Compactified pulsar wind nebula model of gamma-ray loud binary LSI +61 303

0.37 MB·English

by  A. Neronov

Mon.Not.R.Astron.Soc.000,1–10(2005) Printed5February2008 (MNLATEXstylefilev2.2)
Compactified pulsar wind nebula model of γ-ray-loud binary
LSI +61◦ 303.
A. Neronov1,2 ⋆ and M. Chernyakova1,2†
1INTEGRALScienceDataCenter,Chemind’E´cogia16,1290Versoix,Switzerland
2GenevaObservatory,UniversityofGeneva,51ch.desMaillettes,CH-1290Sauverny,Switzerland
7
0
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2 Received<date>;inoriginalform<date>

n
a
J ABSTRACT
We show that radio-to-TeVpropertiesof the binarysystem LSI +61◦ 303 can be explained
5
byinteractionof the compactobject(a youngpulsar)with the inhomogeneitiesof the wind

1 fromcompanionBestar.Wedevelopamodelscenarioof”compactified”pulsarwindnebula
v formed in result of such interaction. To test the model assumptions about geometry of the
4 systemwere-analyzetheavailableX-rayobservationstostudyinmoredetailsthevariations
4 ofthehydrogencolumndensityonlong(orbital)andshort(severalkilosecond)timescales.
1
1 Key words: pulsars : individual: LSI +61◦ 303 – X-rays: binaries – X-rays: individual:
0 LSI+61◦303 –gamma-rays:theory
7
0
/
h
p 1 INTRODUCTION systemsenablestomakeaconjectureaboutsimilarmechanismof
-
o TheBestarbinaryLSI+61◦303isoneofthethreecurrentlyknown theiractivity.
r IftheactivityofLSI+61◦ 303ispoweredbyayoungpulsar,
t γ-ray-loudX-raybinaries.Thespectrumofhigh-energy emission
s from the system extends up to TeV energies (Albertetal. 2006) theradio-to-γ-rayemissionisgeneratedinthecourseofcollisionof
a relativisticpulsarwindwiththewindfromcompanionstar.Interac-
: andthepoweroutputofthesourceisdominatedbyemissioninthe
v γ-rayenergyband. tionofthepulsarandstellarwindsleadstoformationofa”scaled
i down”analogofthepulsarwindnebulae(PWN)inwhichtheen-
X The origin of the high-energy activity of the source is not
ergyofthepulsarwindisreleasedattheastronomicalunit,rather
completely clear. The problem is that most of the ”prototypical”
r thanonparsec,distancescale(Neronov&Chernyakova2006).
a accretion-powered X-ray binaries do not reveal significant γ-ray
InthispaperweexplorethestructureofcompactifiedPWNof
activity.Itispossiblethatthe”γ-ray-loudness”ofanX-raybinary
LSI+61◦303.Weshowthatdifferentphysicalprocessesdetermine
canberelatedtoitsspecialorientationwithrespecttothelineof
coolingofhigh-energyparticlesatdifferentdistancesfromBestar.
sight(byanalogywiththecaseofγ-ray-loudnessofactivegalactic
Thisleadstoan”onion-like”structureinwhichtheregionofdomi-
nuclei)(Mirabel&Rodriguez1999).Otherwise,itispossiblethat
nanceofCoulomblossesisembeddedintotheregionofdominance
thethreeX-raybinariesdetectedinγ-raybandsofararefundamen-
tallydifferentfromtheconventional X-raybinaries.Forexample, of inverse Compton (IC) losses which is, in turn, situated inside
theγ-ray-loudbinariescanbepoweredbyadifferentmechanism, theregionofdominantsynchrotronloss.Thedensityandinhomo-
geneity of the stellar wind determine the speed of escape of the
than the conventional X-ray binaries. In fact, one of the three γ-
high-energy particles injected in the region of pulsar/stellar wind
ray-loudbinaries,PSRB1259-63, isknowntobepoweredbythe
interaction.Anisotropyofthestellarwindleadstothedependence
rotationenergyofayoung pulsar,ratherthanbyaccretionontoa
oftheescapespeedontheorbitalphaseand,asaconsequence,to
blackholeoraneutronstar(Johnstonetal.1992).Intwootherγ-
ray-loud X-ray binaries, namely LS 5039 and LSI +61◦ 303, the thevariationsoftherelativeimportanceofCoulomb,IC,andsyn-
chrotronlossesalongtheorbit.Weshowthatsuchmodelexplains
pulsedemissionfromthepulsarwasnotdetected,sothereisnodi-
rectevidenceforthepulsarpoweringtheactivityofthesesources1. thepuzzlingbehaviourofradio,X-rayandγ-raylightcurvesofthe
system (shifts of the maxima from the periastron, shifts between
However,similarityofthespectralenergydistributionsofthethree
the maxima of X-ray and radio lightcurves, shifts of themaxima
fromorbittoorbit).
Inordertobetterconstrainourmodelwere-analyzetheexist-
ingX-rayobservationsofLSI+61◦ 303andfindamarginallyde-
⋆ E-mail:[email protected]
tectedvariationsofthehydrogencolumndensityonlong(orbital)
† M.ChernyakovaisonleavefromAstroSpaceCenteroftheP.N.Lebedev
andshort(kilosecond)timescales.Theshorttimescalevariations
PhysicalInstitute,Moscow,Russia
1 Thebinaryorbitsofthesetwosourcesaremuchmorecompactthanthat reveal the small-scale inhomogeneities of the stellar wind which
ofPSRB1259-63.Inthiscasetheradiopulsedemissionisabsorbedinthe leadtothemacroscopicmixingofpulsarandstellarwinds.Ifcon-
windfromcompanionstar firmedwithmoreX-rayobservationsattheorbitalphasescloseto
2 A. Neronovand M. Chernyakova
theperiastron,thedetectedorbitalmodulationofthehydrogencol-
umndensityconstrainsthegeometryofthesystem.
φ
=
0
.5
2 INTERACTIONOFTHEPULSARWINDWITHA
CLUMPYWINDFROMCOMPANIONSTAR.
2.1 BasicpropertiesofLSI+61◦303. φ=0.7
Radioemissionfromthesystemisknowntobeperiodicwiththe
Be star
period of T = 26.4960 d which isassociated with thebinary or- Φ=0ο φ=0.3
bital period(Gregoryetal. 2002). Opticaldata allow toconstrain
the orbital parameters of the system revealing the eccentricity of
the orbit, e ≃ 0.55 (Grundstrometal. 2006). The binary orbit of φ=0.9
LSI+61◦ 303turnsouttobeverycompact,withtheperiastronat
just1.3radiiofBestar(seeFig.1).
pulsar
The existing measurements are not sufficient to determine 1
thenatureof the compact object (neutron staror black hole), be- wind φ=0.
causetheinclinationoftheorbitispoorlyconstrained.Thisuncer- ar
taintyallowstodiscusstwo possiblemodels for theoriginof the puls
LSI+61◦ 303activity.Modelsofthefirsttype,firstintroducedby
Taylor&Gregory(1984)(seeBosch-Ramonetal.(2006)forare-
centreference),assumethatactivityofthesourceispoweredbyac-
Figure1.Schematicrepresentation ofinteractionofthepulsarwindwith
cretionontothecompactobject.Inthesecondclassofmodels,first aclumpywindfromcompanionBestar.Relativistic pulsarwindhitsthe
proposedbyMaraschi&Treves(1981),theactivityofthesourceis nearestclumpsofthestellarwind.Insteadofasmoothbow-shapedcon-
explainedbyinteractionsofayoungrotationpoweredpulsarwith tact surface (red dashed line) the interaction region (the set of the dark
thewindfromthecompanion Bestar.Thefactthatnopulsations shaded clumps) is irregularly shaped. Ellipse represents correctly scaled
from thesystem has been found can be explained by theabsorp- pulsar binary orbit around the companion star. x and y axes are in units
tionofthepulsedradioemissionintheBestarwind.Theabsence oftheradiusofcompanionstar.Reddotsalongtheorbitmarkintervalsreg-
ularlyspacedinphase,∆φ=0.05.Theperiastroncorrespondstoφ=0.3
of the break up to the 100 keV in the X-ray spectrum and simi-
(Grundstrometal.2006).
larityoftheradio-to-TeVγ-rayspectralenergydistributionofthe
sourcetotheoneofPSRB1259-63favorthe”hiddenpulsar”model
(Chernyakovaetal. 2006). The ”hidden pulsar” model is also fa-
region,asitisshowninFig.1.Themacroscopicmixingofthepul-
voredbytherecentVLBAmonitoringoftheLSI+61◦303 which
sarandstellarwindsslowsdowntheescapeoftheshockedpulsar
revealsanextendedsourcewhoseirregularmorphology varieson
wind.
theorbitaltimescale(Dhawanetal.2006).
Different regimes of escape of the high-energy particles in-
jected into the region of interaction of pulsar and stellar winds
leadtosignificantlydifferentpredictionsaboutthespatialstructure
2.2 Basicmodelofpulsar/stellarwindinteraction.
ofthecompactifiedPWNinthe”homogeneous stellarwind”and
In the most simple version, the model of interaction of the pul- ”clumpystellarwind”scenaria.Thereasonforthisisthestrongra-
sar wind withthe wind fromcompanion Be star assumes that an dialgradientofthedensitiesofmatterandsoftphotonsaroundthe
isotropic relativistic outflow from the pulsar hits a homogeneous system.Dependingonthevelocityofescape,high-energyelectrons
(but anisotropic, in the case of Be star companion) outflow from canloosedifferentfractionoftheirenergyontoCoulomb,ICand
the star along a regular bow-shaped surface. Such a model was synchrotron loss. Below we explore indetails the relativeimpor-
developed in details e.g. in relation with the PSR B1259-63 by tanceofdifferentcoolingmechanismsandproposeamodelofthe
Tavani&Arons(1997)andappliedtothecaseofLSI+61◦303by compactPWNofLSI+61◦303.
Dubus (2006). Geometry of the interaction surface is determined
bythepressurebalancebetweenthepulsarandstellarwinds.Inthe
settings of ”homogeneous stellar wind” scenario, the pulsar and 2.3 Inhomogeneityofthestellarwindandshorttimescale
stellarwinddonotmixmacroscopically,whichallowstheshocked variabilityofthesource.
pulsarwindtoescapefromthesystemwiththespeed∼1010cm/s,
LongXMM-Newtonobservation ofLSI+61◦ 303in2005hasre-
muchhigherthanescapevelocityoftheshockedstellarwind.
vealed variability of the system at ∆t ≃ (several) ks time scale,
Theassumptionofhomogeneityofthestellarwindisinitially
whichismuch shorter thantheorbital period(Sidolietal.2006).
adopted for the sake of simplicity of the model, rather than by
Variabilityatasimilartimescaleisobservedalsoinradioenergy
physical motivations. At thesame time, it isknown that intrinsic
band(Peracaulaetal.1997).Theobservedvariabilityindicatesthat
instabilitiesof the winds from massive stars lead to formation of
the characteristics of interaction of the pulsar and stellar winds
large inhomogeneity of the winds, observationally seen in X-ray
change at distance scales which are smaller than the size of the
emissionfrommassivestarsandinline-profilevariability(seee.g.
binaryorbit.Distancescalerelevantforthefastvariabilitycanbe
Pulsetal. (2006)). Significant inhomogeneity of the stellar wind
estimatedas
leads,ingeneral,todisappearanceoftheregularbow-shapedcon-
tact surface of the pulsar and stellar winds and to a macroscopic v ∆t
R ∼v∆t∼1011 cm (1)
mixing of the two winds inside an irregularly shaped interaction w "107cm/s#"10ks#
Compactified pulsarwind nebulamodelof γ-ray-loudbinaryLSI +61◦ 303. 3
where v is typical velocity scale (e.g. the orbital velocity of the (L istheluminosityofBestar)and/orsynchrotroncoolingtime,
∗
pulsar,orthespeedofthestellarwind).
If one assumes that the power output in the pulsar wind is t ≃3×105 1G 2 10MeV s (7)
constant,thedistancescalerelatedtothefastvariabilityhastobe synch " B # " Ee #
associatedwiththepulsarwindinteractionswithinhomogeneities
onefinds that ifthemagnetic fieldintheclump is B ∼ 1 G,
(clumps)ofthestellarwind.ChangesintheX-rayluminositycan clump
then electrons captured in the clumps can efficiently cool before
becausedbythevariaitonsofthenumberofinhomogeneitiesex-
theyescape.
posedtothepulsarwindand/or variationsofcharacteristics(den-
ThebinaryorbitofLSI+61◦ 303isverycompact,sothatat
sity,size,magneticfieldstrength)ofinhomogeneities.
periastronthepulsarapproachestheBestarascloseas1.3stellar
Asitisexplainedabove,theinhomogeneityofthestellarwind
radii,D≃ 5×1011 cm(Grundstrometal.2006).Closetothesur-
leads the disappearence of a regular bow-shaped surface of pul-
faceofthestar,notonlythedensityofphotonbackgroundishigh,
sar/stellarwindinteraction.Theirregularityofthecontactsurface
butalsothedensityofthestellarwind,n.Inthiscase10MeVelec-
of wind interaction leads to a change in the regime of escape of
trons suffer from the strong Coulomb energy loss. The Coulomb
high-energyparticlesfromthevicinityofBestar.Namely,contrary
losstime,
tothethe”homogeneouswind”scenario,inwhichtheshockedpul-
sar wind escapes along the bow-shaped contact surface in a time t ≃2×103 1010cm−3 Ee s (8)
tesc ∼ 3D/c ∼ 30 1012cm/D s(which depends on the distance Coul " n #(cid:20)10MeV(cid:21)
D),theescapeofthhepulsarwiindmixedwiththestellarwindcan
becomesshorterthantheICcoolingtime(6)belowthe”Coulomb
slowdownto
break”energy
1012cm 107cm/s
tesc ∼D/vw ∼105" D #" vwind # s (2) E ≃20 L∗ 1/2 D n 1/2 MeV (9)
ThewindfromtheBestarisknowntohaveanisotropicstructure e,Coul "1038erg/s# (cid:20)1012cm(cid:21)(cid:20)1010cm−3(cid:21)
withadenseand”slow”wind(vw∼106÷107cm/s)intheequato- Mostofthepowerinjectedintheformofelectronswithenergies
rialregionandrarefied”fast”wind(vwind ∼ (several)×108 cm/s) belowEe,Coul goesintoheatingofthestellarwind,ratherthaninto
fromthepolarregions.Thedifferenceintheescapetimechanges thesynchrotronandICemission.Ifthedensityofthewindis
the balance between the synchrotron, IC and Coulomb energy
lossesofhigh-energyelectronsinjectedintheregionofinteraction n>1010 1012cm 2 cm−3 (10)
ofpulsarandstellarwinds. " D #
boththeX-rayemissionintheenergybandǫ <10keVandradio
IC
2.4 Coolingofhigh-energyelectronsintheclumps. emissionbelowνsynch ∼10GHz(correspondingtoEe ≃20MeV)
are suppressed. In other words, the innermost part of the stellar
Following Maraschi&Treves (1981) and Chernyakovaetal. windis”X-ray/radiodim”.
(2006) we assume that X-ray emission from the system at ener- Radialgradientsofthedensitiesofthestellarwindandofthe
giesǫIC ∼1−10keVisdominatedbytheICscatteringofoptical soft photon background lead to significant variations of the rela-
photonsfromtheBestarbyelectronsofenergiesabout tiveimportanceofCoulomb,ICandsynchrotronenergylosseswith
2×104K 1/2 ǫ 1/2 distance.This,inturn,leadstoacomplicatedradialstructureofthe
E ≃10 IC MeV (3) compactPWN.
e,IC " T∗ # (cid:20)4keV(cid:21)
(T isthetemperatureof theBestar).Radiofluxatthefrequency
νsynch is produced via synchrotron emission from electrons with 2.5 Structureofthecompactifiedpulsarwindnebula.
roughlythesameenergies,
RecentVLBAmonitoringofthesourceoveranentireorbitalcycle
E ≃10 1G 1/2 νsynch 1/2 MeV (4) (Dhawanetal.2006)revealsanextendedradiosourceofavariable
e,synch " B # (cid:20)7GHz(cid:21) morphologywiththeoverallsizeof DPWN ∼ (several)×1014 cm
whichcanbeidentifiedwitha”compactified”PWN,similartothe
(Bisthemagneticfieldstrength).
compactified PWN of PSR B1259-63 (Neronov&Chernyakova
10 MeV electronsareinjectedintheinhomogeneities of the
2006). The radio emission at 3-13 cm wavelengths (synchrotron
stellarwindirradiatedbytherelativisticpulsarwind.Suchinjection
photonenergiesǫ ∼(1÷3)×10−5eV)peaksoutsidethebinary
canbeeithertheresultofshockaccelerationofelectronsfromthe synch
orbit,atthedistanceD ∼(several)×1013cm.Thepositionofthe
stellarwindortheresultofcoolingofhigherenergyelectronsfrom r
peakmovesaroundthecentralBestarontheorbitaltimescales.
thepulsarwind.
Thevariabilityofmorphologyofthesourceontheorbitaltime
Thehigh-energyparticlescanberetainedininhomogeneities
scaleshowsthatthecoolingtimeof10MeVelectronsresponsible
withstrongenoughmagneticfield.Assumingthatelectronsdiffuse
fortheradiosynchrotronemissionist 6106s.Comparingthis
indisorderedmagneticfield,onecanestimatetheescapetime(in PWN
time with synchrotron and inverse Compton cooling times in the
theBohmdiffusionregime)as
regionofthesizeD onecanfindthatt isshorterthanthe
PWN PWN
R 2 B 10MeV inverse Compton cooling time, t (D ,10MeV) (6). Equating
t ≃3×106 w s (5) IC PWN
diff (cid:20)1011cm(cid:21) (cid:20)1G(cid:21)" Ee # tPWN ≃ tsynch (7) one can find the magnetic field strength in the
compactifiedPWN,B ≃1G.
ComparingtheescapetimewiththeinverseComptoncoolingtime PWN
Suppression of the radio synchrotron emission from the re-
L D 2 10MeV gion D < D can be explained by the fact that in the inner part
t (D,E )≃104 ∗ s (6) r
IC e "1038erg/s#(cid:20)1012cm(cid:21) " Ee # ofthePWNtheinverse Comptonenergy lossdominates over the
4 A. Neronovand M. Chernyakova
synchrotronloss.Indeed,from(6),(7)onecanfindthatifthemag-
neticfieldinthenebuladoesnotrisesignificantlytowardthecen-
ter, t < t for the distances D < 1013 cm. Thus, the inverse
IC synch
Comptonemissioncomespredominantlyfromtheinnerpartofthe
nebula,whilethesynchrotronemissionmostlycomesfromitsout-
skirts.
High-energy electrons responsible for the radio and X-ray Φ
X1
emission are initially injected in the region of interaction of the
Φ
r2
pulsar and stellar wind. Taking into account that the binary orbit
of LSI +61◦ 303 is very compact, D ∼ (several) ×1012 cm, Be star
orb
stoissifiuipncoeceensdhepfidon”ennefvedalbdyss>utitli”nhafDaettbhhtsPecWeetfhayoNsperh/seehpot.sirctyEgehnkchaieeht-dyhde≃efnlporreouo1ermlgs0lseey8actrachterlwmleloetinc/chnsost.edrmoti,Trrnpsahesovaencecrtetleharnwiagantyrifijtveehlocl∼tnttwtihhos1oeeyn0nnpe1rc0neoehatgscirrsirmoloieybtn/rcisrol,oweintomliiaretetphsmitavhtfciiheosstye-r-- Φr1 IC C ΦX2Be disk equatorial plane
areinjectedintothenebuladuringtheperiodswhenthepulsarin- S
teractswiththeclumpsofthefastpolarwindfromBestar,which
hasthevelocityv >108cm/s.
wind
IntheformercaseitwouldbedifficulttoexplainwhytheIC Figure2.Onion-like structureofthecompactified pulsarwindnebulaof
luminosityofthesystemismuchhigherthanthesynchrotronlumi- LSI +61◦ 303. The Coulomb losses dominate in the innermost region
nosity.Indeed,electronswouldleavethecompactregionofthesize markedas”C”.TheinverseComptonlossdominatesintheregionmarked
D<1013 cminlessthanonekilosecond.Thistimeisshorterthan ”IC”.Thesynchrotronlossdominates intheouterregionmarkedas”S”.
Theellipseshowsthebinaryorbit.Thesuggestedorientationoftheequa-
theICcoolingtimeandinsuchscenariomostofthepowerofhigh
torialplaneofthegeometricallythickBestardiskatthemomentof1996
energyelectronswouldbereleasedviasynchrotron,ratherthanvia
RXTEobservations isshownbythethicksolidblueline.Thepartofthe
ICemission.
binary orbit behind the equatorial plane ofBe star disk is shown bythe
Tothecontrary,ifelectronsescapewiththespeedofthestellar dashedline.ΦX1 andΦr1 markthephasesofmaximaofX-rayandradio
wind,theperiodsoffastescapeinthepolarwindareintermittent lightcurves(seetext).
withtheperiodsofslowescapeinequatorialwind.Thevelocityof
theequatorialwindisv ∼106÷107cm/s.Ittakeselectronssome
wind
106 stoescapetothedistancesbeyond1013 cmduringthepulsar fluxshowslargervariations,byafactorof10.Theonion-likestruc-
propagationthroughtheequatorialwind.Sincetheescapetimeis tureofthecompactifiedPWNdiscussedintheprevioussubsection
larger thantheICcooling timeinthiscase, most of thepower is gives a clue to understand the behaviour of the X-ray and radio
releasedviaICX-rayemission. lightcurves.
ItisclearfromtheabovediscussionthattheX-rayICemission TheX-rayfluxincreaseswhenthepulsarmoves deeper into
fromthesystemisproducedinthecompactregion,D 6 1013 cm, theslowequatorialwindofBestar.Duringtheseperiodsthehigh-
inthedirectvicinityoftheBestar.However,closetothestellarsur- energy electrons responsible for the X-ray emission are retained
face,electronscansufferfromthesevereCoulombloss.Sincethe for a longer time in the vicinity of Be star where they efficiently
radialprofileofthedensityofthestellarwindintheequatorialdisk, looseenergyviaICscattering.Insuchamodelthemaximumofthe
n ∼ D(−2)÷(−4) (e.g. Watersetal. (1988)), is steeper than the soft X-raylightcurveat thephase Φ ≃ 300◦, observed withRXTE,
X1
photondensityprofile,nph ∼ D−2,theCoulomblosscandominate hastobeidentifiedwiththemomentofthepulsarpassagethrough
over the IC loss in the innermost part of the compactified PWN. the equatorial plane of the Be star disk (in 1996). This fixes the
TheX-rayICemissionfromthisinnermostpartissuppressedand orientationofthediskisfixedasitisshowninFig.2.
most of the energy of 10 MeV electrons goes intoheating of the Orbit-to-orbitfluctuations/shiftsofthepositionofX-raymaxi-
stellarwindviaCoulombcollisions. mumareexpectedbecausethepulsarpassesthroughtheinnermost
Thus,theentirecompactifiedPWNhasthe”onion-like”struc- part of the disk and the pulsar wind is injected into several indi-
ture shown in Fig. 2: in the innermost region of the nebula the vidualclumpsinthedisk,ratherthanintothe”average”equatorial
dominant Coulomb loss suppresses both the IC and synchrotron windofBestar.DriftofthephaseofX-raymaximum,Φ on4.6yr
X1
luminosity, in the intermediate region 1012 cm< D < 1013 cm year time scale of variability of Be star disk (Zamanov&Marti
theICenergylossdominateswhileintheouterpartofthenebula 2000)isexpectedife.g.thediskprecessesorasignificantasimutal
1013cm<D<(several)×1014cmthesynchrotronlossdominates. inhomogeneityofthediskchangesitsorientation.
By analogy with the case of PSR B1259-63, one expects to
findasecondmaximumoftheX-raylightcurveatthephase(Φ =
2.6 X-rayandradiolightcurvesofthesource. X2
Φ −180◦ ≃ 120◦ (assumingthe1996diskposition),duringthe
X1
X-ray(and radio) lightcurve of the system exhibit asingle maxi- pre-periastronpassageofthepulsarthroughtheequatorialplaneof
mumperorbit,seeFig.3.InthisfigureweplottheX-rayfluxas Bestardisk. However, during thesecond diskpassage the pulsar
afunctionofthe”angular”orbitalphase,Φ,definedinsuchaway ismuch closer tothesurface of Bestar, wheretheCoulomb loss
thattheapastronisatΦ=0◦,whiletheperiastronisatΦ=180◦. candominateovertheICloss.Inthiscasemostoftheilluminated
ThephaseofthemaximalX-rayfluxisknowntobeshiftedwithre- clumpsofBestar windare”X-raydim”, asitisexplained inthe
specttothemaximuminradioband.Besides,thephasesofX-ray previoussub-section.ThedecreaseofamountoftheX-raybright
and radio maxima shift from orbit to orbit. The overall variation clumpsleadstothesuppressionofthesecondmaximumoftheX-
oftheX-rayfluxfromthesystemisbyafactorof2-3.Theradio raylightcurve.
Compactified pulsarwind nebulamodelof γ-ray-loudbinaryLSI +61◦ 303. 5
Itisclearfromtheprevioussubsectionthat10MeVelectrons 2.7 γ-raylightcurve.
can fill theentire compact PWNand efficiently emit synchrotron
Electrons responsible for the TeV γ-ray emission from the sys-
radiation in radio band only if they escape from the compact in-
tem are not able to fill the whole volume of the compactified
jectionregionwiththespeedv > 108 cm/s,muchlargerthanthe
PWN because of the very short cooling time. The shortest cool-
velocityoftheequatorialwind.Thisleadstoasuggestionthatthe
ing time is achieved at electron energies E ∼ 10 − 100 GeV,
radio luminosity of the system peaks during the period when the e
t (1012cm,10GeV) ≃ 10 s (see Eq. (6)). The IC scattering on
pulsar windcan beefficientlyinjected intothe clumpsof thepo- IC
100GeV–TeVelectronsproceedsintheKlein-Nishinaregime.In
larwindofBestar.Theseperiod(s)correspondtotheorbitalphase
thisregimetheelectroncoolingtimegrowswithenergy
whenthepulsar movestoitshighest positionaboveor below the
equatorialplane.(Ifthebinaryorbitwouldbecircular,thephaseof L ǫ 0.7 D 2
theradio maximum would be Φr1 ≃ ΦX1 +90◦. Theeccentricity tIC(KN) ≃102"1038e∗rg/s#(cid:20)1TIeCV(cid:21) (cid:20)1012cm(cid:21) s (11)
oftheorbitshiftsthehighestelevationabovethedisktoearlieror
Propagation of electrons with the energy about 100 GeV is
laterphase.Thephaseoftheradiomaximumisfoundbydrawing
affectedbydevelopmentofthepairproductioncascadeinthefield
atangenttotheellipseparalleltotheassumedlineofintersection
ofsoftphotonsfromBestar(withtypicalenergyǫ ∼1−10eV).
of the orbital plane withtheequatorial plane of Bestar disk (the ∗
Thedensityofsuchphotonsis
dashed linesinFig.2).Similarlytothecase of X-raylightcurve,
the second maximum of the radio lightcurve, naively expected at L 1012cm 2
Φr2 (see Fig. 2), is missing because of the close approach of the nph ∼1013"1038e∗rg/s#" D # cm−3 (12)
binaryorbittothesurfaceofBestar.
Taking into account that the pair production cross-section peak
at the value σ ≃ 10−25 cm2 for γ-rays of energy E ≃
γγ γ
Thephaseofthemaximumoftheradiolightcurveisknownto [10eV/ǫ ]100GeV, one can estimate the optical depth for the
∗
exhibitaperiodicdriftonthe4.6yrtimescale(Gregoryetal.2002). 100GeV–1TeVγ-raysas
Withintheproposedmodel,thedriftoftheradiomaximumisasso-
L 1012cm
ciatedtothedriftofthephaseofthehighestelevationofthebinary τ ≃1 ∗ (13)
orbitabovetheequatorialplaneoftheBestardiskor,equivalently, γγ "1038erg/s#" D #
oftheorientationofthelineofintersectionoftheequatorialplane Thelifetimeofboth100GeV-TeVand10GeVelectronsismuch
oftheBestardiskwiththeorbitalplane.Suchdriftoftheorienta- shorter than the escape time to the region of dominance of syn-
tionofthelineofintersectioncanbee.g.duetotheprecessionof chrotron loss, D > 1013 cm. Thecascade electrons andpositrons
thediskwiththeperiodof4.6yr. looseenergymostlyviaIC(ratherthanviasynchrotron)emission2.
Thismeansthatdevelopmentofelectromagneticcascadeefficiently
channels thepower initiallyinjectedinto100 GeV – TeVenergy
Itisinterestingtonotethatthepatternofthesystematic4.6yr
bandtotheenergybandaround∼10GeV.
drift of thephase of theradio maximum agrees wellwiththeas-
Since the optical depth with respect to the pair production
sumption of the change of the mutual orientation of the orbital
ishighest near theperiastron, oneexpects toobserve a”deep” in
plane and the equatorial plane of Be star disk. Suppose that the
the 100 GeV – TeV lightcurve of the source around the phases
lineofintersectionofthebinaryorbitwiththeequatorialplaneof
BestardiskrotateswithaperiodofP≃2×4.6yr≃9.2yr.Suppose Φ ∼ 180◦ (φ ∼ 0.3) in agreement with MAGIC observations
(Albertetal.2006).Theshapeofthesourcelightcurveintheen-
thatataninitialmomentthelineofintersectionofequatorialplane
ergybandbelow∼ 10GeVshouldbequalitativelydifferentfrom
ofBestardiskcoincideswiththemajoraxisofthebinaryorbit.At
the one in the 100 GeV-TeV energy band. Since (a) the 10 GeV
thismomentthephaseofthemaximalelevationofthebinaryorbit
abovethediskisroughlyequaltoΦ ∼290◦orΦ ≃70◦(forthe emission is not uppressed by the pair production and (b) a part
r1 r1
particularcaseofthebinaryorbitofLSI+61◦ 303,seeFig.1and ofthe10GeVpowerisgeneratedviathedevelopmentofelectro-
magneticcascadeinthesource,themaximumofthe10GeVflux
2.Ifthelineofintersectionoftheorbitalplanewiththeequatorial
isexpected roughly during thephases ofmaximal suppression of
planeofBestardiskprecessesinthecounter-clockwisedirection,
the100 GeV-TeVflux. Inother words, futureobservations of the
the phase of the highest elevation of the orbit above the equato-
rialplaneofthediskgrowsfrom∼ 290◦ to360◦ andthenfurther sourcewithAGILEand GLASTareexpected tofindamaximum,
from0◦ to≃ 70◦.Intermsoftheorbitalphase,thephaseofradio ratherthanminimumofthe1-10GeVlightcurvearoundthephases
Φ∼180◦(φ∼0.3).
maximumchangesfrom0.45to1.andthenfrom0.to0.1.Atthe
momentwhenthelineofintersectionoforbitalandequatorialplane
againcoincideswiththemajoraxisoftheorbit,thephaseofthera-
2.8 Modelsummary.
diomaximumshouldmakea“flip”fromΦ ≃70◦ toΦ ≃290◦
r2 r1
(from φ ≃ 0.1 toφ ≃ 0.45). Since the line of intersection of or- To summarize, we find that the “compactified PWN” model can
bitalandequatorialplanescoincideswiththemajoraxistwiceper explainallobservationalpropertiesofLSI+61◦303,fromradioto
period,the“flips”ofthephaseofradiomaximumareexpectedto
happenwithaperiodofP/2≃4.6yr.Longtermradiomonitoring
ofthesystem(Gregoryetal.2002)showsthatindeedthephaseof 2 ForelectronswithenergiesEe>1TeVthesynchrotronlosstime(7)can
becomparableorshorterthantheICcoolingtime(11)evenintheregion
theradiomaximumdriftsfromφ≃0.45toφ≃1.andsubsequently
D<1012cm.TeVelectronsemitsynchrotronradiationin100keVenergy
flipsfromφ≃0.1toφ≃0.45withperiodicityof4.6yr.Theradio
band (see Eq. (4)). Taking into account that luminosity of the system at
maximumneverappearswithin0.1< φ< 0.4,ingoodagreement
∼100keVenergiesisjustbyafactorofseveralhigherthanitsTeVlumi-
withtheproposedexplanationintermsofthephaseofthehighest nosity,onecanfindthatanon-negligible synchrotroncontributioncanbe
elevationofthebinaryorbitabovetheequatorialplaneofBestar presentinthe100keVenergybandduringthephasesaroundthemaximum
disk. ofTeVlightcurve.
6 A. Neronovand M. Chernyakova
TeVγ-rayenergyband.Themainassumptionofthismodelisthat
the nebula has an ”onion-like” structure. The inner region of the
nebulaat D < 1012 cmischaracterizedbythedensestellarwind
anddensephotonbackground.Inthisregionthe10MeVelectrons,
responsiblefortheradioandX-rayemissionlooseenergymostly
onheatingofthestellarwind,while100GeV-TeVelectronsinitiate
thedevelopmentofpaircascades.Thisexplainsthesuppressionof
theradio,X-rayandTeVγ-rayluminosityofthesourceclosetothe
periastron.TheX-rayluminositypeakswhenthepulsarmovesout
ofthedensecentralpartintotheintermediateregionD>1012 cm
wherethedominantenergylossmechanismistheICscatteringof
softphotons fromBestar.Theradioluminosityreachesitsmaxi-
mumduringthephaseswhen10MeVelectronsareefficientlysup-
pliedbythefastpolarwindfromBestartotheoutermost partof
thenebula,D>1013cm,wherethesynchrotronlossdominates.
Qualitative analysis of behaviour of the X-ray and radio
lightcurvesofthesystemhasenabledusatentativedetermination
oftheorientationoftheequatorialplaneofthewindfromBestar
(atthemomentof1996RXTEobservations),inwhichthedensity
ofthestellarwindishighest.Sincethecorrelationofthemaximum
ofX-raylightcurvewiththemomentofthepulsarpassagethrough
themiddleofthediskisoneofthekeypointsofourmodel,itis
interestingtofindaconfirmationforthisfactinexperimentaldata.
Acomplimentaryinformationabouttheorientationofthebi-
nary orbit with respect to the Be star disk can be obtained from Figure3.Spectralvariabilityofthesystemin2-10keVenergyrange,asob-
servedwithXMM-Newton(red,blueandmagentapoints)andRXTE(green
theobservationthatthepassageofthepulsarfrombelowtoabove
opensquares).Observationsdoneduringoneandthesameorbitalcycleare
(fromthepointofviewofobserverontheEarth)thedensestpart
connectedwithaline.Thetimeisassumedtoincreasefromlefttoright.
ofthediskisassociatedwiththedecreaseofthecolumndensityof
Verticalthickbluelinesshowthesuggestedpositionofintersectionofequa-
matter between the position of the pulsar and the observer. Pas-
torialplaneoftheBestarwindwiththebinaryorbitatthemomentof1996
sage of the pulsar from above to below the densest part of the RXTEobservations.Thinblackverticallinemarksthepositionofperiastron
disk is then associated with the rise of the column density. Tak- (Φ=180).
ing into account that the estimated density of the stellar wind in
the most compact part of the PWN is n > 1010 cm−3, while the
thicknessofthedenselayerofthewindisaboutthesizeofthestar, the variations of the photon index, Γ, in the commonly accepted
∼ R ∼ (several)×1011 cm,oneexpectstofindthevariationsof X-rayspectralmodelofthesource,whichconsistsofapowerlaw
∗
themattercolumndensity spectrummodifiedatlowenergiesbythephoto-absorption. Since
thevariationsofN by(several)×1021 cm−2 areonlymarginally
H
∆NH ∼nR∗>1021cm−2 (14) detectable with such instruments as XMM-Newton, all previously
publishedanalysesassumedforsimplicityaconstantvalueofN .
Suchvariationsare,inprinciple,detectablebyXMM-Newton. H
ThesetofavailableX-raydataconsideredinour re-analysis
Asitismentionedabove,inourmodelthesuppressionofthe
consistsofthemonitoringofthesourceoverasingleorbitalcycle
X-ray maximum during the pre-periastron passage of the pulsar
with RXTE (Harrisonetal. 2000), five short XMM-Newton 2002
through the densest part of the Be star disk is explained by the
observations (Chernyakovaetal. 2006) and a long (50 ks) 2005
dominanceoftheCoulomblossintheinnermostpartofthestellar
XMM-Newtonobservation(Sidolietal.2006).Table1givesasum-
wind. The dominance of the (energy independent) Coulomb loss
maryoftheobservationsanalyzedbelow.
leadstohardeningofelectronspectrumwhich,inturn,shouldlead
tothehardeningofICemissionspectrumwhichisalsoapotentially
detectableeffect.
3.1 RXTE/PCAobservations
Inthefollowingsectionwemakeanattempttofindboththe
variationofN andthehardeningofthespectrumassociatedtothe In1996RXTEhascloselymonitoredLSI+61◦303 making11ob-
H
pulsarpassagethroughthedensestpartofthediskintheavailable servationsalongthesingleorbit,seeTable1.Forthefirsttimethese
observationaldataonLSI+61◦303.There-analysisofexperimen- datawerepresentedinthepaperofHarrisonetal.(2000)wherethe
taldataallowsustofurtherconstrainthegeometryof thesystem authorsreportthatnosignificantvariabilityofthespectralshapeis
(inparticular,itsorientationwithrespecttothelineofsight). visible in RXTE data. Our reanalysis of the RXTE data with the
latestLHEASOFT5.2providedbytheRXTEGuestObserverFa-
cilityreveals,tothecontrary,asystematicvariationofthespectrum
alongtheorbit.Namely,theriseofX-rayfluxisassociatedtothe
3 RE-ANALYSISOFTHEARCHIVEX-RAYDATA.
hardeningofthespectrumwiththephotonindexdecreasingfrom
InthepreviouslypublishedanalysisoftheavailableX-raydata,the Γ≃1.9downtoΓ≃1.5.
hydrogencolumndensitywasfixedduringthemodelfittingofthe TheresultsofourspectralanalysisperformedwithXSPECv
data,sothatnoinformationaboutpresence/absenceoftheshortand 11.3.2aregiveninTable1,andaregraphicallyrepresentedinthe
longtimescalevariationsofN isavailable.Themainreasonfor Fig.3.ThelineofintersectionoftheequatorialplaneoftheBestar
H
thisisthedifficultyof”disentanglement”ofvariationsofN from diskwiththeorbitalplane(atthemomentofRXTEobservations),
H
Compactified pulsarwind nebulamodelof γ-ray-loudbinaryLSI +61◦ 303. 7
Table1.JournalofRXTEobservationsofLSI+61◦303
Date Exposure Orbital Γ Flux(2-10keV) χ2(dof)
(ks) Phase (×10−11ergscm−2s−1)
1996Mar0113:57-19:25 7.9 0.79 1.65+0.10 1.17+0.24 16.93(23)
−0.12 −0.21
1996Mar0421:38-01:36 8.0 0.90 1.72+0.08 1.49+0.23 11.61(23)
−0.09 −0.20
1996Mar0723:33-03:28 7.9 0.01 1.86+0.12 0.99+0.22 26.09(48)
−0.13 −0.20
1996Mar1001:07-05:02 7.7 0.11 1.83+0.16 0.94+0.28 14.79(23)
−0.12 −0.17
1996Mar1304:06-08:19 8.7 0.23 1.55+0.10 1.15+0.22 17.10(23)
−0.11 −0.19
1996Mar1600:54-06:06 8.3 0.34 1.62+0.08 1.62+0.22 25.16(48)
−0.08 −0.21
1996Mar1810:44-15:48 8.3 0.43 1.56+0.06 2.15+0.23 19.67(23)
−0.06 −0.20
1996Mar2407:55-13:10 10.2 0.65 1.93+0.09 1.17+0.19 17.20(20)
−0.10 −0.17
1996Mar2600:56-06:46 12.3 0.71 1.84+0.10 1.06+0.19 16.99(20)
−0.11 −0.17
1996Mar3004:07-10:40 12.9 0.87 1.89+0.10 0.93+0.17 16.50(20)
−0.11 −0.16
Table2.JournalofXMM-NewtonobservationsofLSI+61◦303 Table 3. Spectral parameters for XMM-Newton Observations of
LSI+61◦303.∗
DataObservational Date MJD OrbitalExposure(ks)
Set ID (days) Phase
Data F(2-10keV) Γ NH χ2(dof)
X1 0112430101 2002-02-05 52310 0.55 6.4 Set 10−11ergs−1 (1022cm−2)
X2 0112430102 2002-02-10 52315 0.76 6.4
X1 1.30±0.02 1.60±0.03 0.49±0.02 261(264)
X3 0112430103 2002-02-17 52322 0.01 6.4
X2 1.24±0.03 1.55±0.03 0.52±0.02 272(252)
X4 0112430201 2002-02-21 52326 0.18 7.5
X3 0.60±0.03 1.83±0.05 0.57±0.03 182(165)
X5 0112430401 2002-09-16 52533 0.97 6.4
X4 0.44±0.03 1.48±0.08 0.47±0.04 70(76)
X6 0207260101 2005-01-27 53397 0.61 48.7
X5 1.25±0.03 1.58±0.03 0.56±0.02 277(284)
X6a 1.10±0.02 1.66±0.02 0.54±0.01 653(649)
X6b 0.37±0.01 1.78±0.02 0.49±0.01 435(458)
suggestedfromthequalitativeconsiderationsoftheprevioussec-
tion,isshownbytheverticalbluelinesshiftedby∆Φ=180◦.The
∗Givenerrorsrepresent1σ(68.3%)confidenceintervaluncertainties.
orientationofthediskischoseninsuchawaythatthemaximumof
RXTElightcurveatΦ ≃ 300◦ correspondstothemomentofthe
X1
pulsarpassagethroughtheequatorialplaneofthedisk(ingeneral, The XMM-Newton Observation Data Files(ODFs) were ob-
thediskisgeometricallythick,sothepulsarcanspendmostofits tained from the on-line Science Archive3; the data were then
orbitinsidethedisk).Asitisdiscussedintheprevioussection,the processed and the event-lists filtered using  within
secondmaximum,atΦ ≃120◦ issuppressedbecausethepulsar the Science Analysis Software () v6.0.1. We re-analyzed
X2
passesclosertotheBestarwherethediskissodensethatmostof the XMM-Newton observations using the methods described in
thepower of high-energy electronsinjectedinthediskgoes onto Chernyakovaetal.(2006).
Coulomb heating of the disk, rather than on the IC X-ray emis- Theresultsof our spectral analysisare given inTable3and
sion.FromthemiddlepanelofFig.3onecanclearlyseethatthe Fig.3.FollowingtheworkofSidolietal.(2006)wehavesplitob-
suggested moment of thepre-periastronpassage of theequatorial servationX6intotwo,X6a andX6b. X6a corresponds tothefirst
planeatΦX2 ≃ 120◦ isassociatedwiththehardeningoftheX-ray 20ksoftheobservations,andX6btotherest.
spectrumofthesystem.
Unfortunately, the PCA data do not allow one to find un-
ambiguously the true value of hydrogen column density N , be- 3.3 SystematicvariationofX-rayspectrumalongtheorbit.
H
causeofthelackofsensitivityattheenergiesbelow 3keV. Only
Fromthebottompanel ofFig.3onecanseeahintof systematic
the information about the variations of photon index, Γ, is avail-
variationof N overtheorbit(theprobabilityofnon-variable N
H H
able. For the purpose of the model fit we have fixed the value
is0.02%).FromobservationsX1–X5onecanguessthatthecol-
N =0.49×1022cm−2derivedbyChernyakovaetal.(2006)from
H umndensityappearstobeloweraroundtheorbitalphasescloserto
2002XMM-Newtondata.
periastronandhigher atapastron. Atthesametime,thevariation
of N at 50 ks scale during the observation X6 is comparable to
H
thescatterofN valuesontheorbitaltimescale.Obviously,more
3.2 XMM-Newtonobservations H
observations, especiallyaround theperiastron, 120◦ < Φ < 240◦
XMM-Newton has observed LSI +61◦ 303 with the EPIC instru- wouldhelptoclarifythequestionofthesystematicorbitalmodu-
ments five times during 2002, and once in 2005. Four 2002 ob- lationofequivalenthydrogencolumndensity.
servations have been done during the same orbital cycle, and the A possible interpretation of the orbital modulation of N is
H
fifthonehasbeendonesevenmonthslater.ThelogoftheXMM- that the part of the binary orbit around the periastron is situated
NewtondatadiscussedinthispaperispresentedinTable2.These closertoobserver.Inthiscasethepartoftheorbitaroundtheapas-
data have already been analyzed in papers of Chernyakovaetal. tronisembedded”deeper”intothestellarwind,fromtheviewpoint
(2006) and Sidolietal. (2006). In these works it was shown that
simplepowerlawwithphotoelectricabsorptiondescribesthespec-
trumofLSI+61◦303well,withnoevidenceforanylinefeatures. 3 http://xmm.vilspa.esa.es/external/xmm dataacc/xsa/index.shtml
8 A. Neronovand M. Chernyakova
1.9
1.8 X3
X6b
X6a
1.7 X6b
X6a
1.6
X5
X1
X2
1.5
X6b
X4
X6a
1.4
0.45 0.5 0.55 0.6 0.65
Figure4.Confidencecontourplotsofthecolumndensity NH andofthe Figure5. Columndensity (toppanel)andspectral slope(bottom panel)
photonspectralindexΓuncertainties forapower-lawfittoXMM-Newton dependenceonthe2-10keVFluxduringtheX6observationwithabin
observations. Thecontours give 1σ(68.3%) and 2σ (95.4%) confidence timeof2000s.Arrowsshowthedirectionoftimeevolution.
levels.
binsofdurationof2ks,extractedthespectrumofthesourcesep-
ofobserverandphotonshavetopassthroughahighermattercol- aratelyin each timebinand fittedit withthe absorbed powerlaw
umndensity. model.TheresultingdependenceofthecolumndensityN andthe
H
ThemiddlepanelofFig.3showstheorbitalevolutionofthe photon index Γ on the flux from the source is shown in top and
photonindexΓ.Onecanseethataspectralhardeningsimilartothe bottompanelsofFig.5.
onefoundinRXTEdataisobservedatthephasesΦ ∼ 120◦ (sug- InthetoppanelofFig.5onecanseethatthecolumndensity
gestedpre-periastronpassageoftheequatorial planeofthedisk). decreases when the fluxdrops. The probability that N wascon-
H
However, contrary to theRXTE observations, no softening of the stantduringthewhole50ksobservationislessthan0.03%.Such
spectrumisobservedafterthephaseΦX1 ≃300◦.Ingeneral,alarge behaviour is natural to expect within the ”clumpy wind” model:
scatterofthevaluesofX-rayfluxandphotonindexesarefoundat lower densityof theclumpsand/or smallernumber of active(i.e.
thephasesΦ > 300◦.Thiscanbeexplainedeitherbyphysicalar- illuminatedbythepulsarwind)clumpsshouldresultinthelower
guments(closetotheapastronthedensityofclumpsinthestellar luminosityofthesystem.
winddecreaseswhichleadstothesignificantvariabilityoftheflux BottompanelofFig.5showsthedependenceofthespectral
and the spectral index), or, possibly, by the low quality of X-ray index Γ on the 2 - 10 keV flux. Softening of the spectrum is ob-
data(theprecisionofthemeasurementscanbeinsufficienttomea- servedduringtheperiodofalowerfluxvalues.Suchbehaviouris
sureindependentlyNH andΓ). alsoexpectedinthe”clumpywind”model:coolingofhigh-energy
Toaddressthisquestioninmoredetails,andtoavoidthepos- electronsattheendofactivityofaclumpleadstothesofteningof
sibledegeneracyonthedeterminationofthephotonindexandcol- thespectrumofICemission,asitisdiscussedinChernyakovaetal.
umn density we show in Fig. 4 the 1σ and 2σ confidence levels (2006).
contour plotsofspectralindexΓversus N .Fromthisfigureone
H
canseethatun-freezingofN inthemodelfitdoesnotpreventthe
H
detectionofvariabilityofΓovertheorbit. 3.5 Highandlowfluxspectralstatesofthesource.
ThedirectionofthetimeevolutioninFig.5isshownwitharrows.
Inbothpanelsdatapointsclearlysplitintotwogroups,correspond-
3.4 ShorttimescalevariationsofN .
H ingtoX6a(groupofpointsontherightwithhighflux),andX6b
ThelongXMM-Newtonobservationofthesystemin2005aswell (group of points on the left with low flux) observations on the
asradioobservationsbyPeracaulaetal.(1997)revealthevariabil- Fig. 3. The qualitative behaviour of the system is somewhat dif-
ityofthefluxfromthesystemonthetimescalesofseveralkilosec- ferent in the ”high flux” and ”low flux” state. In the ”high flux”
onds.Interpretationoftheshorttimescalevariabilityinthemodel statethecharacteristicsofthesystem”stabilize”,inthesensethata
ofinteractionofthepulsarwindwiththeclumpsofthestellarwind welldefinedvalueofthephotonindex,closetoΓ≃1.5,andofN ,
H
suggeststhatboththephotonindexΓandthecolumndensity N closetoN ≃ 0.5×1022 cm−2,arefound.Tothecontrary,inthe
H H
shouldvaryatthistimescale. ”low flux” statethe systemsexhibits an irregular behaviour, with
Tostudythepossibleshorttimescalevariabilityofboth N significantlyvariableN andΓ.
H H
andΓwehavesplitthelongXMM-Newtonobservationontotime To test the distinction between the low and high flux states
Compactified pulsarwind nebulamodelof γ-ray-loudbinaryLSI +61◦ 303. 9
2 5 1.7
1
4
p5
6
3 p6
1.6 p1 p4
p3
low−hard
1.5
high
1.4
p2
low−soft
1.3
0.4 0.5 0.6 0.7
Figure6. Hardnessratioversuscountrateinthe0.3-12keVenergyrange Figure7. Confidencecontourplots(1σlevel)ofthecolumndensityNH
forallXMM-Newtonobservations(EPICpn)ofthesource,withabintime andofthephotonspectralindexΓuncertaintiesforapower-lawfittoeach
of1000s.Thehardnessratioisdefinedasratiobetweencountsinthehard 1000sofX5observation.
2-12keVandsoft0.3-2keVenergyrange.Openredsquarescorrespondto
observationX1,filledbluecirclestoX2,opengreentrianglestoX3,cyan
filedsquarestoX4,filledmagentatrianglestoX5,andopenblackcircles
toX6.Arrowsshowthedirectionoftimeevolution.Theenlargedviewof
theX5observationisshownontheinset.
3.6 Mini-flaresproducedbyactivityofindividualclumps.
The short, kilosecond time scale, variability of N can be asso-
H
we have investigated the spectral evolution of the system on the ciated with the variations of the density of the stellar wind from
1kstimescalealsointhe5shortXMM-Newtonobservations.Fig. clumptoclump.Indeed,ifthetypicaldensityoftheclumpisn ∼
6showstheresultsof our analysis. Tomake thedefinitionof the 1010 cm−3,whileitssizeisR ∼ 1011 cm,oneexpectstofind
clump
”spectralstate”ofthesystemindependentofthemodelusedtofit thefluctuationsofthecolumndensityδN ∼nR ∼1021 cm−2
H clump
thespectrum,weplotinthisfigurethemodel-independent ”hard- whentheactivityofindividualclumpsswitcheson/off.
nessratio”,asafunctionoftheX-rayflux.Onecanseethatindeed, Theactivityofindividualclumpsresultsintheshort,several
inthe”highflux”state,thehardnessratiodoesnotexhibitsignif- kilosecond,”mini-flares”observedintheX-raydata.Examplesof
icant variations, which signifies that the spectrum of the system such”mini-flares”arevisiblee.g.intheXMM-Newtonobservations
isstable.Tothecontrary,the”lowflux”statescanbedividedonto X6bandX5whichisshowninmoredetailsintheinsetintheright
”lowsoft”and”lowhard”states,correspondingtothedifferentval- topcornerofFig.6andinFig.7.Similarlytothecaseofthelong
uesofthehardnessratio. XMM-Newtonobservation,wesplitthewholeobservationX5into
Thepatternofthesourcevariabilityinthe”hardnessratiovs. 6intervalsofdurationof1ksandextractthespectraofthesource
flux”diagramturnsouttobequalitativelydifferentfromthepattern separatelyineachinterval.OnecanseethattheX-rayfluxstarted
observedinconventionalaccretingX-raybinaries(Zdziarskietal. togrowduringthesecondksinterval,reacheditsmaximuminthe
2004). Namely, the spectral evolution proceeds along the ”high- 5thintervalandsubsequentlydroppedtoitsinitialvalueduringthe
hard”→”intermediate-soft”→”low-soft”→”low-hard”→”high- interval 6. The column density N started to grow exactly at the
H
hard” loop. This loop is exactly opposite to the ”low-hard” → moment of the on-set of the mini-flare, during the interval 2 and
”intermediate-soft”→”high-soft”→”intermediate-hard”→”low- continued to grow until the flux reached its highest value during
hard”evolutionloopobservedinaccretionpoweredX-raybinaries. interval 5. The scale of the variation of N during the mini-flare
H
Theobserved evolutionpattern inthehardness ratiovs. flux enables to estimate the column density of the clump responsible
diagramcanbenaturallyexplainedinthecompactifiedPWNmodel fortheflare,N ≃1021cm−2.Remarkably,thespectrumofthe
H,clump
ofthesource.Astationaryspectrumofthe”high”stateisachieved sourcehardenedonthetimescaleof1ksatthemomentoftheonset
whenasufficientlylargenumberofthestellarwindclumpsisillu- ofthemini-flare(period2)andstabilizedatthevalueΓ≃1.6after-
minatedbythepulsar wind. Dropofinjectionof thehigh-energy wards.Suchtransienthardeningofthespectrumonthetimescale
electronsattheendofactivityoftheclumpsleadstothesoftening shorterthantheinverseComptoncoolingtime(6)canbeexplained
oftheX-rayspectrumduringthefluxdecrease.Hardspectrumin by the fact that the equilibrium electron spectrum determined by
thelowfluxstateisachievedduringthepulsarpassagethroughthe thebalanceofinjectionandcoolingratesisestablishedonlyatthe
densestpartofthestellarwind. timescalesofaboutt (6).
IC
10 A. Neronovand M.Chernyakova
4 CONCLUSIONS. JohnstonS.,Manchester R.N.,LyneA.,BailesM.,KaspiV.M.,
QiaoGuojun,D’AmicoN.,1992,ApJ,387,L37
Wehavedevelopedamodelofthe”compactified”pulsarwindneb-
MaraschiL.,TrevesA.,1981,MNRAS,194,1
ulaoftheγ-ray-loudbinarysystemLSI+61◦303.Itturnsoutthat
MirabelI.F.,RodriguezL.F.,1999,ARA&A,37,409.
thismodelcanexplaintheobservedpropertiesofthesourceinra-
Neronov A., Chernyakova M., 2006, Ap.S.S., accepted
dio, X-ray and γ-ray energy bands. In this model the maximum
[astro-ph/0610139].
of theX-ray fluxfromthesource isreached when pulsar crosses
PeracaulaM.,MartiJ.,ParedesJ.M.,1997,A&A,328,283
themiddleplaneofthedenseequatorialwindfromBestar.Max-
PulsJ.,MarkovaN.,ScuderiS.,StanghelliniC.,TaranovaO.G.,
imumoftheradioemissionisachivedwhenthepulsarwindpen-
BurnleyA.W.,HowarthI.D.,2006,A&A454,625.
etratesmostefficientlyintotheregionoffastpolarwindfromBe
RomeroG.E.,ChristiansenH.R.,OrellanaM.,2005,Ap.J.,632,
star.Maximalluminosityin100GeV-TeVenergybandisachieved
1093
whenthepropagation of very-highenergyγ-raysisleastaffected
Sidoli L., Pellizzoni A., Vercellone S., Moroni M., SMereghetti
bythepairproduction.ThenaivelyexpectedmaximaofX-rayand
S.,TavaniM.,2006,A&A,459,901
radiolightcurvesclosetothephaseofperiastronofthebinaryorbit
TaylorA.R.,GregoryP.C.,1984,ApJ,283,273
aresuppressed because of thedominance of Coulomblossinside
TavaniM.&AronsJ.,1997,Ap.J.,477,439
thedensestpartofthestellarwind.
Waters L.B.F.M., van den Heuvel E. P. J., Taylor, A. R., et al.,
AssociationoftheobservedX-raymaximumwiththeperiod
1988,A&A,198,200
ofthepulsar passagethrough thedensest partof thediskhasen-
ZamanovR.K.,MartiJ.,2000,A&A,358,L55
abledustoconstraintheorientationofthebinaryorbitwithrespect
Zdziarski A., Gierlinski M., Mikolajewska J., Wardzinski G.,
totheanisotropicwindfromBestar.Totesttheconjectureabout
SMithD.,HarmonA.,KitamotoS.,2004,MNRAS,351,791.
theorientationofthebinaryorbitwehavesearchedforthevaria-
tionsofthehydrogencolumndensity(whichcanbeassociatedwith
thepulsarpassagethroughthedensepartofthestellarwind)inthe
availableX-raydata.Wefindamarginallydetectedorbitalvariation
ofN .ThisorbitalvariationofN ,ifconfirmedwithmoreX-ray
H H
observationsaroundtheperiastron,constrainsalsotheorientation
ofthesystemwithrespecttothelineofsight.
Our re-analysis of available X-ray observations of
LSI +61◦ 303 has also revealed that the changes in the spec-
tral state of the source are accompanied by the changes in the
hydrogen column density by ∆N ∼ 1021 cm−2, not only on the
H
long(orbital)timescale,butalsoontheshort(severalkilosecond)
timescale.Thissupportsthehypothesisthattheshort,kilosecond
timescale variations of the source flux are caused by the motion
of compact object through the dense clumps of wind from the
companionBestar.
5 ACKNOWLEDGMENTS
Theauthors would like tothank M. Revnivtsev for the help with
RXTE data analysis. We are also grateful to V.Bosch-Ramon for
usefuldiscussionsofthesubject.
REFERENCES
AlbertJ.,etal.,2006,Science,312,1771.
Bosch-Ramon,V.;Paredes,J.M.;Romero,G.E.;Ribo,M.2006,
A&A,459,L25.
Chernyakova M., Neronov A., A. Lutovinov, J. Rodriguez, S.
Johnston,2006a,MNRAS,367,1201.
Chernyakova M., Neronov A., Walter R., 2006, MNRAS, 372,
1585.
DubusG.,2006,acceptedtoA&A,astro-ph/0605287
Dhawan,Mioduszewski,Rupen,2006,inProc.ofVIMicroquasar
Workshop,Como-2006
GregoryP.C.2002,ApJ,575,427
GregoryP.C.,PeracaulaM.,TaylorA.R.,1999,ApJ,520,376
Grundstrom E. D. et al., 2006, accepted to ApJ
(astro-ph/0610608)
Harrison F., Ray P.S., Leahy D., Waltman E.B., Pooley G.G.,
2000,ApJ,528,454