Wind Accretion onto Compact Objects: Hydrodynamical Studies

Astronomy and Astrophysics – Astronomy

Scientific paper

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Scientific paper

The Burst And Transient Source Experiment on the Compton Gamma Ray Observatory is currently producing beautiful observations of the brightest dozen or so accretion-powered pulsars. These sources, in which a magnetic neutron star accretes matter from a close binary companion, display a rich variety of phenomena. Much of the observed behavior is not currently understood, particularly in the case of accretion from the stellar wind of a massive OB companion star. In these systems, the neutron star shows alternating periods of spin-up and spin-down behavior which last anywhere from one day to more than one hundred days. Previous hydrodynamical calculations in two dimensions have shown that accretion from a stellar wind can be very unsteady, sometimes leading to the formation of accretion disks which rotate alternately in either direction. While these simulations are suggestive, particularly for bright X-ray transients in which a Be star undergoes episodes of equatorial mass ejection, they are inadequate for modelling accretion from a true three-dimensional stellar wind. We present results from two- and three-dimensional hydrodynamical calculations of wind accretion by a compact object. These calculations have been performed using an Eulerian hydrodynamics code which implements the piecewise parabolic method (PPM) to integrate the fluid variables on a fixed grid in spherical coordinates. The code was run on the Intel Touchstone Delta parallel-processing supercomputer. These calculations represent the first high-resolution three-dimensional models of wind accretion by a compact object which include transverse density gradients caused by the binary orbital motion. These models promise to shed new light on the observed erratic spin behavior of pulsars in high-mass X-ray binary systems.

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