The X-Ray Absorption Spectrum of VELA X-1

Abstract

Exosat observations of the massive X-ray binary Vela X-1 are spectrally analyzed. The absorbing column density shows erratic factor of 10 increases up to 5 x 10 to the 23rd H/sq cm that can occur at any orbital phase and last from minutes to many hours. These variations occur on top of a base level of absorption that increases from 3 x 10 to the 22nd H/sq cm to 15 x 10 to the 22nd H/sq cm from orbital phase about 0.2 to 0.85. It is suggested that these changes are due to intermittent formation of a tenuous accretion disk around the neutron star. The X-ray spectra show a low-energy excess above that expected from cold photoelectric absorption. The character of this excess depends on orbital phase and two different modifications to the simple absorption model are required.