Spectral energy distribution of the gamma-ray microquasar LS 5039

0.2 MB·English

by  J. M. Paredes

Astronomy & Astrophysics manuscript no. lsastro February 5, 2008
(DOI: will be inserted by hand later)
Spectral energy distribution of the γ-ray microquasar LS 5039
J. M. Paredes1, V. Bosch-Ramon1, and G. E. Romero2,3,⋆
1 Departamentd’AstronomiaiMeteorologia,UniversitatdeBarcelona,Av.Diagonal647,08028Barcelona,Spain;
[email protected], [email protected].
2 Instituto Argentino de Radioastronom´ıa, C.C.5, (1894) Villa Elisa, Buenos Aires, Argentina;
6 [email protected].
0 3 Facultad de Ciencias Astron´omicas y Geof´ısicas, UNLP, Paseo del Bosque, 1900 La Plata, Argentina;
0 [email protected].
2

n thedate of receipt and acceptance should beinserted later
a
J
Abstract. ThemicroquasarLS5039hasrecentlybeendetectedasasourceofveryhighenergy(VHE)γ-rays.This

5 detection,thatconfirmsthepreviouslyproposedassociationofLS5039withtheEGRETsource3EGJ1824−1514,

makes of LS 5039 a special system with observational data covering nearly all the electromagnetic spectrum. In
2
order to reproduce the observed spectrum of LS 5039, from radio to VHE γ-rays, we have applied a cold matter
v
dominated jet model that takes into account accretion variability, the jet magnetic field, particle acceleration,
5
adiabaticandradiativelosses,microscopicenergyconservationinthejet,andpaircreationandabsorptiondueto
9
theexternalphotonfields,aswellastheemissionfromthefirstgenerationofsecondaries.Theradiativeprocesses
0
9 taken into account are synchrotron, relativistic Bremsstrahlung and inverse Compton (IC). The model is based
0 on a scenario that has been characterized with recent observational results, concerning the orbital parameters,
5 theorbitalvariabilityatX-raysandthenatureofthecompactobject.Thecomputedspectralenergydistribution
0 (SED) shows a good agreement with theavailable observational data.
/
h
p Key words. X-rays: binaries – stars: winds, outflows – γ-rays: observations – stars: individual: LS 5039 – γ-rays:
- theory
o
r
t
s 1. Introduction who place the system at a distance of 2.5±0.1 kpc. The
a
: radius of the companion star is moderately close to the
v Microquasars are radio emitting X-ray binaries with rel-
Lagrangepointduringperiastronbutwithoutoverflowing
i
X ativistic radio jets (Mirabel & Rodr´ıguez 1999). LS 5039 the Roche lobe. In addition, Casares et al. (2005) state
wasclassifiedasmicroquasarwhennon-thermalradiation
r that the compact object in LS 5039 is a black hole of
a produced in a jet was detected with the VLBA (Paredes mass MX = 3.7+−11..30 M⊙. At X-rays, the spectrum of the
et al. 2000). New observations conducted later with the
source follows a power-law and the fluxes are moderate
EVN and MERLIN confirmed the presence of an asym-
and variable, with temporal scales similar to the orbital
metric two-sided persistent jet reaching up to ∼1000 AU
period.Thevariabilityislikelyassociatedwithchangesin
on the longest jet arm (Paredes et al. 2002, Rib´o 2002).
theaccretionrateduetothemotionofthecompactobject
LS 5039 is a high mass X-ray binary (Motch et al. 1997)
along an eccentric orbit (Bosch-Ramon et al. 2005a).
whose optical counterpart is a bright (V ∼ 11) star of
spectral type O6.5V((f)) (Clark et al. 2001). Although Among microquasars,LS 5039is specially relevantfor
McSwain et al. (2001, 2004) first derived the period and its proposed association with 3EG J1824−1514 (Paredes
theorbitalparametersofthesystem,newvaluesoftheor- etal.2000),anunidentifiedsourceinthe3rdEGRETcat-
bitalperiod(P =3.9060±0.0002days,withperiastron alog of high-energy γ-ray sources (>100 MeV; Hartman
orb
passageassociated to T =HJD 2451943.09±0.10),eccen- et al. 1999). Recently,Aharonianet al.(2005a), using the
0
tricity(e=0.35±0.04),phaseofperiastronpassage(0.0), High Energy Stereoscopic System (HESS), have detected
and inclination (assuming pseudo-synchronization, i = LS 5039 at energies above 250 GeV. This detection, that
24.9±2.8◦) have been reported by Casares et al. (2005), confirmsthehigh-energyγ-raynatureofLS5039(Paredes
et al. 2000), gives strong support to the idea that mi-
Send offprint requests to: J.M. Paredes croquasars are a distinctive class of high- and very high-
e-mail: [email protected] energy γ-ray sources. The confirmation of LS 5039 as a
⋆ Member of CONICET high-energyγ-raysource,combinedwiththe factofbeing
2 Paredes, Bosch-Ramon & Romero: Broad-band emission from LS 5039
arunawaymicroquasarejectedfromtheGalacticplaneat EGRET sources (Collmar et al. 2003, Zhang et al. 2005).
∼150km s−1 (Ribo´ etal.2002), stronglyraisesthe ques- Thehigh-energyγ-raydatahavebeentakenfromthethird
tion about the connection between some of the remain- EGRETcatalogofhigh-energyγ-raysources(>100MeV;
ing unidentified, faint, variable, and soft γ-ray EGRET Hartman et al. 1999). The VHE γ-ray (>100 GeV) data,
sources and possible microquasar systems above/below obtained with HESS, are from Aharonian et al. (2005a).
the Galactic plane (e.g., Kaufman Bernad´o et al. 2002, The values of the magnitudes and fluxes have been trans-
Romero et al. 2004, Bosch-Ramon et al. 2005b). formed in luminosities using the updated value for the
LS 5039 was also proposed to be the counterpart of a distance of 2.5 kpc (Casares et al. 2005). All these obser-
COMPTEL source (∼1 MeV) (Strong et al. 2001) and vational data are plotted in Fig. 3.
BATSE detected it at soft γ-rays (Harmon et al. 2004).
In addition to LS 5039, there is another microquasar,
2.1. X-ray lightcurve
LS I +61 303 (Massi et al. 2004), that is likely associ-
ated with an EGRET source (Kniffen et al. 1997). Both The X-ray lightcurve of LS 5039 observed by RXTE,
microquasars have been extensively studied at different foldedinphasewiththeneworbitalperiodvalue,showsa
wavelengths,andbothhaveaverysimilarspectralenergy clear smooth peak atphase 0.8 and another marginalone
distribution. Bosch-Ramon & Paredes (2004a,b) have ex- atphase0.3(Bosch-Ramonetal.2005a).Thislightcurveis
plored with a detailed numerical leptonic model whether hardly explained by accretion through a spherically sym-
these systems can produce the level of emission detected metric wind of a typical O type star, due to the high ve-
by EGRET (>100 MeV), and the observed variability. locitiesofthewindatinfinity,andalsobecauseofthecon-
At VHE, Aharonian et al. (2005b) argue in favor of straints on the stellar mass loss rate (<∼ 10−6 M⊙ yr−1,
hadronicoriginofTeVphotonsincasethatγ-raysarepro- Casares et al. 2005). It has been proposed that certain
duced within ∼ 1012 cm, and Dermer & B¨ottcher (2005) asymmetriesinthewindcouldproducethesepeaksinthe
point to combined star IC and synchrotron self-Compton X-ray lightcurve (Bosch-Ramon et al. 2005a).
(SSC) emission to explain the different epoch detections
by EGRET and HESS, respectively.
3. Modeling
To explain both, the observed spectrum of LS 5039
from radio to VHE γ-rays and the variability, we have
3.1. An accretion model for LS 5039
applied a broad-band emission model, based on a freely
expandingmagnetizedjetdynamicallydominatedbycold To reproduce approximately the variations in the X-rays,
protons and radiatively dominated by relativistic leptons we have assumed the existence of a relatively slow equa-
(Bosch-Ramonet al. 2005c). In this paper we present the torial wind (outflowing velocity ∼ 6×107 cm s−1) that
results obtained after applying such model to LS 5039. would increase the accretion rate up to the (moderate)
The compiledobservationaldatacoveringthe full electro- levels required by the observed emission in the context
magnetic spectrum is given in Sect. 2. In Sect. 3 a brief of our model, and would produce a peak after perias-
descriptionofthemodelisprovided,andtheparametersof tron. Although the existence of low velocity flows is not
the modelarediscussed.TheresultsobtainedforLS5039 yet well established, it seems that it would not be a rare
are presented in Sect. 4, and the work is summarized in phenomenon among the massive wind accreting X-ray bi-
Sect. 5. naries. This is, for instance, the case of the O9.5V star
BD +53◦2790, whose ultraviolet spectrum reveals an ab-
normally slow wind velocity for its spectral type (Ribo´
2. Observational data of LS 5039
et al. 2005). Only a small fraction of the wind being
We compile here the observational data from the lit- slower at the equator is enough to increase significantly
erature. The radio data, obtained at 20, 6, 3.5 and the accretion rate along the orbit if one assumes pseudo-
2 cm wavelengths with the VLA, are from Mart´ı synchronization. In this case, the orbital plane and the
et al. (1998). The optical and infrared data, covering the plane perpendicular to companion star rotation axis will
bands UBVRIJHKL, are from Clark et al. (2001) and be likely the same (Casares et al. 2005), allowing for the
Drilling (1991), and have been corrected for absorption compact object to accrete from this slow equatorial wind
(A = 4.07, Casares et al. 2005). The X-ray data (3– all along the orbit.
V
30 keV), obtained with RXTE, are from Bosch-Ramon To explain the second peak we have introduced a
et al. (2005a), being diffuse background subtracted. The stream,withavelocityof5×106cms−1andanadditional
(20–430 keV) data, obtained with the BATSE Earth oc- masslossrateof∼1%ofthe normalwind,thatisformed
cultation technique, are from Harmon et al. (2004). The nearperiastronpassagewhentheattractionforcebetween
(1–30 MeV) data are from Zhang et al. (2005), obtained thetwocomponentsofthesystemisatitsmaximum.This
with COMPTEL, whose flux values, however, should be slower matter stream would produce a very delayed peak
taken as an upper limit. This is due to the source region at phase 0.7. The formation of a matter stream, due to
defined by the COMPTELγ-raysource,GRO J1823−12, gravitational stress at the smallest orbital distances, is
at galactic coordinates (l=17.5◦, b = −0.5◦), contains notunusualinclosebinarysystemslikeLS5039.Wehave
in addition to LS 5039/3EG J1824−1514 two additional used the approach followed by Leahy (2002) to explain
Paredes, Bosch-Ramon & Romero: Broad-band emission from LS 5039 3
energy, directly related to the kinetic energy carried by
8 shocks in the plasma. The number of relativistic particles
that can be produced along the jet is constrained by the
7 limited capability of transferring energy from the shock
itself to the particles,and by the fact that the relativistic
M/yr]sun 6 ppraerstsiculreemmauxsitmbuemkeepnterbgeyloiwsltihmeitceodldbpyrtohteonacpcreelsesruarteio.nTehfe-
−9x10 5 fiTchieenaccycealnedratthioenloecffialcieennecrygydelopsesnedssinonthsehaovcakilapbrolepesprtaicees:.
dt [ shock strength, shock velocity, magnetic field direction,
/
Macc(cid:19) 4 and diffusion coefficient (e.g., see Protheroe 1999). It has
d been parametrized due to the lack of knowledge of the
specific details of the shock physics for this case.
3
2
0 0.2 0.4 0.6 0.8 1 Inourscenario,thejetformationismainlyconstrained
orbital phase
by a characteristic launching radius, where the ejected
matter gets extra kinetic energy from the accretion reser-
Fig.1. Accretion curve along an orbital period inferred
voir, and by the accretion energy reservoir itself, which
from the observed X-ray lightcurve in Bosch-Ramon
varies along the orbit. The modeling of the accretion rate
etal.(2005a),adoptingtheaccretionmodeldevelopedfor
changesallowstointroducevariabilityinaconsistentway.
this work.
Moreover, since the interaction angle between jet leptons
andstarphotonschangeswith the orbitalphase,variabil-
theX-raylightcurveofGX302–1,alsowithapeakbefore ity naturally appears at very high energies (see below).
periastronpassage.Although there is still a shift of 0.1in Finally, we have approximately computed the effects pro-
phase between the model peaks and the observed ones, it duced on the SED by the photon absorption under the
is importantto note that there is likely anaccretiondisk, external photon fields as well as the IC radiation of the
andthe matter thatreachesthe disk spends some time in first generation of secondaries inside the binary system.
ituntilpartofthismatterisejectedasajet(alower-limit In Fig. 2, we show the photon absorption coefficient as a
fortheaccretiontimeisthefreefalltime,implyingashift function of the photon energy, at different distances from
of ∼0.06 in phase, when falling from 1012 cm). The ac- the compact object, during periastron and apastron pas-
cretion rate evolution obtained from the model described sages. It is seen that the corona and disk photon fields
here is shown in Fig. 1. (regions of ∼ 108 cm) attenuate slightly the high-energy
γ-ray photons in the inner part of the jet. At VHE, star
photonabsorptiondominatesfordistances<∼1013cm,i.e.,
3.2. A leptonic jet model for LS 5039 broad-band
within the binary system. At greater distances, absorp-
emission
tion goes down to negligible values. Photon absorption is
A new model based on a freely expanding magnetized higher at periastron than at apastron passage,due to the
jet, whose internal energy is dominated by a cold proton dependence of the stellar photon density with the orbital
plasma extracted from the accretion disk, has been de- distance, although for large distances from the compact
veloped in a previous work (Bosch-Ramon et al. 2005c). object,the opacitiesatbothorbitalphases becomeequal.
The cold proton plasma and its attached magnetic field, Our results are similar to those obtained by B¨ottcher &
thatis frozento matter andbelow equipartition,provides Dermer (2005) and Dubus (2005), that present a detailed
aframeworkwhereinternalshocksaccelerateafractionof analysisofthe effectofγγ absorptionofVHE γ-raysnear
theleptonsuptoveryhighenergies.Theseacceleratedlep- the base of the jet of LS 5039. Moreover, the energy de-
tons radiate by synchrotron, relativistic Bremsstrahlung pendence of the opacity on the ambient photon fields to
with the stellar wind (external) and within the jet (inter- the γ-ray propagation and its variation with the distance
nal), and IC processes. In this model, the external seed is not much different from that obtained for the micro-
photons that interact with the leptons of the jet by IC quasarLSI +61303atits periastronpassage(Romero et
scatteringcome fromthe star,the disk and the corona.A al. 2005).
blackbody spectrum is assumed for the star and the disk,
and a power-law plus an exponential cut-off spectrum for
the corona.The SSC radiationhasalsobeencomputedas Finally, we note that our model does not take into ac-
well as the Bremsstrahlung and Compton self-Compton count the radiative contribution from relativistic protons
radiation, being the radiation of the last two mechanisms that could be accelerated along with the leptons. These
negligible. protons can cool through interactions with ions from the
The dissipated shock kinetic energy that goes to rela- stellar wind producing additional γ-ray emission at high
tivisticparticlescomesfromthemeanbulkmotionkinetic energies (Romero et al. 2003, 2005).
4 Paredes, Bosch-Ramon & Romero: Broad-band emission from LS 5039
2 Table 1. Primary parameter values.
108 cm
1012 cm
1 1014 cm
1015 cm Parameter and unit value
periastron e:eccentricity 0.35a
apastron a:orbital semi-major axis [cm] 2.2×1012 a
0 θ: jet viewing angle [◦] 24.9a
τ)γγε m˙w:stellar mass loss rate [M⊙ yr−1] ∼7×10−7 a
log ( −1 MR⋆x::sctoemllaprarcatdoibusje[cRt⊙m]ass [M⊙ ] 93..37aa
M⋆: stellar mass [M⊙] 22.9a
L⋆: stellar bolometric luminosity [erg s−1] 7×1038 a
−2 T⋆: stellar surface temperature [K] 3.9×104 a
p:electron power-law index 2.2b
c
Γjet: jet injection Lorentzfactor 1.1
χ:jet semi-opening angle tangent 0.1c
−3
8 9 10 11 12 13 kT : disk innerpart temperature[keV] 0.1
disk
log (photon energy [eV])
pcor: corona photon index 1.6
z0: Jet initial point [RSch] 50
Fig.2. The photonabsorptioncoefficient as a function of κ: jet-accretion rate parameter 0.1d
the energy for photons emitted in the jet at different dis-
tances from the compact object, at periastron and apas- a b c
Casares et al. (2005), Bosch-Ramon et al. (2005a), Paredes
tron passages. d
et al. (2002), Fender(2001), Hujeirat (2004).
3.3. Parameters of the model
Table 2. Secondary parameter values.
Themodelinvolvesalargenumberofparametersbecause
it aims at reproducing the broad-band spectrum by tak-
ingintoaccountseveralphysicalmechanisms.However,in Parameter and unit value
the case of LS 5039 an important fraction of the param-
ξ: shock energy dissipation efficiency 0.5
eters are well established from observational data or can
fB: B to cold matter energy density ratio 0.01
beeasilyestimatedfromsomereasonableassumptions.We η: acceleration efficiency 0.1
call these parameters ‘primary parameters’. The remain- Vw:equatorial wind velocity [cm s−1] 6×107
ing ‘secondary’ parameters can be reduced to a manage- m˙jet: jet injection rate [M⊙ yr−1] 4×10−10
ablenumber.InTable1wepresenttheprimaryparameter ζ: max. ratio hot to cold leptons 0.1
valuesofourmodel.The firsthalfofthem arethe param- rl: launchingradius [RSch] 9
eters describing the binary system and their components. Vstream: periastron stream velocity [cm s−1] 5×106
WeadoptupdatedvaluestakenfromCasaresetal.(2005).
The electron power-law index (inferred from X-ray pho-
ton indices found by Bosch-Ramon et al. 2005a), the jet
ejectionLorentzfactor(Paredesetal. 2002)andthe tan- producedifferentspectralandvariabilitypropertiesofthe
gent of the jet semi-opening angle (Paredes et al. 2002) source. They are summarized in Table 2. The shock en-
are parameters estimated and/or adopted previously by ergy dissipation efficiency, ξ, that gives the energy dissi-
other authors. Regarding the jet ejection Lorentz factor, pated by shocks in the jet, has been taken(Bosch-Ramon
although it seems well established that the jet is mildly et al. 2005c) to be at most the half of the kinetic energy
relativistic (Paredes et al. 2002), no direct jet velocity es- of the shock to get enough luminosity at energies beyond
timate hasbeenobtaineduntilnowandwehavefixedthe X-rays1. The ratio of the magnetic field (B) energy den-
Lorentzfactorto1.1.Thevaluesadoptedforthediskinner sity to the cold matter energy density has been fixed to
parttemperatureandthecoronaphotonindexcorrespond fB =0.01takingintoaccountX-rayflux constraintsfrom
totypicalvaluesfoundinothermicroquasars,becausethe observations (Bosch-Ramon et al. 2005a). The accelera-
X-rayspectrumofLS5039onlyprovidesclearupperlimits tion efficiency parameter, η (×qeBc), has been fixed such
totheluminositiesofbothdisk(L ≃1034 ergs−1)and that the maximum energy photons reach the energies at
disk
corona(L ≃1034 ergs−1) (Bosch-Ramonetal.2005a). which the source has been detected by HESS. This value
cor
The jet-accretion rate parameter, κ, that gives the ratio
1 This does not mean that half of the shock kinetic energy
between the jet matter injection rate and the accretion
is dissipated through shocks, but that it might be so in case
rate,hasbeentakentobesimilartoobservationalandthe-
that the energy losses would require it, i.e., the shock could
oretical estimates (see, e.g., Fender 2001, Hujeirat 2004). be radiatively efficient up to a 50% of the kinetic energy in
Next, we describe the secondary parameters of the the regions where energy losses are very strong (the total jet
model, which have been adopted in order to better re- energy lost through radiation is about 15%).
Paredes, Bosch-Ramon & Romero: Broad-band emission from LS 5039 5
for η is in agreement with that of a strong and transrela- thejet,ourcomputedfluxesat5GHzareabout10,6and
tivisticshockwithamagneticfieldparalleltoitsdirection 0.1 mJy for the inner part of the jet (< 1 AU), at mid-
of motion and a diffusion coefficient close to the Bohm dle distances fromthe origin(1–1000AU) andbeyond(>
limit (Protheroe 1999). The stellar wind velocity in the 1000 AU), respectively. This roughly reproduces the ex-
vicinity of the compact object (V ), estimated in Sect. 2, tended emission of the radio source observed using VLBI
w
is likely affected by peculiarities of the star (e.g., close to techniques (Paredes et al. 2002).
but not suffering Roche-lobe overflow, fast rotation, etc; AttheX-rayband,mostoftheemissionissynchrotron
seeCasaresetal. 2005).Thisvelocityimplies,alongwith radiation coming from the relativistic electrons of the jet
the knownstellar mass loss rate (see Table 1) and assum- and only a marginal contribution comes from the disk
ing isotropic flux, a mean accretion rate along the orbit and almost a null contribution from the corona compo-
of m˙acc ∼4×10−9 M⊙ yr−1 (i.e. ∼5% of the Eddington nents (see also, e.g., Markoff et al. 2001). Although the
rate).Fromthis value andκ, the jet matter injectionrate XMM-Newton data does not show evidences of a thermal
canbederived:m˙jet ∼4×10−10M⊙ yr−1.Inordertotry diskcomponent(Martocchiaetal.2005),wehaveadopted
tosatisfyboth,the assumptionthatthe jetiscoldmatter in our model a disk with moderate luminosity. The role
dominated and the observed optically thin nature of the played in the model by this component is negligible, and
radio spectrum, we have fixed the maximum number of its contribution appears in Fig. 3 as a small bump over
relativistic leptons2 to be ζ = 0.1 of the total number of the power-law spectrum below 1 keV. The computed X-
particles in the jet. This is higher than the value used by rayluminositiesandphotonindicesareveryclosetothose
Bosch-Ramon et al. (2005c). The implications of this are detected so far (see next section and also Bosch-Ramon
discussed in Sect. 4. Finally, the launching radius can be et al. 2005a).
estimatedtobe∼9R ,sincetheenergytoejectthejetis Gallo et al. (2003) have proposed that radio emitting
Sch
assumedtobeaccretionenergytransferedandtheLorentz X-raybinaries,wheninthelow-hardstate,followapartic-
factor is roughly known (Bosch-Ramon et al. 2005c). ular correlation concerning radio and X-ray luminosities.
We note that we are not fitting data but reproducing Inthecontextofthiscorrelation,LS5039isprettyunder-
a broadband spectrum as a whole. This implies that the luminousatX-rays,presentingapermanentlow-hard-like
values of the parameters used by the model are approx- X-raystate.Thiscouldbeexplainedbythefactthatitsra-
imated. The parameters in Tables 1 and 2 not obtained dioemissionis opticallythin,comingmainlyfromregions
directly from observations have a range of validity within outsidethebinarysystem,insteadofbeingopticallythick
10% for p, Γ , kT , p , V and V ; and within a and completely core dominated (as in the case of mod-
jet disk cor w stream
factor of a few for z0, κ, ξ, fB, η, m˙jet, ζ and rl. els such as the one presented by Markoff et al. 2001, see
also Heinz & Sunyaev 2003). As noted above,it might be
qualitatively explained by the increasein relativistic elec-
4. Results
trons predicted by our model, although the issue requires
4.1. Spectral energy distribution of LS 5039 further investigation since our radio spectrum is still too
hard.
Figure3showsthe computedSED(continuousthickline)
At BATSE and COMPTEL energies, the main con-
of LS 5039 at the periastron passage (phase 0.0) using
tributionstill comesfromsynchrotronemissionofthe jet.
the physicalparameterslistedin Tables 1 and2.We have
Thereisasmalldisagreementbetweenthepredictedlumi-
plotted also the contributions from the star, disk, corona
nosityandtheoneobservedbyBATSE.However,wenote
andsynchrotronradiationaswellas the inverseCompton
that these data are averaged along many days (Harmon
emissionandrelativistic Bremsstrahlung.Inthe samefig-
et al. 2004), whereas the plotted SED is computed at
ure we plot also the observationaldata.
phase 0.0, when the accretion is significant though not
There is a general agreement between the model and
the highest. At the COMPTEL energy range, as noted
the observed data, although there are some discrepancies
above,the moderate disagreementbetween the computed
that require some explanation. First at all, we note that
and observed data is likely produced by the fact that
the multiwavelength data were not taken simultaneously,
the MeV radiation coming from the COMPTEL source
and this can be an important source of discrepancy for GROJ1823−12,to whichLS5039is associated,mightbe
any time-dependent model.
a superposition of the MeV emissions of the three differ-
Attheradioband,thecomputedspectrumhasaspec-
ent EGRETsourcesthatare inthe fieldof view (Collmar
tral index of 0.3, instead of the observed ∼0.5 (Mart´ı
et al. 2003). This means that the data from COMPTEL
et al. 1998; where Fν ∝ ν−0.5). It seems that an addi- shouldbeactuallytakenasanupperlimit.Insuchacase,
tional process, not contemplated in our scenario, rises up
our computed values would be consistent with the data.
thenumberofelectronsemittingopticallythinradioemis-
Athigh-energyγ-rays(0.1–10GeV),thecomputedlu-
sion(e.g.,interactionswiththewindortheenvironment).
minosities are similar to those observedby EGRET while
Concerningthespatialdistributionofradioemissionalong
thecomputedphotonindicesareslightlyharder.Thisfact
2 The numberof relativistic leptonsis fixedbyξ,ζ, andthe could be naturally associatedwith the low temporal reso-
condition that their relativistic pressure cannot dominate the lutionofEGRET,withtheshortestviewingperiodsspan-
cold particle pressure. ning for one week and providing time averaged spectra
6 Paredes, Bosch-Ramon & Romero: Broad-band emission from LS 5039
38
star
VLA RXTE BATSE COMPTEL EGRET HESS
36
) star IC
]
s
/
g 34
r sync.
e
[ corona

ε
L SSC
ε
(
g
o 32 corona IC
l
int. Bremsstr.
disk
30
ext. Bremsstr.
disk IC
28
−5 −3 −1 1 3 5 7 9 11 13
log (photon energy [eV])
Fig.3. ComparisonofthespectralenergydistributionofLS5039computedfromthepresentmodel,attheperiastron
passageandusingtheparametersofTable1,withtheobserveddata.Weshowthedifferentcomponentsoftheemission
aswellasthesumofallofthethem,whichisattenuatedduetoγγabsorption(solidline).Thepointsobservedarefrom
Mart´ı et al. (1998) (VLA, diamond), Clark et al. (2001) and Drilling (1991) (optical, circle, corrected of absorption),
Bosch-Ramon et al. (2005a) (RXTE, square), Harmon et al. (2004) (BATSE, diamond filled), Collmar et al. (2003)
(COMPTEL,squarefilled),Hartmanetal.(1999)(EGRET,circlefilled)andAharonianetal.(2005a)(HESS,triangle
down filled). The arrows in the EGRET and HESS data represent upper limits (3σ).
that could mask harder ones. We note however that, be- the periastron passage due to a higher photon density as
cause of the possible effect of pair creation in the jet, the well as a smaller interaction angle between the electrons
slopes of the SEDs athigh-energyγ-rays,unlike the com- and the photons, which influences the IC scattering. The
putedluminosities,arenotveryreliable(seeBosch-Ramon contribution from the the SSC component is minor, al-
et al. 2005c). The dominant component at this band of thoughis more significantatapastronthanatperiastron.
thespectrumisstarIC,whereasthesynchrotronradiation The VHE spectrum is strongly affected by the γγ opac-
is only important at the lower part of EGRET energies. ity due to pair creation within the binary system, being
Theemissionofsecondarypairscreatedwithinthebinary the absorption stronger at the periastron passage. It ap-
system is not significant. pears therefore unavoidable an important decrease of the
emission as well as an additional source of variability at
AtVHEγ-rays,starICemissiondominates.Thespec-
those energies. Also, a hardening of the emission, as seen
trum reaches several TeV, and it is limited by the max-
in Fig.3,couldtake place.Our modelunderestimates the
imum particle energy, which can reach ∼ 10 TeV. The
TeV emission, although the difference is less than one or-
contribution of the star IC scattering is favored during
Paredes, Bosch-Ramon & Romero: Broad-band emission from LS 5039 7
derofmagnitude.Possibly,somehighenergyprocessesin-
30.70
volvingparticleaccelerationandemissionaretakingplace 5 GHz
inthejetatmiddlescales,wherethe100GeVphotonab- 30.30
sorption is not important. This could also be related to
29.90
the opticallythinnature ofthe radioemission.Thisouter
emission would not be as variable as the emission from
29.50
the compact jet. In any case, a deeper treatment at this 34.70
5 keV
energy band concerning VHE emission and pair creation
34.50
phenomena is required (Khangulyan & Aharonian 2005).
The results obtained by applying other leptonic mod-
34.30
e2l0s05(e).gd.oBnoostchd-iRffearmsounbsettanatli.a2ll0y0f5rbo,mDtehremceorm&prBeh¨oettncshiveer erg/s]) 34.10
model presented here, which can explain the broad-band L [ε 34.40 300 MeV
ε
emission from LS 5039. Otherwise, hadronic models can g ( 34.00
o
reproduce the emission at VHE energies (Romero et al. l
33.60
2003) though it is hard to explain how typical EGRET
fluxescouldbe achieved(Romeroetal.2005).Comparing 33.20
32.40
both types of model, it seems that the leptonic ones can
500 GeV
explainbetterthanthehadroniconesthebroadbandspec-
32.20
trumatleastupto∼1GeV,beingbothsuitableathigher
energiessincetheenergeticrequirementsarenotasstrong 32.00
as those to explain the whole emission from the source.
31.80
0.0 0.2 0.4 0.6 0.8 1.0
orbital phase
4.2. Variability properties of LS 5039
Fig.4. Thepanelshowsthefluxvariationsalongtheorbit
Once the accretion model has been applied to reproduce
for the four energy bands considered here. We note that
roughlytheX-raylightcurve,weexplorewhatarethecon-
emissionatveryhigh-energyγ-raysisstronglyattenuated
sequences at the other bands of the spectrum. In Fig. 4
by the photon absorption in the star photon field.
we show the flux variations along the orbit at four en-
ergy bands. At radio and X-ray energies, the predicted
flux variations are well correlated with accretion, since At phases 0.2-0.3,a smaller peak is seen, which might be
they have similar shapes to the accretion orbital curve detected eventually in future VHE observations. We note
(see Fig. 1). The predicted amplitude variations are of that, as it is seen in Fig. 2, if TeV emission comes from
about one order of magnitude in radio and a factor of distancestypicallylargerthanthesemi-majororbitalaxis
three at X-rays, although we note that, if the radio emit- itwouldnotbesignificantlyattenuatedbyphotonabsorp-
ting region is large enough, say 1015 cm, the variations tion. In such a case, variability would be limited by the
at these wavelengths will be significantly smoothened by size of the emitting region.
light crossingtime limitations (as it seems to be the case,
see Rib´o (2002). The variations of the radio spectral in-
5. Comments and conclusions
dicesalongtheorbitarenottreatedhere,duetothemodel
limitations at radio. However, we mention that the X- We have applied to the microquasar LS 5039 a cold mat-
ray photon index shows a trend similar to that found by terdominatedjetmodelthattakesinto accountaccretion
Bosch-Ramonetal.(2005a),i.e.higherfluxeswhenharder variability,thejetmagneticfield,particleacceleration,mi-
spectra. Nevertheless, the anticorrelation is not as strong croscopicenergyconservationinthejet,andpaircreation
as that observed. Emission at EGRET energies varies al- and absorption due to the external and internal photons,
most one order of magnitude, although the shape of the as well as the emission from the first generation of secon-
lightcurve is different because of the higher rate of IC in- daries under the external photon fields.
teractionduringphase0.0duetoangulareffects.AtVHE, ThecomputedSEDfromradiotoVHEγ-rayshasbeen
however, the emission is strongly absorbed by the stel- compared with the available observationaldata, gathered
lar field, producing a dip around periastron passage. At atdifferent bands of the electromagnetic spectrum,show-
phases0.6–0.9,thereisalongstandingpeakthatisrelated ing in general a good agreement. Although the model is
to the stream related peak at phase 0.7 (0.8) that is not limited by its phenomenological nature, we can extract
close enough to the stellar companion for banishing due some general conclusions that could be useful to under-
to γγ absorption. A similar peak seems to be present in standthephysicsbehindtheseobjectsaswellasforgoing
HESSdata(Aharonianetal.2005a)whenfoldedinphase deeperinmorebasictheoreticalaspectsconcerningthejet
withthe new ephemerisofCasaresetal.(2005),although plasmacharacteristics,particleaccelerationandjetforma-
furtherobservationsshouldbecarriedouttoconfirmthis. tionphenomena.Inparticular,inthecontextofourmodel,
8 Paredes, Bosch-Ramon & Romero: Broad-band emission from LS 5039
wecanstatethatthejetsareradiativelyefficient(∼15%) Dermer, C.D. & B¨ottcher, M. 2005, ApJ, submitted,
andsitesofparticleaccelerationuptomulti-TeVenergies. (astro-ph/0512162)
Otherwise, accretion processes are radiatively inefficient. Drilling, J.S., 1991, ApJS, 76, 1033
The strong absorption of VHE γ-rays by the stellar Dubus,G., 2005, A&A,submitted,(astro-ph/0509633)
Fender, R.P. 2001, MNRAS,322, 31
photon field seems to make hard the detection of high
Gallo, E., Fender, R. P., & Pooley, G. G. 2003, MNRAS,344,
fluxes of radiation if they are generated in the inner re-
60
gions of the jet, closest to the compact object. Possibly,
Harmon, B. A., Wilson, C. A., & Fishman, G. J. et al. 2004,
since the source is not very faint at these energies, some
ApJS,154, 585
high energy processes involving particle acceleration and
Hartman, R. C., Bertsch, D. L., & Bloom, S. D. et al. 1999,
emissionaretakingplaceinthejetatmiddlescales,where ApJS,123, 79
the 100 GeV photon absorption is not important. This Heinz, S. & Sunyaev,R. A.2003, MNRAS,343, L59
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Changesintheaccretionrateduetotheorbitaleccen-
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Leahy,D. A.2002, A&A,391, 219
thespectralandvariabilitypropertiesobservedatX-rays.
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It hints to a companion stellar wind that is not spheri-
Mart´ı, J., Paredes, J. M., & Rib´o, M. 1998, A&A,338, L71
cally symmetric nor fast in the equator, since otherwise
Martocchia,A.,Motch,C.,Negueruela,I.2005,A&A,430,245
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ferentparameters,our model seems to be goodenoughas Wingert, D. W.2001, ApJ, 558, L43
todescribethegeneralfeaturesofthespectrumofLS5039 McSwain,M.V.,Gies,D.R.,Huang,W.,Wiita,P.J.,Wingert,
fromradiotoTeVγ-rayenergies,theleptonicmodelbeing D.W., & Kaper, L. 2004, ApJ, 600, 927
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Acknowledgements. We thank Mark Rib´o for fruitful discus- Paredes, J.M., Mart´ı, J., Rib´o, M., Massi, M. 2000, Science,
sions as well as useful comments and suggestions on the sub- 288, 2340
ject studied here. J.M.P. and V.B-R. acknowledge partial Paredes, J. M., Rib´o, M., Ros, E., et al. 2002, A&A393, L99
support by DGI of the Ministerio de Educaci´on y Ciencia Protheroe, R.J.1999, in:Topics in Cosmic-Ray Astrophysics;
(Spain) under grant AYA-2004-07171-C02-01, as well as addi- Horizons in World Physics, Vol. 230, Edited by Michael
tionalsupportfromtheEuropeanRegionalDevelopmentFund A.DuVernois.PublishedbyNovaSciencePublishers,Inc.,
(ERDF/FEDER). During this work, V.B-R has been sup- Commack, New York,1999, p.247
ported by the DGI of the Ministerio de (Spain) underthe fel- Rib´o, M. 2002, PhD Thesis, Universitat deBarcelona
lowship BES-2002-2699. G.E.R is supported by the Argentine Rib´o, M., Paredes, J. M., Romero, G. E., et al. 2002, A&A,
Agencies CONICET (PIP 5375) and ANPCyT (PICT 03- 384, 954
13291). Rib´o, M., Negueruela, I., Blay, P., Torrej´on, J. M., Reig, P.
2005, A&A,in press
Romero, G. E., Torres, D. F., Kaufman Bernad´o, M. M., &
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