Table Of ContentAstronomy&Astrophysicsmanuscriptno.6568 February5,2008
(DOI:willbeinsertedbyhandlater)
XMM-Newton Observations of the Black Widow Pulsar PSR
B1957+20
7
0 Hsiu-HuiHuangandWernerBecker
0
2
Max-Planck-Institutfu¨rextraterrestrischePhysik,Giessenbachstrasse,85748Garching,Germany
n
a
ReceivedOctober13,2006;acceptedJanuary9,2007
J
4
Abstract.WereportonXMM-Newtonobservationsofthe“BlackWidowpulsar”,PSRB1957+20.Thepulsar’sX-rayemis-
2
sionisnon-thermalandbestmodeledwithasinglepowerlawspectrumofphotonindex2.03+0.51.NocoherentX-raypulsations
0.36
2 atthepulsar’sspin-periodcouldbedetected,though astrongbinary-phase dependence of−theX-rayfluxisobserved forthe
v firsttime.Thedatasuggestthatthemajorityofthepulsar’sX-radiationisemittedfromasmallpartofthebinaryorbitonly.We
1 identifiedthispartasbeingneartowheretheradioeclipsetakesplace.ThiscouldmeanthattheX-raysfromPSRB1957+20
1 aremostlyduetointra-shockemissionwhichisstrongest whenthepulsarwindinteractswiththeablatedmaterialfromthe
6 companionstar.
1
0 Keywords.pulsars:individualPSRB1957+20—stars:neutron—X-rays:stars—binaries:eclipsing
7
0
/
h 1. Introduction blackwidowpulsar.Interestingly,theradioemissionfromthe
p
pulsariseclipsedforapproximately10%ofeachorbitbymate-
- Untilnow,morethan1700rotation-poweredradiopulsarsare
o rialexpelledfromthewhitedwarfcompanion.Foraradiodis-
detected. Among them are about 10% which are millisecond
r persionmeasureinferreddistanceof1.5kpc(Taylor&Cordes
t pulsars(MSPs)(Manchesteretal.2005).Theyformaseparate
s 1993)thepulsarmovesthroughtheskywith asupersonicve-
a population.ThemajorityofthemresidesinGlobularClusters
locityof220km/sec.Theinteractionofarelativisticwindflow-
:
v (c.f.Bogdanovetal. 2006).MSPsare presumedto havebeen ing away from the pulsar with the interstellar medium (ISM)
i spunupinapastaccretionphasebymassandangulartransfer
X producesanH bowshockwhichwasthefirstoneseenaround
froma binarycompanion(Alpar etal. 1982).Only aboutone α
a“recycled”pulsar(Kulkarni&Hester1988).
r
thirdofthemareseentobesolitary.Itisbelievedthattheylost
a
their companion,e.g. in a violent supernovaevent. All MSPs In 1992 Kulkarni et al. published a contour map of the
possessveryshortspinperiodsoflessthan20msandshowa X-ray emission of PSR B1957+20 which was derived from
highspinstabilitywithperiodderivativesintherange 10 18 ROSAT PSPC observations. Although this ROSAT data were
−
10 21. MSPs are generally very old neutron stars w≈ith spin−- verysparseinstatisticsitledtheauthorstopredictfaintdiffuse
−
down ages τ = P/2P˙ of 109 1010 years and low surface X-rayemission with constantsurfacebrightnessto be present
magneticfieldsintherang∼eB −p(PP˙) 108 1010G. along a cylindrical trail formed when the relativistic pulsar
∝ ∼ − wind expands into pressure equilibrium with the interstellar
At present, about 50% of all X-ray detected rotation-
mediumbehindthenebula.Themuchimprovedsensitivity of
powered radio pulsars are MSPs (c.f. Bogdanov et al. 2006
the Chandra and XMM-Newton observatories made it possi-
and references therein). Among them an extraordinarily rich
bletoprobeandinvestigatethestructureandpropertiesofthis
astrophysicsbinarysystemwhichisformedbythemillisecond
uniquebinarysysteminmuchhigherdetailthanitwaspossible
pulsarPSRB1957+20andits0.025M lowmasswhitedwarf
⊙ withROSAT,ASCA orBeppoSAX.AnarrowX-raytailwith
companion(Fruchter,Stinebring,& Taylor 1988).The binary
theextentof16arcsecandtheorientationtothenorth-eastwas
periodofthesystemis9.16hours.Thespinperiodofthepulsar
detected from it in deep Chandra observationsby Stappers et
is 1.6 ms which is the third shortest among all knownMSPs.
Itsperiodderivativeof P˙ = 1.69 10 20 s s 1 impliesaspin- al.(2003).
− −
down energyof E˙ = 1035 erg s ×1, a characteristic spin-down SearchinginROSATdataforamodulationofthepulsar’s
−
age of > 2 109 years, and a dipole surface magnetic field X-ray emission as a function of its orbital phase revealed a
×
of B = 1.4 108 Gauss.OpticalobservationsbyFruchteret suggestive but insignificant increases in flux before (at phase
⊥ ×
al.(1988)andvanParadijsetal.(1988)revealedthatthepulsar φ 0.17) and after (at phase φ 0.4 0.5) the pulsar radio
∼ ∼ −
wind consisting of electromagneticradiation and high-energy eclipse(φ= 0.25)(Kulkarnietal.1992).TakingChandradata
particles is ablating and evaporating its white dwarf compan- intoaccountrevealeda hintthatthelowestandhighestfluxes
ionstar.Thisrarelyobservedpropertygavethepulsarthename arelocatedduringandimmediatelyaftertheradioeclipse,re-
2 Hsiu-HuiHuangandWernerBecker:X-rayobservationofPSRB1957+20
spectively. The statistical significance of this modulation ob-
servedbyChandra,though,isonly98%andthuspreventsany
finalconclusiononit(Stappersetal.2003).
In this paper we reporton XMM-Newton observations of
PSR B1957+20and its white dwarf companion.The paper is
organizedas follows. In 2 we describe the observations and
§
dataanalysis. 3givesasummaryanddiscussion.
§
2. ObservationsandDataAnalysis
PSRB1957+20wasobservedwithXMM-NewtononOctober
31,2004fora 30kseceffectiveexposure.Inthisobservation,
theEPIC-MOS1andMOS2instrumentswereoperatedinfull-
frame mode using the thin filter to block optical stray light.
TheEPIC-PN detectorwas setup to operatein the fast timing
mode.Becauseofthereducedspatialinformationprovidedby Fig.1. XMM-Newton MOS1/2 image of the PSR B1957+20
the PN in timing mode we use the MOS1/2 data for imaging -WD system with contour lines overlaid. The contour
and spectral analysis of the pulsar and its diffuse X-ray neb- lines are at the levels of (5.1,6.1,9.4,16.3,32.4,69.0)
×
ulawhilethePNdatahavingatemporalresolutionof0.02956 10 6ctss 1arcsec 2.Thepositionofthepulsarisindicated.
− − −
ms allowed us to search for X-ray pulsationsfrom the pulsar.
All the data were processed with the XMM-Newton Science
AnalysisSoftware(SAS)package(Version6.5.0).Spatialand
spectralanalyseswererestrictedtothe0.3 10.0keVenergy
−
bandwhilefortiminganalysiseventswereselectedfortheen-
ergyrange0.3 3.0keV. neartothepulsarposition.Responsefileswerederivedbyus-
−
ingtheXMM-NewtonSAStasksRMFGENandARFGEN.
2.1.SpatialAnalysis Aftersubtractingbackgroundphotons,intotal338sources
counts were available for a spectral analysis. The extracted
Figure1showsthecombinedEPIC-MOS1/2imageofthePSR
spectra were binned with at least 30 source counts per bin.
B1957+20system.Theimagewascreatedwithabinningfac-
Assuming thatthe emission originatesfrom the interactionof
tor of6 arcsec andby usingan adaptivesmoothingalgorithm
the pulsar wind with the ISM or with the stellar wind we ex-
with a Gaussian kernel of σ < 4 pixels in order to better
pectsynchrotronradiationtobetheemissionmechanismofthe
make visible faint diffuse emission. The extent of the diffuse
detectedX-rays.Totestthishypothesiswefittedthespectrum
emissionwithits orientationtothe north-eastis about16arc-
witha powerlawmodel.Indeed,thismodeldescribestheob-
sec which is consistent with the previous result derived from
servedspectrumwithareducedχ2 of1.09(for8D.O.F.).The
the Chandra Observations. However, the detailed structure of photon-indexisfoundtobeα=2.ν03+0.51.Thecolumnabsorp-
the X-ray emission from XMM-Newtoncan not be as clearly 0.36
tionN is8.0 1020 cm 2.Forthen−ormalizationat1keVwe
H −
seen asfromChandradueto the 10timeswiderPointSpread find1.5+0.9 10×5photonskeV 1cm 2sec 1(1-σconfidencefor
Function(PSF)ofXMM-Newton. 0.3× − − − −
1param−eterofinterest).Thespectrum(dataandmodel)andthe
InspectingtheXMM-NewtonMOS1/2imagetwofaintfea-
fitresidualsareshowninFigure2.
tures (denoted as A and B) which contribute to only about
3% of the total X-ray flux are apparent. In order to investi- The unabsorbed X-ray fluxed derived from the best fit-
gatewhetherthisfaintfeaturesareassociatedwithnearbystars ting model parameters is fx = 8.35 10−14 erg s−1 cm−2
×
we inspected the Digitized Sky Survey data (DSS) and the and fx = 7.87 10−14 erg s−1 cm−2 in the 0.3 10 keV and
× −
USNO-B1.0Catalogueforpossiblesources.Thesecatalogues 0.1 2.4keV energyband,respectively.The X-rayluminosi-
−
whicharelimiteddownto22mag(Krongold,Dultzin-Hacyan tiesintheseenergybands–calculatedforapulsardistanceof
& Marziani2001)and 21 mag (Monetet al. 2003)do not re- 1.5 kpc – are Lx(0.3 10.0keV) = 2.24 1031 erg s−1 and
− ×
veal possible counterparts. These features are not seen in the Lx(0.1 2.4keV)=2.12 1031ergs−1,respectively.Thecon-
Chandraimagethough(Stappersetal.2003). version−efficiency Lx/E˙ i×n the 0.1 2.4 keV bandis foundto
−
be 2.12 10 4.
−
∼ ×
Inordertocheckwhetherthespectralemissioncharacter-
2.2.SpectralAnalysis
isticschangesforphotonsdetectedinasmallercompactregion
Combined EPIC-MOS1/2 data of PSR B1957+20 were ex- of10arcsecradiusatthepulsarpositionweappliedaspectral
tractedfromacircleof30arcsecradiuscenteredatthepulsar fit to this events only. The encircled energy within 10 arcsec
position (RA (J2000)= 19h59m36s.77,Dec = 20 4815”.12). is 60 %. In total, 186 sourcescounts were available for spec-
◦ ′
Theselectionregioncontainsabout85%ofallsourcecounts. tralfits. We didnotfindanysignificantchangein thespectral
Background photons were selected from a source-free region parametersthanreportedabove.
Hsiu-HuiHuangandWernerBecker:X-rayobservationofPSRB1957+20 3
limitof9%(1-σ)isdeducedbyassumingasinusoidalpulsed
profile.
Arons & Tavani (1993) predicted that depending on the
flow speed and the degree of absorption and/or scattering by
thecompanionwindtheX-rayemissionfromPSRB1957+20
increasesbyuptoafactorof2.2attheorbitalphasebeforeand
after the radio eclipse. In order to test this predictionwe cre-
atedalightcurvebybinningalleventsinbinsof1.5ksecwidth.
Withaneffectiveexposuretimeofabout30ksec andanorbit
periodof9.16-htheXMM-NewtonMOS1/2andPNdatacover
roughly83%and92%ofonebinaryorbit,respectively.Table
1liststhefirstandthelastphotonarrivaltimesrecordedbythe
MOS1/2andEPIC-PNdetectorsandthecorrespondingpulsar
binaryorbitalphase.ThelightcurvesresultingfromtheEPIC-
PNandMOS1/2dataareshowninFigure3.Forthelightcurve
Fig.2.Energyspectrumof PSR B1957+20obtainedfromthe deducedfromthePN-data(uppersolidcurve)itisclearlyseen
XMM-NewtonMOS1/2 data.The plotshowsthe X-rayspec- thatbeforetheradioeclipse,i.e.betweentheorbitalphase0.1
trum fitted with an absorbed power law model (upper panel) and 0.25, the X-ray emission increases. The emission in the
andcontributiontotheχ2fitstatistic(lowerpanel). highestbinisaboutafactorof3.0strongerthanatotherorbital
phaseanglesbeforeandaftertheradioeclipse.Thisisinagree-
mentwiththepredictionsmadebyArons&Tavani(1993).
Although it is the first time that a significant X-ray flux
2.3.TimingAnalysis modulationfromPSRB1957+20isobserved,thefluxincrease
nearthephaseoftheradioeclipseisvirtuallyonlyseeninthe
The EPIC-PN camera observed the pulsar in the fast timing
PNdata.Owingtotheshortobservationtimeof30ksec,which
mode.Inthismodethespatialandspectralinformationfroma
is less than the time of one full binary orbit, the orbit angles
64 199CCDpixelarrayiscondensedintoaonedimensional
× 0.30 0.38and0.18 0.36arenotcoveredbyEPIC-PN and
64 1 pixel array (1D-image), i.e. the spatial information in − −
× MOS1/2dataatall.Itthusturnsoutthatitwasagreatfortune
Y-directionislostduetothecontinuousread-outoftheCCD.
thattheEPIC-PN,moreorlessbychance,coveredtheorbital
Thecompletephotonflux(sourceplusDCemissionfromfore-
phase rangeof the radioeclipse and providedusevidencefor
ground or background sources located along the read-out di-
thefluxenhancementwhiletheMOS1/2detectorswerealready
rection) is accumulated and collapsed in the final 1D-image,
switchedoff.
severely reducingthe signal-to-noiseratio of pulsed emission
andpreventingthedetectionofweakX-raypulsationsfromthe
targetofinterest. 3. SummaryandDiscussion
In order to search for X-ray pulsations from PSR
Interactionbetweenrelativisticpulsarwindswhichcarryaway
B1957+20 we extracted 1639 counts from the CCD columns
therotationalenergyofpulsarsandthesurroundingmediumis
33 41 in which the pulsar got located. In order to increase
expectedtocreatedetectableX-rayemission.Indeed,thereare
−
thesignal-to-noiseratiowerestrictedtheanalysistotheenergy
about30pulsarwindnebulae(PWNe)currentlydetectedinthe
range 0.3 3.0 keV. Below and beyond this energy band the
X-rayband(e.g.Kaspi,Roberts&Harding2006,Gaensler&
−
accumulativeskyandinstrumentbackgroundnoiseexceedsthe
Slane2006,Kargaltsev&Pavlov2006).However,thesePWNe
contributionfromthe pulsaritself (c.f.Becker & Aschenbach
areallpoweredbyyoungandpowerfulpulsarswithspin-down
2002). However, still 80 % of the counts are estimated to be energies of more than 3.6 1036 erg s 1. Until now, only
−
derivedfromthebackground. ∼ ×
two MSPs are known to have X-ray nebulae. They are PSR
The photon arrival times were corrected to the solar sys- B1957+20 (Stappers et al. 2003) and PSR J2124-3358 (Hui
tembarycentrewiththeBARYCENtool(version:1.17.3,JPL &Becker2006).Bothofthemhavetail-likestructuresbehind
DE200Earthephemeris)oftheSASpackage.Asthepulsaris the moving pulsars. These trails could be associated with the
inabinarywecorrectedfortheorbitalmotionofthepulsarby shocked relativistic wind confined by the ram pressure of the
usingthemethodofBlandford&Teukolsky(1976). ambientISM.
As millisecond pulsars are known to be extremely stable TheXMM-NewtondataofPSRB1957+20haveprovided
clocksweusedthepulsarephemerisfromtheATNFCatalogue, observationalevidenceforastrongdependenceofthepulsar’s
f =622.122030511927 Hzand f˙= 6.5221 10 15sec 2(at X-ray emission on its binary orbital phase. It is the first time
− −
− ×
MJD=48196.0)toperformaperiodfolding.ByusingtheZ2 thata significantX-rayflux modulationfromPSR B1957+20
n
statistics(Buccherietal.1983)withtheharmonicsnumber(n) is observed. The emission near to the radio eclipse is sup-
fromonetotennosignificantsignalwasdetectedattheradio posed to be beamed in a forward cone because the shocked
spin period extrapolated for the epoch of the XMM-Newton fluidisacceleratedbythepressuregradientasitflowsaround
observation.Restrictingtheperiodsearchtothevarioussmaller theeclipseregion.Relativisticbeamingwouldtendtogivethe
energybandsdidnotchangetheresult.Apulsedfractionupper maximum flux just before and after the pulsar eclipse which
4 Hsiu-HuiHuangandWernerBecker:X-rayobservationofPSRB1957+20
Table1.ListofthefirstandthelastphotonarrivaltimesanditscorrespondingorbitalphaseofPSRB1957+20fortheMOS1/2
and PN detectors. Arrival times are corrected to the solar system barycentre. The orbital phase is measured from the time of
ascendingnode.
Dateset 1stphoton lastphoton Duration(s)
Time(MJD) OrbitalPhase Time(MJD) OrbitalPhase
MOS1 53309.9688 0.3605 53310.2830 0.1830 27143.4
MOS2 53309.9666 0.3548 53310.2846 0.1871 27467.1
PN 53309.9791 0.3873 53310.3301 0.3063 30327.4
MJD sion. Such analysis would allow us to investigate with higher
53309.95 2h 4h 6h 8h 9.16h
250 accuracy than currently possible whether the X-ray emission
PN from PSR B1957+20is presentduringall orbitanglesor vir-
MOS1/2
tuallyonlyneartotheradioeclipsewhilediffuseX-rayemis-
200
sionfromthePWNispresentatanyorbitalangle.Indeed,the
laterissuggestedbythecurrentdataandaconfirmationwould
150 haveasevereimpactonourunderstandingofthepulsar’sX-ray
Counts/bin emiAsssiotnheprporpeseertnitesX.MMobservationcoversbarelyonebinary
100
orbit,westressthatitcannotbefullyexcludedthattheincrease
inphotonfluxneartheorbitalangleoftheradioeclipseisdue
50 to a single “burst like” event or that the peak flux emission
varies from orbit to orbit or on longer time scales. Clearly, a
repeatedcoverageofthebinaryorbitinalongerXMM-Newton
0
0.32 0.4 0.6 0.8 1.0 0.2 0.32 observationandacomparisonwiththe2004datawouldanswer
Orbital Phase
thisquestionimmediately.This,inaddition,wouldprovideus
Fig.3. X-ray emission from PSR B1957+20 within 0.3 3.0 not only a better photon statistic but would also allow us to
−
keV as function of the pulsar’s orbitalphase (φ). We mapped determinetheemissiongeometrywithamuchhigheraccuracy
onecompleteorbitalperiodofthissystematthestartingpoint thancurrentlypossible.
of MJD = 53309.95,i.e.φ = 0.32.φ = 1.0correspondstothe
ascending node of the pulsar orbit. The upper curve was ob- Acknowledgements. Thiswork made use of the XMM-Newton data
tainedfromtheXMM-NewtonEPIC-PN(backgroundlevelat archive.Thefirstauthoracknowledgestherecipeoffundingprovided
66cts/bin).ThelowerlightcurveisobtainedfromtheMOS1/2 by the Max-Planck Society in the frame of the International Max-
data. The gray strip between the orbitalangle 0.21 0.29in- PlanckResearchSchool(IMPRS).
−
dicates the eclipse of the pulsar. Phase bins with zero counts
correspondtophaseanglesnotcoveredintheobservation.
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