Black Hole Accretion in Transient X-Ray Binaries

Recent work on the modes of accretion onto black holes (BHs) in Soft X-Ray Transients (SXTs) is reviewed, with an emphasis on uncertainties affecting models of accretion during quiescence (inner hot flow, outer thin disk). Various interpretations of the quiescent X-ray luminosity difference between systems containing neutron stars (NSs) and systems containing BH candidates are also summarized. A new scenario, which does not require BH candidates to possess an event horizon, is presented here. This scenario may be ruled out in the future, from detailed X-ray spectroscopic diagnostics or from the absence of type I X-ray bursts in systems containing BH candidates.Comment: 15 pages, Proceedings of the 2nd KIAS Astrophysics Workshop "Current High-Energy Emission around Black Holes

Atmospheric Circulation and Composition of GJ1214b

The exoplanet GJ1214b presents an interesting example of compositional degeneracy for low-mass planets. Its atmosphere may be composed of water, super-solar or solar metallicity material. We present atmospheric circulation models of GJ1214b for these three compositions, with explicit grey radiative transfer and an optional treatment of MHD bottom drag. All models develop strong, superrotating zonal winds (~ 1-2 km/s). The degree of eastward heat advection, which can be inferred from secondary eclipse and thermal phase curve measurements, varies greatly between the models. These differences are understood as resulting from variations in the radiative times at the thermal photosphere, caused by separate molecular weight and opacity effects. Our GJ1214b models illustrate how atmospheric circulation can be used as a probe of composition for similar tidally-locked exoplanets in the mini-Neptune/waterworld class.Comment: 14 pages, 4 figures, 1 table, accepted for publication in ApJ

Viscosity Mechanisms in Accretion Disks

The self-sustained turbulence which develops in magnetized accretion disks is suppressed in the weakly-ionized, quiescent disks of close binary stars. Because accretion still proceeds during quiescence, another viscosity mechanism operates in these systems. An anticorrelation of the recurrence times of SU UMa dwarf novae with their mass ratio supports spiral waves or shock-waves tidally induced by the companion star as the main process responsible for accretion in the quiescent disks. Other weakly-ionized gaseous disks in systems lacking a massive companion have to rely on yet another transport mechanism or they could be essentially passive.Comment: 7 pages, 2 figures, accepted for publication in Scienc

Structure and properties of transition fronts in accretion discs

We use high-resolution time-dependent numerical simulations of accretion discs around white dwarfs to study the structure and properties of transition fronts in the context of the thermal-viscous disc instability model. The thermal structure of cooling and heating fronts is dominated by radiative cooling and viscous heating, respectively, except in a very narrow precursor region in heating fronts where advection and radial transport of energy dominate. Cooling fronts are much broader than heating fronts, but the widths of both types of fronts scale with the local vertical scale height of the disc. We confirm that during a fair fraction of the propagation time of a cooling front, the structure of the inner disc is close to self-similar. The speed of heating fronts is ~ a few km/s, while the speed of cooling fronts is ~ a fraction of a km/s. We show that direct measurements of the speed of transition fronts probably cannot discriminate between various prescriptions proposed for the viscosity parameter alpha. A natural prediction of the disc instability model is that fronts decelerate as they propagate in the disc, independent of the prescription for alpha. Observation of this effect would confirm that dwarf nova outbursts are driven by the thermal-viscous instability. Most of our results also apply to low mass X-ray binaries in which the accreting object is a neutron star or a black hole.Comment: LateX, 12 pages, 10 figures, uses mn.sty; Submitted to MNRA