http://arxiv.org/abs/1604.08214
Estimates of accretion rate on to compact objects are often based on the well-known, spherically symmetric, steady-state solution due to Bondi. This solution assumes that there is a sink of mass at the center — which in case of a black hole (BH) corresponds to the advection of matter across the event horizon. Other stars, such as a neutron star (NS), have surfaces and hence the infalling matter has to slow down at the surface. We study the initial value problem in which the matter distribution is uniform and at rest at $t=0$ with different inner radial boundary conditions for BHs and NSs: outflow boundary condition (mimicking mass sink at the center) valid for BHs; and {\em reflective} and steady-shock (allowing gas to cross the inner boundary at subsonic speeds) boundary conditions for NSs. We obtain a similarity solution for the flow with inner outflow and reflective boundary conditions (assuming a cold ambient medium) . 1-D simulations show the formation of an outward propagating and a standing shock in NSs for reflective and steady-shock boundary conditions, respectively. Entropy is the highest at the bottom for reflective boundary conditions. In 2-D this profile is convectively unstable and settles down to an isentropic, hydrostatic atmosphere in steady state. With the steady-shock inner boundary conditions, both the pre-shock and post-shock flows are isentropic but with a discontinuity at the shock location. This flow is unstable to the standing accretion shock instability (SASI), which can give rise to coherent, global shock oscillations. These oscillations can lead to periodic variations in the lightcurves of accreting sources and may be responsible for some of the observed quasi-periodic oscillations (QPOs). For steady accretion in the quiescent state, spherical accretion rate on to a NS can be suppressed by orders of magnitude compared to that on to a BH.
P. Dhang, P. Sharma and B. Mukhopadhyay
Fri, 29 Apr 16
57/57
Comments: abstract abridged; 15 pages, 17 figures, 1 table; submitted to MNRAS; comments are welcome
You must be logged in to post a comment.