117 research outputs found
Thin discs, thick discs and transition zones
Accretion onto a compact object must occur through a disc when the material
has some initial angular momentum. Thin discs and the thicker low radiative
efficiency accretion flows are solutions to this problem that have been widely
studied and applied. This is an introduction to these accretion flows within
the context of X-ray binaries and cataclysmic variables.Comment: 27 pages, 2 figures, to appear in the proceedings of the Aussois
summer school "Stades Ultimes de l'Evolution Stellaire", EAS pub. serie
Gamma-ray binaries and related systems
After initial claims and a long hiatus, it is now established that several
binary stars emit high (0.1-100 GeV) and very high energy (>100 GeV) gamma
rays. A new class has emerged called 'gamma-ray binaries', since most of their
radiated power is emitted beyond 1 MeV. Accreting X-ray binaries, novae and a
colliding wind binary (eta Car) have also been detected - 'related systems'
that confirm the ubiquity of particle acceleration in astrophysical sources. Do
these systems have anything in common ? What drives their high-energy emission
? How do the processes involved compare to those in other sources of gamma
rays: pulsars, active galactic nuclei, supernova remnants ? I review the wealth
of observational and theoretical work that have followed these detections, with
an emphasis on gamma-ray binaries. I present the current evidence that
gamma-ray binaries are driven by rotation-powered pulsars. Binaries are
laboratories giving access to different vantage points or physical conditions
on a regular timescale as the components revolve on their orbit. I explain the
basic ingredients that models of gamma-ray binaries use, the challenges that
they currently face, and how they can bring insights into the physics of
pulsars. I discuss how gamma-ray emission from microquasars provides a window
into the connection between accretion--ejection and acceleration, while eta Car
and novae raise new questions on the physics of these objects - or on the
theory of diffusive shock acceleration. Indeed, explaining the gamma-ray
emission from binaries strains our theories of high-energy astrophysical
processes, by testing them on scales and in environments that were generally
not foreseen, and this is how these detections are most valuable.Comment: 71 pages, 23 figures, minor updates to text, references, figures to
reflect published versio
The case for super-critical accretion onto massive black holes at high redshift
Short-lived intermittent phases of super-critical (super-Eddington) growth,
coupled with star formation via positive feedback, may account for early growth
of massive black holes (MBH) and coevolution with their host spheroids. We
estimate the possible growth rates and duty cycles of these episodes, both
assuming slim accretion disk solutions, and adopting the results of recent
numerical simulations. The angular momentum of gas joining the accretion disk
determines the length of the accretion episodes, and the final mass a MBH can
reach. The latter can be related to the gas velocity dispersion, and in
galaxies with low-angular momentum gas the MBH can get to a higher mass. When
the host galaxy is able to sustain inflow rates at 1-100 msunyr, replenishing
and circulation lead to a sequence of short (~1e4-1e7 years), heavily obscured
accretion episodes that increase the growth rates, with respect to an
Eddington-limited case, by several orders of magnitude. Our model predicts that
the ratio of MBH accretion rate to star formation rate is 1e2 or higher,
leading, at early epochs, to a ratio of MBH to stellar mass higher than the
"canonical" value of ~1e-3, in agreement with current observations. Our model
makes specific predictions that long-lived super-critical accretion occurs only
in galaxies with copious low-angular momentum gas, and in this case the MBH is
more massive at fixed velocity dispersion.Comment: Accepted for publication in Ap
DIM light on Black Hole X-ray Transients
The current model for the outburst of stellar-mass black holes X-ray binaries
is the disk instability model (DIM). An overview of this model and a discussion
of its theoretical and observational challenges are given.Comment: 6 pages, 7 figures, sollicited talk for "Astrophysical Sources of
High Energy Particles and Radiation" (Torun, June 20-24, 2005), to be
published in AIP Proceedings Serie
The gamma-ray binaries LS 5039, LS I +61 303 and PSR B1259-63
Three binaries are now established sources of emission at very high energies
(>1e11 eV). They are composed of a massive star and a compact object. The
emission can be due to the interaction of the relativistic wind from a young ms
pulsar with the stellar wind of the companion, by which rotation-power ends up
as non-thermal flux. Variations at VHE energies are explained as due to
gamma-gamma absorption and/or changes in shock location along the orbit.
Resolved radio emission is due to cooling particles trailing the pulsar.Comment: 10 pages, 3 figures, proceedings Vulcano workshop (22nd - 27th May,
2006), Frontier Objects in Astrophysics and Particle Physics, F. Giovannelli
& G. Mannocchi (eds.), Italian Physical Society, Editrice Compositori,
Bologna, Ital
High-energy radiation from the relativistic jet of Cygnus X-3
Cygnus X-3 is an accreting high-mass X-ray binary composed of a Wolf-Rayet
star and an unknown compact object, possibly a black hole. The gamma-ray space
telescope Fermi found definitive evidence that high-energy emission is produced
in this system. We propose a scenario to explain the GeV gamma-ray emission in
Cygnus X-3. In this model, energetic electron-positron pairs are accelerated at
a specific location in the relativistic jet, possibly related to a
recollimation shock, and upscatter the stellar photons to high energies. The
comparison with Fermi observations shows that the jet should be inclined close
to the line of sight and pairs should not be located within the system.
Energetically speaking, a massive compact object is favored. We report also on
our investigations of the gamma-ray absorption of GeV photons with the
radiation emitted by a standard accretion disk in Cygnus X-3. This study shows
that the gamma-ray source should not lie too close to the compact object.Comment: 4 pages, 3 figures, Proceedings of the SF2A conference held in
Marseille, 21-24 June 201
Shining in the Dark: the Spectral Evolution of the First Black Holes
Massive Black Hole (MBH) seeds at redshift are now thought to
be key ingredients to explain the presence of the super-massive () black holes in place after the Big
Bang. Once formed, massive seeds grow and emit copious amounts of radiation by
accreting the left-over halo gas; their spectrum can then provide crucial
information on their evolution. By combining radiation-hydrodynamic and
spectral synthesis codes, we simulate the time-evolving spectrum emerging from
the host halo of a MBH seed with initial mass ,
assuming both standard Eddington-limited accretion, or slim accretion disks,
appropriate for super-Eddington flows. The emission occurs predominantly in the
observed infrared-submm () and X-ray () bands. Such signal should be easily detectable by JWST around
up to , and by ATHENA (between and
, up to ). Ultra-deep X-ray surveys like the
Chandra Deep Field South could have already detected these systems up to . Based on this, we provide an upper limit for the MBH
mass density of assuming standard Eddington-limited accretion. If accretion
occurs in the slim disk mode the limits are much weaker, in the most
constraining case.Comment: Submitted for publication in MNRA
Colliding wind binaries and gamma-ray binaries : relativistic version of the RAMSES code
Gamma-ray binaries are colliding wind binaries (CWB) composed of a massive
star a non-accreting pulsar with a highly relativistic wind. Particle
acceleration at the shocks results in emission going from extended radio
emission to the gamma-ray band. The interaction region is expected to show
common features with stellar CWB. Performing numerical simulations with the
hydrodynamical code RAMSES, we focus on their structure and stability and find
that the Kelvin-Helmholtz instability (KHI) can lead to important mixing
between the winds and destroy the large scale spiral structure. To investigate
the impact of the relativistic nature of the pulsar wind, we extend RAMSES to
relativistic hydrodynamics (RHD). Preliminary simulations of the interaction
between a pulsar wind and a stellar wind show important similarities with
stellar colliding winds with small relativistic corrections.Comment: Proceeding of the 5th International Symposium on High-Energy
Gamma-Ray Astronomy (Gamma2012). arXiv admin note: text overlap with
arXiv:1212.404
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