Timing and spectral studies of LMC X-4 in high and low states with BeppoSAX: detection of pulsations in the soft spectral component

Abstract

We report here detailed timing and spectral analysis of two BeppoSAX observations of the binary X-ray pulsar LMC X-4 carried out during the low and high states of its 30.5 day long superorbital period. Timing analysis clearly shows 13.5 s X-ray pulsations in the high state of the superorbital period, which allows us to measure the mideclipse time during this observation. Combining this with two other mideclipse times derived earlier with ASCA, we have derived a new estimate of the orbital period derivative. Pulse-phase-averaged spectroscopy in the high and low states shows that the energy spectrum in the 0.1-10 keV band comprises a hard power law, a soft excess, and a strong iron emission line. The continuum flux is found to decrease by a factor of ~60 in the low state, while the decrease in the iron-line flux is only by a factor of ~12, suggesting a different site for the production of the line emission. In the low state, we have not found any significant increase in the absorption column density. The X-ray emission is found to come from a very large region, comparable to the size of the companion star. Pulse-phase-resolved spectroscopy in the high state shows a pulsating nature of the soft spectral component with some phase offset compared to the hard X-rays, as is known in some other binary X-ray pulsars.