A Model of Sub-Eddington Accretion onto a Highly Magnetized Neutron Star









Andrei M. Bykov and Alexandre M. Krassilchtchikov



A.F.Ioffe Institute for Physics and Technology, 26 Politekhnicheskaya, St. Petersburg 194021, Russia

Published in Astronomy Letters, v. 30, p. 309, 2004.









Abstract

A non-stationary one-dimensional collisionless two-fluid model of sub-Eddington column accretion onto a magnetized neutron star is developed. Evolution of an accreting flow is studied for a range of accretion rates and magnetic fields within a first-order Godunov scheme with source terms splitting. Strong shocks accompanied by hot plasma regions are found to develop on time scales of about 10-5 s and be stable up to 10-2 s or even more. Cyclotron line transfer in the column based on the constructed flow profiles is considered. It is shown that a substantial part of the CNO nuclei is likely to survive in modeled columns and this may be important for modeling of the first type of X-ray bursts. A comparison of the model predictions with observations of X-ray binary systems by modern X-ray and gamma-ray missions can reveal typical conditions in an accretion column as well as answer a fundamental question of hydrodynamical character of accreting flows.

The main external parametres of the model are:


M*, r* - mass and radius of the NS
B* - magnetic field strength on the surface of the NS
[M\dot] / A0 - accretion rate per unit surface area of a polar cap



The main physical processes we take into account are:




Simulated flow profiles, emerging spectra, and destruction probabilities for a carbon nucleus are shown on Fig.1-Fig.5 for a range of parametres. [M\dot]15 is the accretion rate in 1015 g s-1, while B12 is the surface magnetic field in 1012 G.

The flow profiles demonstrate a strong and stable shock that heats the flow. At first the ions are heated more than the electrons, as they contain most of the kinetic energy of the flow, but then they pass a part of their energy to the electrons via Coulomb collisions.

The emerging spectra demonstrate broad absorption features due to cyclotron line transfer and photoabsorption.

The dependence of carbon destruction probabilities on the accretion rate has a maximum at [M\dot]15 = 3. This is due to the fact that as the accretion rate is increased, the predominant factor is first the growth of the column density of hot plasma traversed by a nucleus, and then the decrease of the total time spent by a nucleus in the hot region.













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