LMC X-4 - GINGA OBSERVATIONS AND SEARCH FOR ORBITAL PERIOD CHANGES

We report on the results of a 2.4 day observation of the binary X-ray pulsar LMC X-4 with the Ginga satellite. A high-quality X-ray spectrum which includes an Fe emission line with an equivalent width of 0.18 keV is presented. Timing analyses of the 13.5 s X-ray pulsations yield a Doppler delay curve, which, in turn, provides the most accurate determination of the LMC X-4 orbital parameters available to date. The phase of the 1.4 day orbit is determined with an accuracy of approximately 60 s. This phase is combined with four previous determinations of the orbital phase to yield an estimate of the rate of change in the orbital period: P(orb)P(orb) = (1.1 +/- 0.8) x 10(-6) yr-1. An interpretation of this measurement and the known decay rate for the orbit of Cen X-3 is made in the context of tidal evolution. During the observation two very large flares were observed with a duration of approximately 30 minutes; the source luminosity, at times, exceeded approximately 5 Eddington luminosities. The flares are described and discussed in some detail.