Magnetic Inhibition of Accretion and Observability of Isolated Old Neutron Stars

Details Magnetic Inhibition of Accretion and Observability of Isolated Old Neutron Stars Magnetic Inhibition of Accretion and Observability of Isolated Old Neutron Stars

Aug 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003apj...593..472t&link_type=abstract

The Astrophysical Journal, Volume 593, Issue 1, pp. 472-480.

Astronomy and Astrophysics

Astronomy

21

Accretion, Accretion Disks, Magnetic Fields, Plasmas, Stars: Magnetic Fields, Stars: Neutron, X-Rays: Stars

Scientific paper

Isolated old neutron stars moving through the interstellar medium capture matter gravitationally. If the star is unmagnetized, the captured matter accretes to the surface of the star. However, the stars are expected to be magnetized. Moreover, some of the stars may be in the ``propeller'' stage of evolution. Both the magnetic field and the rotation act to decrease the accretion rate to the surface of the star. Here we consider stars that are past the propeller stage, so that rotation is unimportant. The influence of the magnetic field on the accretion rate to the star's surface is investigated using axisymmetric, resistive magnetohydrodynamic (MHD) simulations. Matter is taken to inflow at the Bondi rate for a nonmagnetized star, and we verify that stationary Bondi accretion flows occur in the absence of a magnetic field. For a magnetized star we find that an outward-propagating shock wave forms and that a new stationary, subsonic accretion flow is set up inside this shock, as first pointed out by Toropin et al. in 1999. Accretion to the surface of the star M occurs along two columns aligned with the magnetic axis of the star. Only a fraction of the Bondi flux MB accretes to the surface of the star. The empirical dependences we find are M/MB~(R*/RA)5 for RA/R*~6-10, where RA is the Alfvén radius. In terms of the star's magnetic moment μ, we find M/MB~μ-3. The accretion rate decreases as the magnetic diffusivity of the plasma ηm decreases, M~(ηm)0.6. We conclude that even a very small residual magnetic field, B~106-108 G, may significantly reduce the accretion rate to the surface of the star and thereby make the accretion luminosity undetectable. The possibility of enhanced accretion owing to three-dimensional instabilities remains to be investigated. The results presented here may also be applicable to wind-fed X-ray stars in binary systems.

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