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Session 84 - Neutron Stars and Their Friends.
Oral session, Thursday, June 13
Historical Society,
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.
