Coordinated X-ray and optical observations of the pulsating X-ray transient A 1118-61.

Abstract

An EXOSAT observation of the field of the X-ray transient A 1118-61 reveals the presence of a weak hard X-ray source in the detectors of the medium energy experiment (ME). The detection by the Einstein IPC instrument of a similar hard X-ray source at a position consistent with that of the Be star He3-640, the proposed optical counterpart of A 1118-61, supports the conclusion that the ME flux is the quiescent emission of the transient source. The ME flux implies a quiescent X-ray luminosity L_x = 0.5 - 3.0×10³⁴erg s^-1. This indicates that accretion still takes place and that the mechanism of centrifugal inhibition is not at work, in accord with the long spin period of the neutron star. Coordinated optical observations of He3-640 show that the strong Balmer emission of the Be star increased between 1976 and 1985. It is shown that the presence of a dense envelope is compatible with the low quiescent X-ray emission if the orbital period is longer than ≡100 days.