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Session 113 - Neutron Stars & Pulsars.
Oral session, Saturday, January 10
International Ballroom East,
Highly sub-Eddington 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 (\ll 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_ødot and R=10 km, we attempt to find self-consistent solutions for given choices of the mass accretion rate (\dot M) and ionizing spectrum.
We have two main results:
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 \sim few years (\sim flow time from the accretion radius at \sim 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 J1856-3754 and RX J0720-3125.
