPSRB1259–63system
A6
wasdonebyROSAT in1992(Cominskyetal.1994).Themostre-
cent published observations of thesystem inX-rays werecarried X3 θ Be A2
out with the ASCA satellitein 1994 and 1995 (Kaspietal. 1995;
Hirayamaetal. 1999). These observations show that X-rayemis- A1
sionisapproximatelytwiceashighatthetimeofthedisccrossing X10
X4 X9
than at periastron. In soft gamma-rays the system was observed X7X8
only after the periastron passage with CGRO/OSSE and INTE- X5 S1 X6
GRAL/ISGRI(Grove1995;Shawetal.2004).Thecombinedsoft
gamma-andX-rayspectrumisconsistentwithapowerlawofpho-
Figure1.SchematicrepresentationofPSRB1259-63binarysystemwith
tonindex∼ 1.7.Thisindexalsocoincideswiththeradiospectral thelocationsofthepulsarduringXMM-Newton BeppoSAX andASCAob-
indexoftheunpulsedemission.NopulsedX-rayemissionwasde- servations.Shadedareashowsthegeometryofthediskinferredfromthe
tected from the system. With the standard epoch-folding method X-raydata.Darkerandlightershadingcorrespondsto1and2half-opening
theupperlimittoapulsedcomponentin2–10Kevenergyrange anglesofthedisk,respectively. Angleθistheorbitalphaseofthepulsar
wasestimatedtobeabout10%oftheunpulsedcomponentcloseto referredinthetext.
theperiastron,andtoabout 40%duringtheapastron(Kaspietal.
1995;Hirayamaetal.1999).Notethatsearchforthepulsationsin
emission as the inverse Compton emission from the pulsar wind
P - P˙ space allowed Kaspietal. (1995) to reduce an upper limit
electrons.
closetotheperiastrontolessthan2%underacertainassumptions
Anothersurprisingresultofouranalysisthatisnotpredicted
ontheshapeofpulseprofile.
inthetheroeticalmodelscitedaboveisthepersistentlyhardX-ray
The 2004 periastron passage was observed in TeV gamma- spectrum(photonindexΓ≃1.2−1.3)observedduringthe10-day
raysbyHESS(Aharonianetal.2005).TeVlightcurveofthesource
periodwhenthepulsarentersthedisk.
showsanunexpectedbehaviour.Contrarytothepredictionthatthe
Thispaper isorganizedasfollows:inSection2wedescribe
TeVfluxshouldbemaximumattheperiastron(Kirketal.2002),
thedetailsoftheXMM-NewtonandBeppoSAXdataanalysis.The
thefluxapparentlyhasalocalminimumaroundthismoment.
resultsarepresentedinSection3,anddiscussedinSection4.
Several theoretical models were put forward to explain the
behaviour of the X-ray lightcurve of the system. In the model
ofTavani&Arons(1997),theenhancement ofsynchrotronX-ray
2 OBSERVATIONSANDDATAANALYSIS
emission during the disk passage is attributed to enhancement of
magneticfieldatthepositionofthecontact surfaceofpulsar and InFigure1weshowaschematicdrawingofthepulsarorbitalong
stellar winds. In the model of Chernyakova&Illarionov (1999, withtheapproximatelocationofthepulsarpositionduringXMM-
2000)theenhancementoftheinverseComptonX-rayemissiondur- Newton,BeppoSAX,andASCAobservations.TheX-rayobserva-
ingthediskpassageissupposedtobeduetotheeffectofthemacro- tions are more-or-less homogeneously distributed over the whole
scopic mixing of the stellar and pulsar winds in the disk which rangeofpulsarorbitalphase0 6 θ < 360◦.Thefigurealsoshows
effectively enhances the escape time of high-energy electrons re- thelocationof theBestar disk(parametersof thediskarefound
sponsiblefortheobservedemission. belowfromtheanalysisoftheX-raydata).Onecanseethattheset
TypicalenergiesofelectronsresponsiblefortheX-rayemis- ofobservationsdenotedasX6–X10enablestostudytheentrance
sion are Ee ∼ TeV in Tavani&Arons (1997) model while in tothediskingreatdetails.
Chernyakova&Illarionov (1999) model E ∼ 10 MeV. One can,
e
inprinciple,distinguishbetweenthetwomodelsbycomparingthe
detailsofradio,X-rayandTeVlightcurveofthesystem.Wehave 2.1 XMM-Newtonobservations
organizedacycleoffiveXMM-Newtonobservationsasthepulsar
The log of the XMM-Newton data analyzed in this paper is pre-
approachedthe2004periastron.Inthispaperwepresenttheresults
sented in Table 1. Observations X6 – X10 were obtained as part
of our observations along with other XMM-Newton observations
of the2004 periastron campaign and we have also used all other
of thesystem not previously published. For the completeness we
XMM-Newton observations of the system, which are now public
alsoincludetheanalysisofthepreviouslyunpublishedBeppoSAX
(X1 – X5). In the Table negative τ denotes the number of days
data.The2004periastronpassagewasextensivelyobservedinra-
before the date of 2004 periastron passage (2004 March 7), and
dio(Johnstonetal.2005)andTeV(Aharonianetal.2005)bands.
positivevaluescorrespondtothenumberofdaysafterthe2000pe-
Combinationoftheradio,X-rayandTeVdataopensupapossibil-
riastron(2000October17).θistheorbitalphasecountedasshown
ity,tostudytheevolutionofthespectrumofPSRB1259–63system
inFig.1.
oversome12decadesinenergy.
The XMM-Newton Observation Data Files(ODFs) were ob-
OuranalysisoflargeX-raydatasetenables,forthefirsttime,
tainedfromtheon-lineScienceArchive1;thedatawerethenpro-
toconstrainthegeometricalparametersofthediskofBestar,such cessedandtheevent-listsfilteredusingwithintheSci-
astheopening angleandorientation. Comparing theTeVand X- enceAnalysisSoftware()v6.0.1.Inallobservationsthesource
raylightcurveswefindthattheincreaseoftheTeVfluxsome10
wasobservedwiththeMOS1andMOS2detectors.TheX1–X5
to100daysaftertheperiastronpassageisunambiguously related
tothepulsarpassagethroughtheBestardiskforthesecondtime.
ThischallengestheconventionalinterpretationoftheobservedTeV 1 http://xmm.vilspa.esa.es/external/xmm dataacc/xsa/index.shtml
XMM-Newton observationsofPSR B1259–63near the2004periastronpassage 3
Table1.JournalofXMM-NewtonobservationsofPSRB1259–63 Table2.JournalofBeppoSAXobservationsofPSRB1259–63
Data Date MJD τ θ Exposure Data Date MJD τ θ Exposure
Set (days) (deg) (ks) Set (days) (deg) (ks)
X1 2001-01-12 51921.73 87.2 320.8 11.3 S1 1997-03-22 50529.63 −68.31 44.9 28.622
X2 2001-07-11 52101.31 266.7 342.9 11.6 S2 1997-09-02 50693.75 95.81 322.9 16.985
X3 2002-07-11 52467.24 −604.1 0.5 41.0 S3 1997-09-08 50699.44 101.50 324.1 53.555
X4 2003-01-29 52668.27 −403.1 9.31 11.0 S4 1997-09-17 50708.08 110.14 325.9 51.301
X5 2003-07-17 52837.53 −233.9 19.5 11.0 S5 1997-09-25 50716.05 118.11 327.3 28.719
X6 2004-01-24 53028.79 −42.6 57.4 9.7
X7 2004-02-10 53045.43 −25.0 73.4 5.2
X8 2004-02-16 53051.39 −20.0 83.1 7.7
X9 2004-02-18 53052.02 −18.4 86.5 5.2
X10 2004-02-20 53055.82 −15.6 93.2 6.9
observations were done in the Full Frame Mode, while the 2004
observationswereperformedintheSmallWindowMode,tomini- PSR B1259-63
mizethepile-upproblems.FortheX6–X10observationsPNdata
arealsoavailable.Inallobservationsamediumfilterwasused.
Theeventlistsforspectralanalysiswereextractedfroma45′′
radiuscircleatthesourceposition fortheX1–X5observations, deg-63.9
andfroma22.5′′ radiuscircleforMOS1,2observationsinSmall on,
ati
WindowMode(X6–X10).ForthePNinstrumentaregionof35′′ eclin
aroundthesourcepositionwaschosen.Backgroundphotonswere D
collected from aregion located inthe vicinity of the source with
2RXP J130159.6-635806
thesamesizeareaastheonechosenforthesourcephotons.
Forthespectralanalysis,periodsofsoftprotonflaresneedto
be filtered out. To exclude them we have built light curves with
ahundredsecondbinningandexcludedalltimebinsinwhichthe
-64.0
countrateabove10keVwashigherthan1.5cnt/s.Thesourcecoun-
195.8 195.6 195.4
tratein1–10keVenergyrangevariedfromabout0.07to1.2cts/s
Right ascention, deg
for MOS instruments, and from 0.4 to 4 cts/s for PN. Data from
MOS1,MOS2and, whenavailable, PNdetectorswerecombined
inthespectralanalysistoachievebeststatistics. Figure2.ContourplotoftheXMM-NewtonfieldofviewfortheX6ob-
servation.Atotalof10contourswereusedwithalinearscalebetween5
counts perpixel (outer contour) and 50counts perpixel (inner contour).
2.2 BeppoSAXobservations Inthisobservation2RXPJ130159.6-635806wasfortytimesbrighterthan
PSRB1259–63.ThisfigureistakenfromChernyakovaetal.(2005).
Table 2 lists the observations of PSR B1259–63 made with
BeppoSAXaroundthe1997periastronpassage.IntheTableτde-
notesthenumber ofdaysbeforeorafterthe1997periastronpas- observationsareconsistentwitheachother,andcombinedthemin
sage. ordertoimprovestatistics.Thiscombinedspectrumwedenoteas
WeuseddataofMECSinstrumentsaboardBeppoSAXobser- SChereafter.
vatorywhichprovidesthecoverageintheenergyrange∼1.5-11.0
keV.ReductionofMECSdatawasdonewiththehelpofstandard
tasks of LHEASOFT/FTOOLS5.2 package according to recom- 3 RESULTS
mendations ofBeppoSAX Guest Observer Facility2.Spectrumof
thesourcewasobtainedextractingeventsfromacircleof3′radius 3.1 ImagingAnalysis
aroundthesourceposition.Spectrumoftheinstrumentbackground
IntheXMM-NewtonfieldofviewduringitsPSRB1259–63obser-
wasestimatedconsideringeventsfromacircleofthesameradius
vationprogram,twosourceswereclearlydetected(seeFig.2for
6′ away from the source. Typically the source provided several
thecontourplotofXMM-NewtonfieldofviewfortheMJD53029
hundredsofcountsperobservation.Responsematrices(RMFand
(X6) observation). Besides PSR B1259–63 itself there is second
ARF) of MECS detectors were taken from HEASARC archive3.
source,whichwehaveidentifiedas2RXPJ130159.6-635806.De-
CorrectionsofphotonsarrivaltimetotheSolarsystembarycenter
tailed analysis of this source are presented in Chernyakovaetal.
forsubsequentusageoftheeventlistsintiminganalysisweredone
(2005).Inthispaperitisshowninparticular,that2RXPJ130159.6-
withthehelpoftasksfromSAXDASpackage4.
635806 is highly variable, with an intensity during flares which
WehavecheckedthatallthespectraobtainedintheS2–S5
isseveral times larger than the peak intensity of PSRB1259–63.
Therefore, although many observations of PSR B1259–63 have
2 http://heasarc.gsfc.nasa.gov/docs/sax/abc/saxabc/saxabc.html, beenmadewithRXTE,thefactthatitisanon-imagingtelescope
http://www.asdc.asi.it/bepposax/software/cookbook/ means that it is extremely difficult to determine the flux from
3 http://heasarc.gsfc.nasa.gov/FTP/sax/cal/responses PSRB1259–63alone.WethereforedonotusenumerousRXTEob-
4 http://www.asdc.asi.it/bepposax/software/saxdas/index.html servationsofthe2004periastronpassage,presentedinShawetal.
4 M. Chernyakova et al.
A3
A4
A2
X1
X10 SC
X6
X5
S1
HESS 2004
(> 380 GeV)
1994
1997
2000
2004
Figure3.Comparisonbetween theX-ray(top),TeV(middle) andradio (bottom)lightcurves. The1-10keV X-rayfluxisgiven in10−11ergscm−2s−1.
XMM-Newtonobservationsaremarkedwithtriangles,BeppoSAXoneswithcircles,andASCAoneswithsquares.Dataforfourdifferentperiastronpassages
areshownwithdifferentcolors:red(1994),green(1997),black(2000)andblue(2004).BottomXaxisshowstheorbitalphase,θ,topXaxisshowsdaysfrom
periastron,τ.
(2004).NotethatdespiteintheShawetal.(2004)allRXTEobser- Duringthenexttwodaystheintensitycontinued toincrease,and
vationsareanalysed,theauthorswereabletouseintheiranalysis reacheditspeakvalueatX9.Thenextobservation(X10)wasdone
only data simultaneous to INTEGRAL observations, when it was onlythreedayslater,andtheobservedfluxwasalreadyoneanda
clearthatinfluenceof2RXPJ130159.6-635806issmall. halftimeslower.
ComparisonofX-rayandradiolightcurvesshowsthattheX-
rayfluxfromthesystemvariesinamoreregularwaythanthera-
3.2 TheX-raylightcurve
dio flux: the X-ray data points from different periastron passages
The upper panel of Fig. 3 shows the X-ray lightcurve of the liemore-or-lessonthesamecurve.RapidgrowsoftheX-rayflux
system. For comparison we also show in the same Figure TeV found in our XMM-Newton observations X6 – X10 is correlated
lightcurveof2004HESSobservation(Aharonianetal.2005)and withtherapidgrowthoftheunpulsedradioemissionfromthesys-
radio (Johnstonetal. 1999; Connorsetal. 2002; Johnstonetal. tem. The growth of radio and X-ray flux at these phases can be
2005)lightcurvesofdifferentyearsperiastronpassages. attributedtothepulsarenteringtheBestardisk.
The X-ray fluxfrom the source, inagreement withprevious Unfortunately,TeVobservationsstartsomewhatlateranditis
observations,wasobservedtobehighlyvariable.The1–10keV not possible to see whether the TeV flux growth during the pre-
fluxisatminimumnearapastron(observationX3),andhasmax- periastron disk crossing. However, simple geometrical argument
imummore than20 timeslarger, 18days before periastron (X9). tells that the orbital phase θ at which the pulsar should enter the
DuringobservationsX5,X6,andX7,theintensityoftheemission diskfor thesecond timeshouldbeshiftedby180◦ relativetothe
increased,asthepulsarapproachedperiastronandtheBestardisc. firstentrance.FromFig.3onecaninferthatthefirstpre-periastron
FivedaysafterX7observationthefluxincreasedbyafactorof4. entrancefallsroughlybetweenthephases70◦ < θ < 110◦.Thus,
XMM-Newton observationsofPSR B1259–63near the2004periastronpassage 5
X9
X7
X3
SC
X9
SC
X7
X3
Figure4.TheX-ray(top)andTeV(bottom)fluxasafunctionoftherelative
phase∆θ=θ−θdisk(seetextforthedefinitionofθdisk).Thecurvesshowa
fitwithagaussianofthehalf-width∆θdisk=18.5◦.
Figure5.Folded(upperpanel)andunfolded(bottompanel)PSRB1259–
63spectrafromtheSC,X3,X7,andX9observations.TheXMM-Newton
spectraforX7andX9areforMOS1instrument.TheX3spectrumisfor
PNinstrument.
thepulsar hastoenter the diskagainbetween the phases 250◦ <
θ < 290◦.Surprisingly,onecanclearlyseefromthemiddlepanel
ofFig.3thattheTeVfluxgrowsinthisphaseinterval. 3.3 TimingAnalysis
Thegrowthrateof theX-rayfluxbetweenthephases70◦ <
θ < 110◦ is remarkably close to the decrease rate at the phases For each of the fivePN event filesin 0.2 – 10 keV energy range
290◦ < θ < 340◦. This enables us to make a conjecture that the witha 6 ms time resolution we searched for a possible pulsation
aroundthenominal 47.6msperiodusingtheepoch-folding tech-
observeddecreaseoftheX-rayfluxcanbeassociatedtotheexitof
nique(efsearch toolfromtheXRONOSpackage). Fromtheindi-
thepulsarfromthedisk.Totestthisconjecturewesuperimposethe
vidual observations the best 3−σ upper limit on pulsation at a
pre-periastronX-rayandTeVlightcurvesoverthepost-periastron
periodof47.6msis∼9%,compatiblewithprevioussimilaranaly-
lightcurvesbyshiftingthephaseofthepost-periastrondatapoints
by −180◦. For X-rayswe haveused only data fromthe rise(θ < sismadewithASCA(Kaspietal.1995;Hirayamaetal.1999).In
110◦)andthefall(θ > 290◦)periods.TheresultisshowninFig. ordertoperformadeepersearchforpulsationswecombinedneigh-
boringXMMobservations(namelyX7-X10,see.Table1).Asthe
4.Onecanseethatinsuchrepresentationtheriseanddecreaseof
timespan of these observations covers only small fraction of the
both X-rayand TeV fluxfromthesystem can bewell fittedwith
binary orbit wedidnot make corrections tothebinary motion of
agaussiancurveF(θ) ∼ exp −(θ−θ )2/(2∆θ2 ) .Wefindthat
the best fit is achieved witht(cid:16)he paramdisekter choidcieskθ(cid:17) ≃ 109.1◦, theneutronstar.Usageofthecombined XMMdataimprovesthe
∆θ ≃ 18.5◦ (thecoordinate∆θalongtheX-axisodfiskFig.4is,in upperlimitto∼2%.
disk Data of BeppoSAX/MECS (1.5-11 keV) observations also
fact∆θ=θ−θ ).
disk
weresearchedforpulsations.Fordeepersearchwecombinedob-
Themost obviousexplanation totheshape oftheX-rayand
servations S2-S5 (see Table 2). Due to small time period of per-
TeV lightcurves shown in Fig. 4 is that pulsar crosses the thick
formedobservationswedidnotmakecorrectionforbinarymotion
disk with the half opening angle ∆θ . The “middle” plane of
disk oftheneutronstar.Inspiteof considerably smallereffectivearea
the disk intersects the pulsar orbital plane along the line θ =
(109.1◦ 289.1◦).Theintersectionofthediskwiththeabovepa- ofMECSinstrumentsofBeppoSAXobservatory,theeffectiveex-
posuretimeofobservationsofthesourcewasmuchlarger,which
rametersSwith the pulsar orbital plane is shown schematically in
resulted in acomparable upper limit on X-ray pulsation fraction.
Fig. 1 by a shaded area. Because of the exponential profile, the
Wehavenotfoundpulsationswith3-σupperlimitat∼2%.
diskhasnofixedboundary. DensershadinginFig.1corresponds
totheoneopening angleofthedisk, whilelightershadingcorre-
spondstotwoopeningangles.Thediskisalsoshownbythesame
3.4 SpectralAnalysis
shading in the Fig. 3. From this figure one can see that although
thediskappearsquitesymmetricintermsoftheorbitalphaseθ,it The spectral analysis was done with NASA/GSFC XSPEC soft-
ishighlyasymmetricintermsofthetimeτwhichmeasuresdays ware package. In Fig. 5 the folded and unfolded spectra of
fromtheperiastronpassage.Forexample,duringthesecond(post- PSRB1259–63 forSC,X3,X6,andX9observationsareshown.
periastron) diskpassage, therisephase (250◦ < θ < 290◦)of X- A simple power law with a photoelectrical absorption de-
rayandTeVlightcurvestakesjustsome10days,whilethedecay scribesthedatawell,withnoevidenceforanylinefeatures.InTa-
(290◦ <θ<340◦)takesabout100days. ble3wepresenttheresultsofthethreeparameterfitstotheXMM-
6 M. Chernyakova et al.
-618 -40 -8.5 0 8.5 40 618
X5
S1
A1 A2
A3
A4
X10 X1
X6
SC
X7
Figure6.SpectralparametersofXMM-Newton,BeppoSAX,andASCAobservationsofthePSRB1259–63system.NotationsarethesameasinFig.3.Top
panelshowstheevolutionofthephotonindexΓasafunctionoftheorbitalphaseθ.BottompanelshowstheevolutionofthehydrogencolumndensityNH.
DenserandlightershadedregionshowphaseswithinoneandtwohalfopeninganglesoftheBestardisk.
Newtondata inthe 0.5 –10 keV energy range. The uncertainties thehydrogencolumndensityremainshighstartingfromabout20
aregiven atthe1σ statisticallevel anddonot includesystematic daysbeforetheperiastrontillatleast87daysafterperiastron.
uncertainties.Thegraphicalrepresentationoftheevolutionofthe
PSRB1259–63spectralparametersalongtheorbitisgiveninFig- Thebehaviourofthespectralslopeasafunctionoftheorbital
ure6whichincludesdatafromtheBeppoSAXandXMM-Newton phase θ found in XMM observations differs from the one found
observations, and those from ASCA (taken from Hirayamaetal. in ASCA observations. Whereas in 1994 – 1995 the spectrum at
(1999)). apastron was found to be significantly harder than during perias-
tron,XMM-Newtonobservationsshowmuchsofterspectrumatthe
InFigure7weshowcontourplotsofN versusphotonspec- apastronwiththespectralslopeclosetotheASCAperiastronvalue.
H
tralindexΓ,holdingthe1–10keVfluxfixedatthevaluesgiven
inTable3.Thecontoursplottedarethe68%,90%,and99%con- Themostremarkable featureofthespectral evolutionof the
fidencelevels.FortheX5observationthevaluesof N andΓare systemisthehardeningoftheX-rayspectrumclosetothemoment
H
consistentwithX4observation,andweomittedtheX4contoursto whenpulsarenterstheBestardiskatthephaseθ≃θdisk−2∆θdisk ≃
keeptheFigureclear. 70◦.OnecanseethatthedecreaseofthephotonindexΓissimul-
taneous with the onset of the rapid growth of the X-ray flux. To
Onecanseethat thevariation of N asafunction oforbital thebestofourknowledge,suchbehaviourwasnotpredictedinany
H
phase is correlated with the Be star disk crossing. No variations ofexistingmodelsofX-rayemissionfromthesystem.Moreover,
of N are observed for the XMM-Newton observations X2 to X7 the observed values Γ < 1.5 during the observations X6 – X10
H
whichallfalloutsidethedisk(assumingthediskparametersfound aredifficulttoreconcilewiththemodelsofsynchrotronorinverse
inSection3.2).ThemeanvalueofofN duringtheseobservations Comptonemissionfromshock-acceleratedelectronsbecauseofthe
H
isN (X2−X7)∼0.32×1022cm−2.However,theobservationsX8, toohardelectronspectrumimplied.Possibleerrorindetermination
H
X9,X10andX1arecharacterizedbyabsorptionwhichishigherby ofΓduetotheuncertaintyindeterminationof N (seeFig.7)is
H
afactorofabout1.5,N (X8−X10,X1)∼0.45×1022cm−2.Thus, toosmalltoallowforΓ>1.5atleastforobservationsX7–X9.
H
XMM-Newton observationsofPSR B1259–63near the2004periastronpassage 7
X1X08 XX810
X1 X9
XX54
X6
X2 X7
X4
X3 X5
Figure7.ConfidencecontourplotsofthecolumndensityNHandofthephotonspectralindexΓuncertaintiesforapower-lawfittoXMM-Newtonobservations.
Thecontoursgive68%,90%,and99%confidencelevels.X4observation,markedwithtriangle,isverysimilartotheX5observationandthecontoursarenot
plottedhere.
throughthediskiscausedbythemacroscopical mixingofpulsar
Table3.Spectral parameters forBeppoSAXandXMM-Newton Observa-
tionsofPSRB1259–63.∗ windwiththestellarwind.
MainpredictionofthismodelisthecorrelationofX-raywith
Data F(1-10keV) Γ NH χ2(dof) the radio lightcurves of the system. Although different quality of
Set 10−12ergs−1 (1022cm−2) X-rayandradiolightcurvesdoesnotallowtomakedefinitivecon-
clusions about radio-X-ray correlations, our XMM-Newtonobser-
S1 1.36+0.26 1.88+0.73 0.47+1.48 0.814(201)
SC 6.31+−00..6493 1.39+−00..0262 0.65+−00..1467 0.898(813) vations show that the onset of rapid increase of the X-ray fluxis
−0.80 −0.08 −0.20 simultaneouswithappearanceoftheunpulsedradioemissionfrom
X1 8.51(30) 1.51(3) 0.45(2) 0.97(341)
X2 2.81(23) 1.36(7) 0.39(4) 1.02(122) thesystem(seeFig3).
X3 1.04(49) 1.69(4) 0.29(2) 0.88(207) TheX-rayinverseComptonemissionintheCImodelispro-
X4 1.82(14) 1.82(7) 0.36(3) 1.01(109) ducedbyelectronswithrathermoderategamma-factors,γe,IC ∼10
X5 0.99(9) 1.80(8) 0.32(4) 0.80(62) while the gamma-factors of radio-synchrotron-emitting electrons
X6 2.47(17) 1.41(6) 0.35(3) 0.96(151) arearound γ ∼ 102.Thesynchrotron/inverse Comptoncooling
e,S
X7 4.05(23) 1.20(4) 0.28(3) 0.95(154) timesforsuchelectronsare
X8 12.59(34) 1.31(2) 0.47(1) 1.12(614)
X9 24.27(56) 1.39(2) 0.43(1) 1.07(737) t ≃ 108 0.1G 3/2 10GHz 1/2 s
X10 16.79(37) 1.47(2) 0.45(1) 0.88(772) S " B # " ν #
1erg/cm3 1keV 1/2
∗Numberinparenthesesrepresent68%confidenceintervaluncertaintiesin t ≃ 106 s (1)
IC " U #" ǫ #
thelastdigitquoted. r IC
Botharelargecomparedtothetypicaladiabaticcooling(orescape)
timet ∼R/v∼103s(Risthecharacteristicsizeoftheemission
esc
region,andvisthevelocityoftheflowalongthecontactsurface).
4 DISCUSSION
Anotherimportanttimescalefortheelectronsintheenergyrange
InalltheoreticalmodelsofunpulsedemissionfromPSRB1259–63 relevantintheCImodelistheCoulomblossscale
thecollisionofrelativisticpulsarwindwithnon-relativisticwindof 109cm3 γ
t ≃104 e s (2)
theBestarplaysacrucialrole.Duetotheinteractionofthewindsa coulomb " n #(cid:20)10(cid:21)
systemoftwoshockwavesformsbetweenthestars.ClosetotheBe
As the pulsar enters the disk, the density of the stellar wind in-
starandtothepulsarbothwindsaresupposedtoberadialbutafter
creases.Typicalestimatesofthewinddenistyatthepulsarlocation
passingtheshocksparticlesturnandstarttoflowalongthecontact
(assumingtheradialprofilen∼(R /R)2)resultinthedensityesti-
surface,losingtheirenergyviasynchrotron andinverseCompton ⋆
maten∼109−1010 cm−3.Thus,insidethediskCoulombcooling
(withtheseedphotonsbeingtheBestarsoftphotons)emission.
timeiscomparabletotheescapetime.
Inthemodel ofChernyakova&Illarionov(1999,2000)(CI)
This effect leads to the modification of electron spectrum at
theobservedX-rayemissionisduetoinverseComptonscattering
theenergiesbelowtheso-called“Coulombbreak”energyatwhich
of soft photons fromtheBestar on thesameelectron populaiton
t =t :
thatproduce thenon-pulsed radioemissionfromthesystem.The coulomb esc
n t
intensityofobservedunpulsedX-rayandradioemissiondepends γ ≃1 esc (3)
stronglyontheflowvelocityoftherelativisticparticlesbeyondthe e,coulomb (cid:20)109cm−3(cid:21)(cid:20)103s(cid:21)
shock.Slowerdriftvelocityleadstothehigherconcentrationofthe Namely,sincetheCoulomblossratedE/dt∼γ /t isenergy-
e coulomb
relativisticelectronsbeyondtheshockandhigherX-rayandradio independent, the electron acceleration spectrum with the spectral
luminosity.Decreaseofthedriftvelocityduringthepulsarpassage indexΓ >2hardensto(Γ −1)<2belowthebreakenergy.This
e e
8 M. Chernyakova et al.
of the pulsar wind. Typical gamma-factors of the X-ray emitting
electronsareγ∼106.ModulationoftheX-rayfluxfromthesource
duringthediskandperiastronpassageisduetothecombinedeffect
ofdisplacementofthecontactsurfacebythedisk(whichleadsto
thevariationofthemagneticfieldstrength)andtheincreaseofthe
inverseComptonenergylossclosetotheperiastron.
OneofthemainpredictionsofTAmodelissteepeningofthe
X-rayspectrumduringthediskandperiastronpassage.Theeffect
is due to the fact that synchrotron and/or inverse Compton cool-
ingtimeofX-rayemittingelectronbecomescomparableorshorter
thanthetimeneededtoescapefromtheemissionregion.However,
theobserved behaviour of thespectrum isquiteopposite: theen-
trancetothediscisaccompaniedbythehardeningofthespectrum.
ThevaluesofthephotonindexΓ<1.5foundintheobserva-
tionsX6–X10aretoohardtobeexplainedwiththesynchrotron
emissionfromshock-acceleratedelectronswhichnormallyresultin
theelectronspectrawithpower-lawindexΓ >2and,correspond-
e
inglyinsynchrotronspectrawithphotonindexΓ>1.5.Ofcourse,
overalargefractionofthepulsarorbitthesynchrotroncoolingtime
islargerthanthetimeneededtoescapefromtheemissionregion
along the contact surface. Thus, if one assumes that the produc-
tion spectrum of high-energy electrons responsible for the X-ray
emissionisharder thanΓ = 2,(if,e.g.electronsoriginateinthe
Figure 8. Modification of the broad-band spectra of CI (a) and TA (b) e
coldpulsarwindwiththebulkLorentzfactorγ∼106),itispossi-
models due to the bremsstrahlung emission from shocked stellar/pulsar
ble,inprinciple,toobtainspectrawhichareharderthanΓ = 1.5.
wind electrons during the disk passage. Contributions to the total spec-
trum from synchrotron, inverse Compton and bremsstrahlung processes However,inthiscaseoneexpectstoobserve theveryhardX-ray
are marked with S, IC and BS, respectively. In the case of TA model spectrumwithΓ<1.5overmostofthepulsarorbit.
blacksolidcurves showthespectrum ofemissionfromtheshockedpul- Within the TA model radio synchrotron emission originates
sar wind while the green dashed curves show the contribution from the fromdifferentpopulation ofelectronsthantheX-raysynchrotron
shocked stellar wind. Parameters used for the CI model calculation are emission (radio is convensionally attributed to shock-accelerated
n = 2×109cm−3, B = 0.01G(low)and B = 0.1G(highsynchrotron stellar wind electrons, while X-ray flux is emitted by the pulsar
flux),dNe/dγ ∼γ−2.4exp(−γ/400),withsuchparametersCoulombbreak windelectrons).Althoughitisnotpossibletoexplaintheharden-
is at γ ∼ 50. The same parameters are used for the calculation of the
ingoftheX-rayspectralindexbythechanges inthepulsarwind
shocked stellar wind emission (green dashed curves) in the lower panel.
electronspectrum,onecantrytoascribetheobservedhardeningto
Theemissionfromtheshockedpulsarwindwascalculatedassumingn =
2×1011cm−3,B = 0.15G(low)andB = 1.5G(highsynchrotronflux), thecontributionfromtheshockedstellarwindelectrons.TheX-ray
dNe/dγ∼γ−2.3exp(−γ/4×106).WithsuchparametersCoulombbreakis spectrumwithphotonindexharderthanΓ= 1.5isnaturalifthere
atγ∼500. isabremsstrahlungcontributiontotheX-rayflux.Theincreaseof
thedensityofstellarwindinsidethediskcanleadtotheincrease
ofbremsstrahlungluminosity.
naturallyleadstothehardeningofthespectrumofinverseCompton Theestimateofthebremsstrahlunglosstime
X-rayemissionatthemomentwhenthepulsarentersthedisk,as
1010cm−3
itisshowninFig.8a.OnecannotethattheCoulombbreakenergy t =105 s (4)
brems " n #
(3)isclosetotheenergyoftheX-rayemittingphotons.Thismeans
thattheX-rayphotonindexisverysensitivetotheslightchangesof shows that for radio emitting electrons it is orders of magnitude
thedensityand/oroftheoutflowvelocity(seeFig.8ainwhichthe shorter than the synchrotron cooling time (see (1)). Thus, the
twospectraarecalculatedforγ = 10andγ = 50). bremsstrahlung luminosity should be much higher than the radio
e,coulomb e,coulomb
Sincebothvaryduringthediskpassage(itisassumedthatwithin synchrotronluminosity.FromFig.8onecanseethatasatisfactory
thediskpulsarandstellarwindsaremacroscopically mixed),nu- fittothebroad-bandspectrumcanbeobtainedassumingthedisk
mericalmodelling isrequired topredict thetimeevolutionof the densityn∼1011cm−3.Itisinterestingtonotethatbremsstrahlung
X-ray spectral index. Weleave the detailed modelling of this for canalsoexplainthebehaviouroftheTeVlightcurveinthiscase,if
futurework(Chernyakova&Neronov2006). oneassumesthepossibilityofmixingofthepulsarwindelectrons
IntheoriginalCImodel,onlyelectornswithtypicalgamma- withthepulsardiskmedium(similartotheoneassumedintheCI
factors of 10-100 are considered. The presence of protons with model).
comparable gamma-factors γ ∼ 1− 103 at the contact surface The bremsstrahlung model faces, however, a serious over-
p
wouldleadtothegenerationofTeVemissionduringthediskpas- all energetics problem. From Fig. 8b one can see that the total
sagewhichisduetotheproton-protoninteractionsinthedisk(see bremsstrahlungluminosityisatleast2ordersofmagnitudehigher
Kawachietal.(2004)).Macroscopicmixingofthepulsarandstel- thantheX-rayluminosityatthemomentofX7observation,L ≃
X7
larwindwillleadtotheincreaseoftheproton-proton interaction 1033erg/s(seeFig.3).Asitisdiscussedabove,theenergylossrate
rateinthedisk,whichcanexplaintheformoftheTeVlightcurve of mildlyrelativisticelectronsresponsible for thebremsstrahlung
ofthesystem. emission is dominated by the Coulomb losses. Comparing (4) to
InthemodelofTavani&Arons(1997)(TA)theX-rayemis- (2)onecanseethattheCoulomlossrateisstill2-3ordersofmag-
sionisthesynchrotronemissionfromshock-acceleratedelectrons nitudehigherthanthebremsstrahlungluminosity.Thus,toobtain
XMM-Newton observationsofPSR B1259–63near the2004periastronpassage 9
theX-raybremsstrahlungfluxatthelevelof1033 erg/sonehasto ChernyakovaM.,IllarionovA.,1999,MNRAS,304,359
assumethatthepowerinjectedintothe10−100MeVelectronsis ChernyakovaM.,IllarionovA.,2000,ApSS,274,177
about1038erg/swhichismuchhigherthanthespin-downluminos- Chernyakova M., Lutovinov A., Rodriguez J., Revnivtsev M.,
ityofthepulsarL ≃1036erg/s. 2005,MNRAS,364,455
pulsar
ChernyakovaM.,NeronovA.,2006,inpreparation
CominskyL.,RobertsM.,JohnstonS.,1994,ApJ,427,978
5 CONCLUSIONS Connors T.W., Johnston S., Manchester R. N., McConnell D.,
2002,MNRAS,336,1201
InthispaperwehavepresentedtheXMM-Newtonobservationsof GroveJ.E.,TavaniM.,PurcellW.R.,etal.,1995,ApJ,447,L113
thepulsarPSRB1259-63 duringits2004periastronpassagenear Hirayama M., Cominsky L. R., Kaspi V. M., Nagase F., Tavani
thecompanionBestarSS2883.Combiningourobservationswith M.,KawaiN.,GroveJ.E.,1999,ApJ,521,718
the previous X-ray observations from XMM-Newton, BeppoSAX JohnstonS.,Manchester R.N.,LyneA.,BailesM.,KaspiV.M.,
andASCAweproducedthedetailedlightcurveofthesystemover QiaoGuojun,D’AmicoN.,1992,ApJ,387,L37
thewiderangeoftheorbitalphases. Johnston S., Manchester R. N., Lyne A. G., Nicastro L., Spy-
UsingthelargeX-raydatasamplewewereabletoconstrain romilioJ.,1994,MNRAS,268,430
thegeometryoftheBestardisk.Inparticular,wehavefoundthat Johnston S. Manchester R.N., McConnell D., Campbell-Wilson
thehalf-openingangleofthedisk(projectedonthepulsarorbital D.,1999,MNRAS,302,277
plane)is∆θdisk ≃18.5◦ andthatthelineofintersectionofthedisk JohnstonS.,BallL.,WangN.,ManchesterR.N.,2005,MNRAS,
withthepulsarorbitalplaneisinclinedatabout70◦withrespectto 358,1069
themajoraxisofthepulsarorbit. Kaspi V. M., Tavani M., Nagase F., Hirayama M., Hoshino M.,
Inspection of the X-ray and TeV lightcurves of the system AokiT.,KawaiN.,AronsJ.,1995,ApJ,453,424
hasrevealedcorrelationoftheX-rayandTeVfluxvariations.Our KawachiA.,NaitoT.,PattersonJ.R.,2004,ApJ,607,949
analysis shows that the variations of the TeV flux some 10-100 Kirk,J.G.,Skjæraasen,O.,Gallant,Y.A,2002,A&A,388,L29
daysaftertheperiastronpassagecanbewellfitwiththegaussian McClure-GriffithsN.M.,Johnston S.,StinebringD.R.,Nicastro
curve whose parameters coincide withtheparameters of thedisk L.,1998,ApJ,492L
foundfromtheX-rayanalysis(centeratθdisk =289.1◦,half-width ShawS.,ChernyakovaM.,RodriguezJ.,WalterR.,KretschmarP.,
∆θ = 18.5◦)(seeFig.4).If theobserved behaviour of thefluxis MereghettiS.,2004,A&A,426,L33
indeedduetotheinfluenceofBestardisk,itcannotbeexplained TavaniM.,AronsJ.,1997,ApJ,477,439
withintheinverseComptonscatteringmodelofTeVemissionfrom TaylorJ.H.,ManchesterR.N.,LyneA.G.,1993,ApJS,88,529
thesystem. WangN.,JohnstonS.,ManchesterR.N.,2004MNRAS,351,599
Our XMM-Newtonobservations whichweredone duringthe Waters L.B.F.M., van den Heuvel E. P. J., Taylor, A. R., et al.,
period of first pulsar entrance to the disk (some 40 to 20 days 1988,A&A,198,200
before the periastron) revealed quite unexpected evolution of the WexN.,JohnstonS.,ManchesterR.N.,LyneA.G.,StappersB.
spectrumofX-rayemission.Inparticular,theX-rayspectralslope W.,BailesM.,1998,MNRAS298,997
hardened from Γ ≃ 1.8 down to Γ ≃ 1.2 roughly at the onset of
therapidgrowth of theX-rayluminosity. Thisbehaviour cannot
beexplainedwithinthesynchrotronmodelofX-rayemissionfrom
thesystem.Wehaveshownthattheobservedhardeningcanbethe
result of the hardening of electron spectrum due to the Coulomb
lossesinthediskinthemodelwhereX-rayemissionistheinverse
Comptonemissionfromthepopulationofelectronsresponsiblefor
the radio synchrotron emission. Otherwise, the hardening can be
causedbytheincreaseofthebremsstrahlungemissionfromthese
electrons.However,thebremsstrahlungproblemfacesaseriousen-
ergybalanceproblem.
6 ACKNOWLEDGMENTS
TheauthorsacknowledgeusefuldiscussionswithF.Aharonianand
V.Beskin.AuthorsaregratefultoV.Kaspiforhelpfulcomments.
We are grateful to L. Foschini for helpful advices on the XMM-
Newton data analysis and thank M. Revnivtsev for the help with
the BeppoSAX data reduction. MC thank M. Tu¨rler for the help
withfigureproduction.ALacknowledgesthesupportofRFFIgrant
04−02−17276.
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