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Z. E. Takoy (Kurchatov Inst.), M. V. Medvedev, N. Murray (CITA)
A new hot accretion flow onto a rapidly rotating neutron star has recently been discovered by Medvedev & Narayan. This flow is similar to the standard advection-dominated accretion flow (ADAF) in some respects, namely it is geometrically thick, optically thin, relatively radiatively inefficient, and two-temperature, with cool electrons and hot (nearly virial) protons. However, the physics of the flow is drastically different from ADAF. The settling flow extracts rotational energy of a rapidly rotating star via a strong viscous torque in the boundary layer where the flow meets the star surface. The extracted energy is converted into heat (again, by viscosity) and is radiated away. The star spin-down rate and the flow luminosity strongly depend on the star rotational velocity, whereas they are completely independent of the accretion rate (so long as it is less then a few percents of Eddington). Such a flow forms around neutron stars rotating at velocities of about ten percents of the Keplerian velocity.
One might think that no settling flow may occur around a black hole because of the no-slip condition at the horizon. However, the torque may be transported from the hole via magnetic stresses. If magnetic fields threading the horizon are anchored in the accretion flow, angular momentum which powers the settling flow is extracted from the hole via the Blandford-Znajek process rather than viscous stresses. The settling flow onto a back hole may be accompanied by collimated outlows (jets).
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