Binaries, microquasars and GLAST
Binaries and GLAST
Guillaume Dubus
Laboratoire d’Astrophysique de Grenoble, Université J. Fourier, CNRS, BP 53, 38041 Grenoble, France
Abstract. Radio and X-ray observations of the relativistic jets of microquasars show evidence for the acceleration of particles
to very high energies. Signatures of non-thermal processes occurring closer in to the compact object can also be found. In
addition, three binaries are now established emitters of high (>100 MeV) and/or very high (>100 GeV) energy gamma-rays.
High-energy emission can originate from a microquasar jet (accretion-powered) or from a shocked pulsar wind (rotation-
powered). I discuss the impact GLAST will have in the very near future on studies of such binaries. GLAST is expected to
shed new light on the link between accretion and ejection in microquasars and to enable to probe pulsar winds on small scales
in rotation-powered binaries.
Keywords: Gamma-ray; Pulsars; Black holes; X-ray binaries; Infall and accretion; Jets, outflows and bipolar flows
PACS: 95.85.Pw, 97.60.Gb, 97.60.Lf, 97.80.Jp, 98.35.Mp, 98.58.Fd
INTRODUCTION
Binaries composed of a black hole or neutron star in orbit with a stellar companion are prominent sources of the X-ray
sky. X-ray binaries are usually powered by accretion of matter from the companion. The gravitational energy released
by accretion heats the plasma to temperatures in the range 1 keV < kT < 50 MeV for a stellar-mass black hole, which
is someway below the GLAST energy range. However, part of the power is also emitted non-thermally by particles of
much greater energies. The radio emission from X-ray binaries, due to synchrotron radiation from electrons located in
relativistic outflows, provides clear evidence for this.
These relativistic jets are the most striking demonstration of the analogy between accretion onto the stellar-mass
compact objects in X-ray binaries and onto the supermassive black holes in Active Galactic Nuclei (AGN). X-ray
binaries with relativistic jets have thus been denominated ‘microquasars’. Similarities also exist in timing and spectral
characteristics. The conjecture, at least for black holes, is that the underlying scaling factor of the physical processes
is the gravitational radius Rg=GM/c
2 (and its associated timescale Rg/c).
The similarities have prompted speculation that some X-ray binaries may be analogs of blazars, AGNs dominated
by non-thermal output because their relativistic jet is fortuitously aligned with the line-of-sight. Blazars emit a large
fraction of their power in high energy (HE) gamma-rays (>100 MeV). Notwithstanding the possible issue that the
microquasar population may be too small for a chance alignment to occur, finding genuine ‘microblazars’ could also
prove exceedingly difficult if the scaling strictly holds, since the observed sub-hour TeV flaring in blazars translates
to milliseconds for a microblazar. But even if the jet is misaligned, the high particle energies implied by the radio to
X-ray observations make it likely, if not unavoidable, that HE gamma-ray emission should be present at some level
in generic microquasars. Cataclysmic variables (AE Aqr) and colliding winds in Wolf-Rayet binaries have also been
proposed to emit HE gamma-rays - but these systems will not be addressed here.
GLAST observations will soon open a new window into non-thermal processes in accreting binaries. Three compact
binaries are presently known sources of HE gamma-rays. These three gamma-ray binaries (in that most of their
radiative output is in gamma-rays, regardless of the underlying physics) are probably all rotation-powered by the spin-
down of a young pulsar rather than accretion-powered. The current observational status and GLAST prospects for both
types of sources are reviewed.
GLAST STUDIES OF ACCRETING BINARIES
X-ray jets
The best evidence for particle acceleration to high energies in accreting binaries comes from the observation of
X-ray emission from localized regions in the relativistic jet of two microquasars, XTE J1550-563 and H1743-322
http://arxiv.org/abs/0704.0536v1
[1]. The radio spectrum from these regions connects remarkably well with the X-ray spectrum across nine decades in
energy. The overall spectral slope α ≈−0.6 points to synchrotron emission from a canonical non-thermal distribution
of electrons dN ∝ N−2.2dE over nine orders-of-magnitude. The emission zone is resolved (≈1”). The equipartition
magnetic field is ≈ 500 µG and the X-ray emitting electrons have energies ≈ 10 TeV [1].
Electrons of such energies necessarily emit in the GLAST energy range. However, the prospects for detecting this
emission are not favorable. Synchrotron losses limit the maximum possible electron energy to a few PeV, in which
case the spectrum could extend to a 100 MeV at a level ≈ 10−12 ergs s−1 cm−2, too faint for GLAST observations in
the Galactic Plane. Moreover, the short radiative timescale (≈ 5 days) of PeV electrons would lead to a break or cutoff
in the spectrum below 100 MeV. (The timescale also shows the particle acceleration has to occur in situ, although why
and how it occurs is not understood but could involve internal shocks, magnetic energy dissipation or a shock with the
ISM.) Self-Compton or inverse Compton on CMB photons are also unlikely to lead to detectable levels of emission
for GLAST. In both cases an inverse Compton luminosity greater or equal to the synchrotron luminosity requires
a magnetic field < 3 µG, a hundred times below equipartition. Therefore, although these observations undoubtedly
show the presence of particles of high-energies in microquasar jets, the gamma-rays emitted in those conditions are
not likely to be detected by GLAST.
Large scale jet-ISM interaction
The dissipation of the jet power in the ISM is a possible source of gamma-ray emission for GLAST. The energy
involved can be substantial. For example, radio observations show the a.u.-scale compact jet of Cyg X-1 is prolonged
into a parsec-scale structure associated with the termination shock. The inferred power inconspicuously transported
to the large scales is comparable to the bolometric luminosity of the binary [2]. Jets can therefore quietly inject
large amounts of high energy particles into the ISM, which might be traced by their gamma-ray emission. Heinz &
Sunyaev [3] estimate their contribution could reach 10% of the Galactic cosmic ray luminosity. If there is indeed a
significant component of high energy nucleons, a jet interacting with a nearby molecular cloud (effectively modeled as
an accelerator + beam dump in [4]) would create pions. The subsequent decay, bremsstrahlung and inverse Compton
emission can be detected by GLAST, depending upon the jet power and composition, but also upon the duty cycle of
the ejection process, the distance of the cloud to the source etc. As an example, the emission predicted in Fig. 12 of [4]
is detectable within a year for a microquasar at a distance of 1 kpc (e.g. A0620-00, XTE J1118+480, the latter having
the advantage of a large Galactic latitude b=+62.3o). Such observations would give new clues as to the content and
power of relativistic jets in binaries.
Gamma-ray spectral states and major ejections
Gamma-ray emission originating closer in to the compact object can also be expected. There is reasonable evidence
from CGRO observations for soft power-law tails (spectral slope α ≈-1.5) extending beyond 100 keV, up to several
MeV in some X-ray binaries [5]. These soft tails are a defining property of the very high state (or steep power law
state) together with significant thermal emission (around a keV) and fast variability (QPOs) [6]. This is most clearly
seen in the high state of Cyg X-1 where the power-law extends to 10 MeV. Extrapolating shows the 100 MeV emission
was beyond the reach of EGRET but should be detectable by GLAST within days. Cyg X-1 spends 90% of its time in
the hard state where there is a hint for a similar, but fainter, power-law component that could be detected in a year by
GLAST. These power-laws can be produced in plasmas where a fraction of the accretion energy goes into non-thermal
channels [7]. Models predict a cutoff in the GLAST energy range either because there is a maximum electron energy
or because pair production sets in when the plasma compacity is high.
Changes in X-ray spectral states can therefore be surmised to be associated with changes in HE gamma-ray
luminosity, the very high X-ray state (resp. hard X-ray state) involving high (resp. low) levels of gamma-ray emission.
Interestingly, spectral state changes from hard to very high X-ray states have been conjectured to be associated with
major relativistic ejections [8] and one should note, perhaps naively, that if jets are composed of e+e− pairs then there
has to be some gamma-ray emission linked to the pair production, if only at a few MeV. Radio/IR observations of
discrete ejections in GRS 1915+105 show non-thermal emission cooling with expansion. Extrapolating back to early
times, the fluence expected in a day by Atoyan & Aharonian [9] in HE gamma-rays is detectable by GLAST. Gamma-
ray monitoring of outbursting binaries or sources such as GRS 1915+105, a task that is well-suited to GLAST, can
therefore shed light on how spectral state changes relate to ejection events.
THE OBSERVATIONAL STATUS: GAMMA-RAY BINARIES
The view from space
Although the above (should) demonstrate that there are reasonable grounds to expect HE gamma-rays from compact
binaries, observational confirmation has proved elusive and when found, arguably disconcerting. Several tentative
associations of binaries with EGRET sources were made based on positional coincidence and/or the detection of
variability, with the source 2CG 135+01 figuring prominently as the first and most secure: follow-up observations
carried out after the initial COS B discovery had revealed a high-mass X-ray binary, LS I+61 303, in a 26 day
elliptical orbit showing periodic radio outbursts [10]. The latter feature being rare, this highlighted the system as a
plausible counterpart. Yet, although variability was reported in the EGRET data, neither this nor the position were
enough to formally identify the two (the HE variation not being tied to variability at other wavelengths and the stellar
counterpart being localized only in between the 95% and 99% confidence contours of the HE source). The limited
angular resolution combined with the strong underlying Galactic diffuse HE emission resulted in error boxes of a
several tens of arcmins, too large to pick up the needle in the haystack of possible Galactic Plane counterparts.
The view from the ground
Breakthrough observations were obtained by the ground-based Cherenkov telescopes operated by the HESS and
MAGIC collaborations. These observe at a higher threshold (≥ 100 GeV) but benefit from a larger collecting area and
a better angular resolution. Three binaries were detected: PSR B1259-63, LS 5039 and LS I+61 303 [11, 12, 13]. The
latter two had tentative EGRET associations but not PSR B1259-63, possibly because the HE gamma-ray emission is
highly variable along the 3.5 year orbit and confined to a short period around periastron passage. The next passage
occurs this year, too early for GLAST so searches may have to wait 2010 (unfortunately, periastron passage cannot be
observed by HESS before 2014). The localizations are much more precise: for instance, LS 5039 is coincident with
HESS J1826-148 within the positional uncertainty of 30”, excluding a nearby SNR and a pulsar that were within the
error box of the EGRET source 3EG 1824-1514. More importantly, all three binaries display variability. In LS 5039,
it was demonstrated that the >100 GeV flux is strongly modulated on the orbital period [14] and there is little doubt
that the fluxes measured in PSR B1259-63 and LS I+61 303 also depend on orbital phase.
Gamma-ray binaries
All three binaries have high mass O or Be type companions with compact objects in eccentric orbits. The X-ray
output from these binaries is about 1034 erg/s, rather low in itself compared to typical X-ray binaries, and smaller
or comparable to the emission above 100 MeV. These systems are therefore gamma-ray loud, a first surprise. All of
them display radio emission, resolved in LS 5039 and LS I+61 303 as collimated outflows on milliarcsecond scales,
immediately suggesting a microquasar nature [15]. X-ray and radio variability, when (and if) present is of limited
amplitude (odd in accreting binaries) and occurs on the orbital timescale. The overall spectral energy distributions
are similar for all three systems, showing (in νFν ) a rising spectrum from radio to X-rays flattening around an MeV
and reaching energies of several TeV. However, PSR B1259-63 is not a microquasar but a young 48 ms pulsar with
a spin-down power ≈ 1036 erg s−1. The relativistic pulsar wind is sufficient to quench any wind accretion and the
emission is thought to arise from particles accelerated where the pulsar and stellar wind interact [16]. This paints a
rather different picture than accretion-powered scenarios.
Gamma-ray binaries as compact pulsar wind nebulae
Why the observational properties of the three gamma-ray binaries should bear any resemblance is disconcerting
unless all are actually rotation-powered by a young pulsar [17]. This can explain the low, steady level of emission,
conceivably modulated as the pulsar moves around its orbit. Pulsed emission would be absorbed in the stellar wind
because of the smaller orbital separations in LS 5039 and LS I+61 303 (as observed near periastron in PSR B1259-63).
The pulsar wind is confined by the stellar wind to a cometary nebula pointing away from the massive companion,
producing the collimated radio outflow. Radio VLBI observations of LS I+61 303 recently reported by [18] are
consistent with this picture, showing a periodic sweep of the radio tail with orbital phase which appears irreconcilable
with an accretion-powered jet: LS I+61 303 is almost certainly powered by a pulsar. The small-scale radio morphology
of LS 5039 has been successfully modeled as a pulsar wind nebula [17] but this does not provide the same level of
certainty in the absence of observations at other orbital phases. Other models have proposed the emission arises in
a relativistic jet powered by accretion [19, 20], but that hypothesis seems rather uneconomical to this author. At this
stage, it seems more probable that HE gamma-ray emission from accretion-powered binaries has yet to be detected
and that, when this will be achieved, their observational properties will be clearly different from those of the rotation-
powered binaries.
GLAST STUDIES OF ROTATION-POWERED BINARIES
Gamma-ray orbital modulation
That all three gamma-ray binaries have high-mass stellar companions may be instrumental to generate the HE
emission, as these will provide copious amounts of seed photons for inverse Compton scattering. On the other hand,
the large photon densities at UV energies also imply pair production with TeV photons can be important. The starlight
both provides a source and a sink for gamma-rays. Gamma-ray absorption has a strong orbital dependence as the cross-
section for pair production depends on the angle between the two photons. The effect can be dramatic on gamma-rays
emitted towards the observer and crossing head-on the path of stellar photons. In LS 5039, the tight 4-day orbit brings
the compact object to within a stellar radius from the O6V star. Gamma-rays of >30 GeV emitted close to the compact
object are modulated with peak attenuation at superior conjunction (when the compact object is behind the star as seen
by the observer) [21]. Such an orbital modulation has been observed in LS 5039 by HESS with a peak and trough at
the predicted orbital phases [14].
However, the flux at superior conjunction is not completely absorbed. Furthermore, the >100 GeV spectrum varies
from a soft power law at superior conjunction (low flux) to a hard power law at inferior conjunction (high flux),
whereas pure absorption predicts a dip in the spectrum around a TeV. Other effects must play a role. One is that a pair
cascade is initiated when the newly created e+e− up-scatter star photons back into the absorption range. The magnetic
field has to be low enough (≤10 G) to prevent synchrotron losses from dominating. The radiated energy is redistributed
below the pair production threshold (<30 GeV) i.e. in the GLAST range. Calculations show an anti-correlation of the
HESS and GLAST light-curves [22], detectable within a year [23], that would prove the existence of a cascade.
A second effect is that inverse Compton scattering is also anisotropic. For example, this will decrease the gamma-
ray flux around inferior conjunction, when the (incoming) star photons and (outgoing) gamma-ray photons both go
towards the observer: in this configuration the energy of the outgoing photon and the cross-section for IC are small.
There actually is a hint of a dip in the HESS light-curve at this phase.
Probing pulsar winds
Besides these geometrical effects, the efficiency with which gamma-rays are emitted may also change along the
orbit. The variations in the >100 GeV flux observed by HESS and MAGIC in LS I+61 303 and PSR B1259-63 do not
match the expected light-curve for pure absorption (whose effect is marginal due to the wider orbits) and so must be
intrinsic to the emission process [24]. LS I+61 303 may prove a Rosetta stone for this problem as its orbit is both wide
enough (0.2-0.7 a.u.) to avoid most cascading and absorption, but short enough (26 days) to allow for detailed studies
over many orbits. MAGIC reports a minimum in flux close to periastron (which is close to inferior conjunction, where
absorption is minimal) and a maximum towards apastron. The explanation is straightforward with a compact pulsar
wind nebula. The pulsar wind is contained by the stellar wind of the Be companion. At periastron, the ram pressure
of the dense equatorial wind crushes the PWN nebula to a small distance from the pulsar. The magnetic field at the
shock location is strong and particles lose energy rapidly to synchrotron emission without radiating much inverse
Compton above >10 GeV. At apastron, the stellar wind is polar and diffuse, implying a large shock distance and
weaker magnetic field, allowing for higher particle energies emitting more HE inverse Compton gamma-rays. Hence,
phase-resolved spectral energy distributions from X-rays to VHE gamma-rays can yield information on the magnetic
field at different locations, forming a probe of the relativistic wind as a function of distance to the pulsar.
Population studies of gamma-ray binaries
The pulsar spin-down timescale is short (∼ 105 years for PSR B1259-63) and can only power the binary emission
for a brief period of time. Accretion from the stellar wind is then no longer quenched by a pulsar wind and an X-ray
pulsar turns on. Hence, gamma-ray binaries are the progenitors of the longer-lived, accretion-powered high-mass X-
ray binaries (HMXBs). Because most of their output is in gamma-rays, GLAST all-sky observations provide a unique
way of identifying these progenitors and studying them as a population, constraining the birth rate and evolution of
HMXBs. Population synthesis calculations find there should be around 30 active gamma-ray binaries in order to match
the present-day population of HMXBs, assuming the rotation-powered phase lasts only 104 years [25]. They should be
visible throughout the Galaxy if their luminosity in the GLAST band is comparable to that of the three known systems
(∼ 1035 erg s−1). This estimate does not take into account the reduction in sensitivity to be expected in the Galactic
Plane due to the diffuse emission. In this respect, the Magellanic Clouds, which harbor a comparatively very large
population of HMXBs, might also make for good dedicated studies despite being more distant.
ACKNOWLEDGMENTS
I thank the organizers of the 1st GLAST symposium for their invitation and acknowledge support from the Agence
Nationale de la Recherche.
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	Introduction
	GLAST studies of accreting binaries
	X-ray jets
	Large scale jet-ISM interaction
	Gamma-ray spectral states and major ejections
	The observational status: gamma-ray binaries
	The view from space
	The view from the ground
	Gamma-ray binaries
	Gamma-ray binaries as compact pulsar wind nebulae
	GLAST studies of rotation-powered binaries
	Gamma-ray orbital modulation
	Probing pulsar winds
	Population studies of gamma-ray binaries
