Optical spectroscopy and photometry of the periodic X-ray transient AO 538-66 ( X 0535-668) during an outburst and an OFF state.

Abstract

Optical spectroscopy and photometry of the periodic X-ray transient A0538 - 66 are presented. Extensive spectroscopy performed in March 1982 follows the evolution of the multi-component line profiles through an outburst. Particularly distinctive are the broad, asymmetric emission lines which are interpreted as arising in an extensive stellar wind. Photometric measurements during an X-ray OFF state in May 1983 reveal V-band variability with an amplitude of 0.2 mag on a time-scale of days. During this quiescent interval the H-beta line displays an 'inverse P-Cygni' profile suggesting the presence of circumstellar material in the system even during the inactive state. A radial velocity curve is obtained that appears to confirm the model for A0538 - 66 in which a neutron star is in an eccentric orbit about the primary and generates X-ray and optical flares at periastron passage. The long-term evolution of the optical colors of A0538 - 66 during quiescence reflects the changing nature of the circumstellar matter than envelops the primary star. No evidence of the star's 69-ms X-ray periodicity is found in optical data taken during an outburst in May 1982.