Timing and spectral studies of LMC X-4 in high and low states with Beppo-SAX: Detection of pulsations in the soft spectral component

Abstract

We report here detailed timing and spectral analysis of two Beppo-SAX observations of the binary X-ray pulsar LMC X-4 carried out during the low and high states of its 30.5 days long super-orbital period. Timing analysis clearly shows 13.5 s X-ray pulsations in the high state of the super-orbital period which allows us to measure the mid-eclipse time during this observation. Combining this with two other mid-eclipse times derived earlier with the ASCA, we 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 of 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.