OPTICAL STUDIES OF THE X-RAY TRANSIENT EXO 0748-676.

Abstract

ABSTRACT Identification of the optical counterpart of the transient X-ray source EXO 0748-676 is reported. The identification was made during a search of the X-ray error circle on March 1, 1985, when the optical counterpart was at 17th magnitude. The observations were carried out using the Charge-Coupled imaging Device (CCD) attached to the l-meter Yale telescope at CTIO. CCD images of the field containing the X-ray error circle are provided. Spectroscopic analysis of the CCD images showed absorption features in the wavelengths bands double lambda 7150-7400 (H2O), 7600-7700 (A band), and 8100-8400 (H2O). No absorption or emission features were found in excess of the statistical noise. CCD photometry of EXO 0748-676 is given in a table. It is argued that EXO 0748-676 almost certainly comprises a neutron star receiving matter from a secondary star in close orbit around it. The presence of an accretion disk is inferred from the longer duration of the optical eclipse as compared to the X-ray eclipse. The mass of the secondary star is estimated near a few times 0.1 solar mass. Some similarities between the optical spectrum EXO 0748-676 and the optical spectra of other low-mass binary X-ray transients and luminous cataclysmic variable stars are discussed.