Jet interactions in massive X-ray binaries
To appear in “Massive Stars: Fundamental Parameters and Circumstellar Interactions (2007)”RevMexAA(SC)
JET INTERACTIONS IN MASSIVE X-RAY BINARIES
Gustavo E. Romero 1,2
RESUMEN
Los sistemas binarios masivos de rayos X están formados por un objeto compacto que acreta materia del viento
estelar de una estrella donante de tipo temprano. En algunos de estos sistemas, llamados microcuásares, chorros
de part́ıculas relativistas son eyectados desde las cercańıas del objeto compacto. Estos chorros interactúan
con el campo de fotones de la estrella compañera, con el viento estelar, y, a grandes distancias, con el medio
interestelar. En este trabajo se resumirán los principales resultados de tales interacciones con especial énfasis en
la producción de fotones de alta enerǵıa y neutrinos. El caso de algún sistema particular, como ser LS I +61 303,
será discutido con algún detalle. Además, se presentarán las perspectivas futuras para observaciones a diferentes
longitudes de onda para este tipo de objetos.
ABSTRACT
Massive X-ray binaries are formed by a compact object that accretes matter from the stellar wind of an
early-type donor star. In some of these systems, called microquasars, relativistic jets are launched from the
surroundings of the compact object. Such jets interact with the photon field of the companion star, the stellar
wind, and, at large distances, with the interstellar medium. In this paper I will review the main results of such
interactions with particular emphasis on the production of high-energy photons and neutrinos. The case of
some specific systems, like LS I +61 303, will be discussed in some detail. Prospects for future observations at
different wavelengths of this type of objects will be presented.
Key Words: GAMMA RAYS: THEORY — GAMMA RAYS: OBSERVATIONS — JETS AND OUT-
FLOWS — STARS: BINARIES — STARS: MICROQUASARS
1. INTRODUCTION
Massive stars use to form binary systems. In such
systems one of the stars evolves faster than the other.
At the end of the lifetime of this star a supernova
explosion will occur, and either a neutron star or a
black hole will be left behind. If the system is not
disrupted by the explosion, the compact object will
start to accrete matter from the stellar wind of its
early-type companion. Since the matter has angu-
lar momentum, it will form an accretion disk around
the compact star. The matter will be heated in the
disk, losing angular momentum and falling into the
potential well. The hot disk will cool through the
emission of X-rays. We say then that a massive X-
ray binary (HMXRB) is born. There are around 120
HMXRBs detected in the Galaxy so far (Liu et al.
2006). Some of these systems present non-thermal
radio emission. This emission is thought to be syn-
chrotron radiation produced by relativistic electrons
in a jet that is somehow ejected from the surround-
1Facultad de Ciencias Astronómicas y Geof́ısicas, Univer-
sidad Nacional de La Plata, Paseo del Bosque, 1900 La Plata,
Argentina (romero@fcaglp.unlp.edu.ar).
2Instituto Argentino de Radioastronomı́a, Casilla de
Correos No. 5, (1894) Villa Elisa, Buenos Aires, Argentina
(romero@irma.iar.unlp.edu.ar).
ings of the compact object. When the jet is resolved
at radio wavelengths through interferometric tech-
niques or at X-rays, the HMXRB is called a high-
mass microquasar (Mirabel et al. 1992).
The word ‘microquasar’ (MQ) was coined to em-
phasize the similarities between galactic jet sources
and extragalactic quasars (Mirabel & Rodŕıguez
1998). These similarities, although important,
should not make us to overlook the also important
differences between both types of objects. The main
difference is, of course, the presence of a donor star
in the case of MQs. In high-mass MQs, this star pro-
vides a strong photon field, a matter field in the form
of a stellar wind, and a gravitational field that can
act upon the accretion disk producing a torque and
inducing its precession. The photon and matter field
constitute targets for the relativistic particles in the
jet. The interaction of the jets with these fields can
produce a variety of phenomena that are absent in
the case of extragalactic jets. The aim of the present
article is to review these phenomena.
2. WHAT IS A MICROQUASAR?
A microquasar is an accreting X-ray binary sys-
tem with non-thermal jets. The basic ingredients of
a MQ are shown in Figure 1. They are the compact
http://arxiv.org/abs/0704.0811v1
2 GUSTAVO E. ROMERO
ACCRETION DISC
(hard X-rays)
‘Corona’
CORONA
Accretion
      disc
Accreting
neutron star
or black
(radio - ?)
(optical -
soft X-rays)
Mass-
donating companion
star (IR-optical)
Mass-flow
> 1radio   infrared  optical soft-X  hard-X  gamma-ray
COMPANION
Fig. 1. Sketch showing the different components of a
microquasar and the energy bands at which they emit.
Not to scale. From Fender & Maccarone (2004).
object, the donor star, the accretion disk, the jets,
which usually are relativistic or mildly relativistic,
and a region of hot plasma called the ‘corona’ that
surrounds the compact object. If the star is an early-
type, hot star, the accretion can proceed through
capture of the wind material. In the case of low-
mass stars and in some close systems, the accretion
occurs through the overflow of the Roche lobe. In
what follows we will focus only on high-mass MQs.
The donor star can produce radiation from the IR
up to UV energies. The accretion disk produces soft
X-rays, whereas the corona is responsible for hard
X-rays that are likely generated by Comptonization
of disk photons. The emission of the jets goes from
radio wavelengths to, in some cases like LS 5039,
gamma-rays. MQs, like blazars, can emit along the
entire electromagnetic spectrum. Their spectral en-
ergy distribution (SED) is complex, being the result
of a number of different radiative processes occurring
on different size-scales in the MQs.
MQs present different spectral states at X-rays.
The two basic state are the ‘soft’ state and the ‘hard’
state. In the former the SED is dominated by a grey-
body peak around E ∼ 1 keV, probably due to the
contribution of the accretion disk, which extends in
this state all the way down to the last stable or-
bit around the compact object. In the hard state
the peak in the X-rays is shifted toward lower ener-
gies and a strong and hard power-law component is
present up to energies ∼ 150 keV, in some cases even
beyond. This emission is usually interpreted as soft
X-ray Comptonization in the corona (e.g. Ichimaru
1977), although some authors have suggested that it
could be produced in the jet through external inverse
Compton (IC) interactions (Georganopoulos et al.
2002) or through synchrotron mechanism (Markoff
et al. 2001, 2003).
The sources spend most of the time in the hard
state. It is in this state when a steady, self-absorbed
radio jet is usually observed. The transition form one
state to the other is commonly accompanied by the
ejection of superluminal components, that can be de-
tected as moving radio blobs (Mirabel & Rodŕıguez
1994, Fender et al. 2004).
3. WHAT ARE JETS MADE OF?
One of the most important open issues concern-
ing MQs is the nature of the matter content of
the jets. We know for sure that relativistic lep-
tons with a power-law distribution are present in
the jets since we can detect and measure their syn-
chrotron radiation. The relativistic outflow can be
made of relativistic electron-positron pairs, or al-
ternatively it could be a relativistic proton-electron
plasma. Another possibility is a plasma formed by a
cold electron-proton fluid, where the particles would
have a thermal distribution, plus a relativistic con-
tent, locally accelerated by shocks (Bosch-Ramon,
Romero & Paredes 2006). In this kind of jets, the
bulk of the momentum is carried out by the cold
plasma, which additionally confines the relativistic
component.
In any case, the large perturbations observed in
the interstellar medium (ISM) around some MQs like
Cygnus X-1 (Gallo et al. 2005) and SS 433 (Dubner
et al. 1998), strongly suggest that the jets are bari-
onic loaded. The direct detection of iron lines in the
case of the jets of SS 433 (Kotani et al. 1994, 1996;
Migliari et al. 2002) clearly confirms that they con-
tain hadrons, at least in this particular object. Since
there seems to be a clear correlation between the ac-
cretion and ejection of matter in MQs (Mirabel et al.
1998), it is natural to assume that the content of the
jets does not basically differ in nature from that of
the accreting matter. All these considerations make
quite likely the presence of relativistic protons in the
jets of MQs. Hence, their radiative signatures can
not be neglected in a serious analysis of the radia-
tive processes in these sources.
4. JET INTERACTIONS
What does happen when a relativistic jet pass
through the medium that surrounds a hot, massive
star?. The radiation field of the star penetrates freely
into the jet and the dominant UV photons will in-
teract with relativistic particles in the outflow. The
interaction of the stellar wind with the jet will form
JET INTERACTIONS IN HMXRBS 3
a boundary layer where shocks will likely be formed,
but some level of fluid mixing is expected to occur.
The interaction between relativistic particles from
the jet and thermal particles of the wind will take
place, producing high-energy emission. We can sep-
arate the microscopic jet-stellar environment inter-
actions in two groups, according to whether they are
of leptonic or hadronic nature. Of course, both types
of reactions will occur in a specific system, but ac-
cording to the given conditions, one type or the other
might dominate the high-energy output of the MQ.
Let us briefly discuss both cases.
4.1. Leptonic interactions
Relativistic electrons and positrons in the jet will
IC scatter soft photons up to high energies. The
origin of these photons can be diverse: stellar UV
photons, X-ray photons from the accretion disk and
the hot corona around the compact object, or non-
thermal photons produced in the jet by synchrotron
mechanism. At high energies, the interaction en-
ters in the Klein-Nishina regime, where the cross
section decreases dramatically. Opacity effects to
gamma-ray propagation due to the presence of the
local photon fields can result in the generation of IC
cascades within the binary system (Bednarek 2006a,
Orellana et al. 2007). Relativistic leptons can in-
teract with cold protons and nuclei from the stellar
wind producing high-energy emission through rela-
tivistic Bremsstrahlung. A number of papers have
been devoted to leptonic interactions in MQs in re-
cent years, for instance, Atoyan & Aharonian (1999),
Markoff et al. (2001, 2003), Georganopolous et al.
(2002), Kaufman Bernadó et al. (2002), Romero et
al. (2002), Bosch-Ramon & Paredes (2004), Bosch-
Ramon et al. (2005a, 2006), Paredes et al. (2006),
Dermer & Böttcher (2006), Gupta et al. (2006),
Bednarek (2006b), etc. The reader is referred to
these papers and references therein for detailed dis-
cussions.
In Figure 2 we show the broadband SED ex-
pected from leptonic interactions in a high-mass MQ.
The different contributions are indicated. It can be
seen that the synchrotron emission can extend up to
MeV energies and that in the GeV-TeV range the
dominant process is IC upscattering of stellar pho-
tons. Figure 3 shows a detail of the SED at high
energies. Notice that absorption by photon-photon
annihilation has been taken into account in the fi-
nal curve, yielding a soft spectrum around 100 GeV
(Bosch-Ramon et al. 2006).
−5 −3 −1 1 3 5 7 9 11 13
log(photon energy [eV])
observed SED
IC emission
seed photons
ext. Bremsstr.
int. Bremsstr.
star IC
corona IC
disk IC
sync.
corona
Fig. 2. Spectral energy distribution of a high-mass MQ.
The different contributions to the total SED are shown.
From Bosch-Ramon, Romero & Paredes (2006).
6 7 8 9 10 11 12 13
log(photon energy [eV])
observed SED
star IC
synchrotron
corona IC
Fig. 3. High-energy emission from high-mass MQ. Cour-
tesy of V. Bosch-Ramon.
4.2. Hadronic interactions
The main reaction for proton cooling in a high-
mass MQs is pp interaction, through the channels
pp → p + p + π0 and pp → p + p + ξ(π+ + π−),
where ξ is the π± multiplicity. The neutral pions
decay yielding gamma rays, π0 → γ + γ, whereas
the charged pions produce neutrinos and e± pairs:
π± → µ±νµν̄µ → e
±νeν̄e. The gamma-ray spectrum
will mimic at high-energies the spectrum of the par-
ent relativistic proton population. In general, since
proton losses are not as severe as electron losses in
the inner region of the source, we could expect a
higher energy cutoff in hadronic-dominated sources.
Models for hadronic MQs have been developed
4 GUSTAVO E. ROMERO
 7  8  9  10  11  12  13  14
Log E [eV]
θ = 0º
θ = 30º
θ = 60º
θ = 90º
 0  60  120  180
θ [º]
Beaming factor
Fig. 4. Spectral energy distributions for the hadronic
emission of a high-mass MQ with a smooth stellar wind.
Different curves correspond to different viewing angles.
From Orellana et al. (2007).
by Romero et al. (2003), Romero et al. (2005),
Romero & Orellana (2005) and Orellana & Romero
(2007). Neutrino production in this kind of models
is discussed by Romero & Orellana (2005), Aharo-
nian et al. (2006), Benarek (2005) and Christiansen
et al (2006). For photo-pion production of neutri-
nos, under rather extreme conditions, see Levinson
& Waxman (2001).
Figure 4 shows different SED obtained from pp
interactions for a high-mass MQ with a smooth
spherical wind (Orellana et al. 2007). The vari-
ous curves correspond to different viewing angles.
The total jet power in relativistic protons is Lrel
6 × 1036 erg s−1. The jet is assumed to be perpen-
dicular to the orbital plane, but this constraint can
be relaxed to allow, for instance, for a precessing jet.
Actually, in some systems, the jet could point even
in the direction of the star (Butt et al. 2003, Romero
& Orellana 2005). In such a case, jet-induced nucle-
osynthesis can occur in the stellar atmosphere. The
power of the stellar wind might be, for some stars,
strong enough as to stop the jet creating a stand-
ing shock between the compact object and the star.
Protons and electrons might be re-accelerated there
up to very high energies, producing a detectable TeV
source.
All existing models for hadronic MQs assume a
smooth wind from the star3. However, it would
be quite possible that the wind have some struc-
ture, for instance in the form of clumps, a fact that
3See, nonetheless, the paper by Aharonian & Atoyan
(1996) that, although not framed in the context of MQs, dis-
cusses the interaction of a proton beam with a cloudy medium
around a star.
would lead to gamma-ray variability on short time
scales. If such a variability could be detected by fu-
ture Cherenkov telescope arrays, it might be used to
infer the structure of the wind. The jet would act as
a kind of lantern illuminating the wind in gamma-
rays to the observer.
Hadronic jets can propagate through the ISM
producing hot spots similar to those observed in the
case of extragalactic sources (Bosch-Ramon et al.
2005b). Particles re-accelerated at the termination
point of the jets, can diffuse in the ambient medium,
interacting with diffuse material and producing ex-
tended high-energy sources.
5. THE CONTROVERSIAL CASE OF
LS I +61 303
LS I +61 303 is a puzzling Be/X-ray binary,
which displays gamma-ray variability at high ener-
gies. The nature of the compact object and the
origin of the high-energy emission is unclear. The
detection of jet-like radio features by Massi et al.
(2001, 2004) led to the classification of this source as
a MQ. This has been recently challenged by Dhawan
et al. (2006), who observed the source with the
VLBA at different orbital phases concluding that
the direction the jet-like feature during the perias-
tron passage (opposed to the primary star) supports
the scenario of a colliding wind model where the com-
pact object is an energetic pulsar (wind power∼ 1036
erg/s). The system has been detected by the MAGIC
telescope at E > 200 GeV. The variability is modu-
lated with the orbital period. Contrary to the expec-
tations the maximum of the gamma-ray emission oc-
curred well after the periastron passage. The source
was not clearly detected during the periastron (Al-
bert et al. 2006). The cause of this could be gamma-
ray absorption in the combined photon field of the Be
star and its decretion disk (e.g. Orellana & Romero
2007). Figure 5 shows the electromagnetic cascades
that might develop close to the periastron passage
(which occurs at phase 0.23). According to these
simulations the source should be detectable during
the periastron, but with longer exposures, and the
spectrum will be softer than what was observed at
phases 0.6-0.7.
The pulsar/Be binary interpretation goes not free
of severe problems. The flux at MeV-GeV energies
observed by EGRET (Kniffen et al. 1997) accounts
for a luminosity of ∼ 1036 erg/s, which would imply
an impossible conversion efficiency from wind power
to gamma-rays of ∼ 1. In addition, since the pulsar
wind would be orders of magnitude stronger than the
slow Be equatorial wind, the observed ‘cometary tail’
JET INTERACTIONS IN HMXRBS 5
Fig. 5. Electromagnetic cascades at different phases de-
veloped close to the periastron passage of the X-ray bi-
nary LS I +61 303 (from Orellana & Romero 2007).
radio feature, if interpreted as synchrotron radiation
from electrons accelerated at the colliding wind re-
gion, should point out toward the primary star, and
not opposite to it.
It is clear that LS I +61 303 is a interesting and
peculiar system that deserves more intensive studies
in the near future.
6. CONCLUSIONS
MQs are outstanding natural laboratories to
study a variety of physical phenomena such as par-
ticle acceleration, accretion physics, and particle in-
teractions. Observations of gamma-ray emission of
high-mass MQs can be used to probe the structure
of stellar winds and the nature of the matter content
of relativistic jets.
How many MQs are there in the Galaxy?. It is
difficult to answer this questions, but it seems possi-
ble that a significant number of the yet-unidentified
variable gamma-ray sources located on the galactic
plane (Romero et al. 1999) could be associated with
high-mass MQs (Romero et al. 2004, Bosch-Ramon
et al. 2005a). In the next few years, new Cherenkov
telescope arrays like HESS II, MAGIC II, and VER-
ITAS, along with the satellite observatories AGILE
and GLAST, will continue detecting these extraor-
dinary objects at high energies and helping to pene-
trate into their mysteries.
Acknowledgments
This work has been supported by the Agencies
CONICET (PIP 5375) and ANPCyT (PICT 03-
13291 BID 1728/OC-AR). I thank the organizers for
a wonderful meeting and a warm hospitality.
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