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

What caused the GeV flare of PSR B1259-63 ?

PSR B1259-63 is a gamma-ray binary system composed of a high spindown pulsar and a massive star. Non-thermal emission up to TeV energies is observed near periastron passage, attributed to emission from high energy e+e- pairs accelerated at the shock with the circumstellar material from the companion star, resulting in a small-scale pulsar wind nebula. Weak gamma-ray emission was detected by the Fermi/LAT at the last periastron passage, unexpectedly followed 30 days later by a strong flare, limited to the GeV band, during which the luminosity nearly reached the spindown power of the pulsar. The origin of this GeV flare remains mysterious. We investigate whether the flare could have been caused by pairs, located in the vicinity of the pulsar, up-scattering X-ray photons from the surrounding pulsar wind nebula rather than UV stellar photons, as usually assumed. Such a model is suggested by the geometry of the interaction region at the time of the flare. We compute the gamma-ray lightcurve for this scenario, based on a simplified description of the interaction region, and compare it to the observations. The GeV lightcurve peaks well after periastron with this geometry. The pairs are inferred to have a Lorentz factor ~500. They also produce an MeV flare with a luminosity ~1e34 erg/s prior to periastron passage. A significant drawback is the very high energy density of target photons required for efficient GeV emission. We propose to associate the GeV-emitting pairs with the Maxwellian expected at shock locations corresponding to high pulsar latitudes, while the rest of the non-thermal emission arises from pairs accelerated in the equatorial region of the pulsar wind termination shock.Comment: 6 pages, 3 figures, accepted for publication in A&

International social work field placement or volunteer tourism? Developing an asset-based justice-learning field experience

This paper examines a developing model for building an international social work placement that meets the needs of the host agency and community first. The paper addresses the challenges for social work departments to develop a strong learning environment while also keeping primary the needs of the host community and agency

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

Spectral signature of a free pulsar wind in the gamma-ray binaries LS 5039 and LSI +61\degr303

LS 5039 and LSI +61\degr303 are two binaries that have been detected in the TeV energy domain. These binaries are composed of a massive star and a compact object, possibly a young pulsar. The gamma-ray emission would be due to particle acceleration at the collision site between the relativistic pulsar wind and the stellar wind of the massive star. Part of the emission may also originate from inverse Compton scattering of stellar photons on the unshocked (free) pulsar wind. The purpose of this work is to constrain the bulk Lorentz factor of the pulsar wind and the shock geometry in the compact pulsar wind nebula scenario for LS 5039 and LSI +61\degr303 by computing the unshocked wind emission and comparing it to observations. Anisotropic inverse Compton losses equations are derived and applied to the free pulsar wind in binaries. The unshocked wind spectra seen by the observer are calculated taking into account the gamma-gamma absorption and the shock geometry. A pulsar wind composed of monoenergetic pairs produces a typical sharp peak at an energy which depends on the bulk Lorentz factor and whose amplitude depends on the size of the emitting region. This emission from the free pulsar wind is found to be strong and difficult to avoid in LS 5039 and LSI +61\degr303. If the particles in the pulsar are monoenergetic then the observations constrain their energy to roughly 10-100 GeV. For more complex particle distributions, the free pulsar wind emission will be difficult to distinguish from the shocked pulsar wind emission.Comment: 11 pages, 10 figures, accepted for publication in Astronomy and Astrophysic

Shining in the Dark: the Spectral Evolution of the First Black Holes

Massive Black Hole (MBH) seeds at redshift $z \gtrsim 10$ are now thought to be key ingredients to explain the presence of the super-massive ($10^{9-10} \, \mathrm{M_{\odot}}$) black holes in place $< 1 \, \mathrm{Gyr}$ 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 $10^5 \, \mathrm{M_{\odot}}$, assuming both standard Eddington-limited accretion, or slim accretion disks, appropriate for super-Eddington flows. The emission occurs predominantly in the observed infrared-submm ($1-1000 \, \mathrm{\mu m}$) and X-ray ($0.1 - 100 \, \mathrm{keV}$) bands. Such signal should be easily detectable by JWST around $\sim 1 \, \mathrm{\mu m}$ up to $z \sim 25$, and by ATHENA (between $0.1$ and $10 \, \mathrm{keV}$, up to $z \sim 15$). Ultra-deep X-ray surveys like the Chandra Deep Field South could have already detected these systems up to $z \sim 15$. Based on this, we provide an upper limit for the $z \gtrsim 6$ MBH mass density of $\rho_{\bullet} \lesssim 2.5 \times 10^{2} \, \mathrm{M_{\odot} \, Mpc^{-3}}$ assuming standard Eddington-limited accretion. If accretion occurs in the slim disk mode the limits are much weaker, $\rho_{\bullet} \lesssim 7.6 \times 10^{3} \, \mathrm{M_{\odot} \, Mpc^{-3}}$ in the most constraining case.Comment: Submitted for publication in MNRA

General up to next-nearest neighbour elasticity of triangular lattices in three dimensions

We establish the most general form of the discrete elasticity of a 2D triangular lattice embedded in three dimensions, taking into account up to next-nearest neighbour interactions. Besides crystalline system, this is relevant to biological physics (e.g., red blood cell cytoskeleton) and soft matter (e.g., percolating gels, etc.). In order to correctly impose the rotational invariance of the bulk terms, it turns out to be necessary to take into account explicitly the elasticity associated with the vertices located at the edges of the lattice. We find that some terms that were suspected in the litterature to violate rotational symmetry are in fact admissibl

Hot white dwarfs and the UV delay in dwarf novae

We calculate the effect of illumination of dwarf nova accretion discs by radiation from a hot, central, white dwarf. We show that only for very hot white dwarfs (Teff ~ 40 000$ K) the inner region of quiescent dwarf nova discs are partially depleted so that the delay between the rise to outburst of the optical and UV fluxes would be increased as suggested recently by King (1997). This depletion, however, must create several small outbursts between main outbursts, contrary to observations. Lower white dwarf temperatures may cause the outburts to be of the `inside-out' type removing the UV delay. We conclude that white dwarf irradiation of dwarf nova discs is not very efficient for example because the UV radiation from the hot white dwarf does not penetrate deep enough in the disc atmosphere. The total ablation of the inner disc by e.g. evaporation (possibly related to illumination) appears to be a very promising possibility, accounting for both the EUV delay and the general lightcurves properties.Comment: 6 pages, 8 figures; accepted for publication in MNRA