Optical spectroscopy and photometry of the periodic X-ray transient A0538-66 (X0535-668) during an outburst and an OFF state

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Abstract
We present optical spectroscopy and photometry of the periodic X-ray transient A0538–66. Extensive spectroscopy performed in 1982 March follows the evolution of the multi-component line profiles through an outburst. Particularly distinctive are the broad, asymmetric emission lines which are interpret as arising in an extensive stellar wind. Photometric measurements during an X-ray OFF state in 1983 May reveal V-band variability with an amplitude of 0.2 mag on a time-scale of days. During this quiescent interval the Hβ line displays an ‘inverse P-Cygni’ profile suggesting the presence of circumstellar material in the system even during the inactive state. We obtain a radial velocity curve 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 1982 May.

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