Highly subEddington Spherical Accretion onto Isolated Neutron Stars
Abstract
Highly subEddington accretion may be a commonly occurring process among isolated old neutron stars which have spun down sufficiently to start accreting interstellar gas. We study this type of accretion in a spherically symmetric setting, which is applicable to stars moving at very subsonic speeds (<< 10 km/s) with respect to the ambient medium. We devised an iterative scheme to couple the accretion flow dynamics with the requisite nonequilibrium atomic processes. The dynamics are treated using the standard spherical flow equations while the energetics and ionization are treated using the MAPPINGS II photoionization code. Adopting a neutron star with M=1.4 M_sun and R=10 km, we attempt to find selfconsistent solutions for given choices of the mass accretion rate ({dot M}) and ionizing spectrum. We have two main results: (1) Steady state solutions are possible only for 10(9) < {dot M} \ (g/s) < 10(10) , or, medium densities 0.05 < n_infty \ (cm(3) ) < 0.5. For reference, {dot M}_{Edd}=10(18) g/s. Outside this regime, variability is expected on timescales ~ few years ( ~ flow time from the accretion radius at ~ 10(13) 10(14) cm). (2) For the steady state solutions, {dot M} obeys the Bondi formula (for adiabatic accretion) to a good approximation. We explore the implications of our results for the two candidate accreting isolated neutron stars, RX J18563754 and RX J07203125.
 Publication:

American Astronomical Society Meeting Abstracts
 Pub Date:
 December 1997
 Bibcode:
 1997AAS...19111302W