Nature of the Soft Spectral Component in the X‐Ray Pulsars SMC X‐1 and LMC X‐4

We present here the results of an investigation of the pulse averaged andpulse phase resolved energy spectra of two high luminosity accretion poweredX-ray pulsars SMC X-1 and LMC X-4 made with ASCA. The phase averaged energyspectra definitely show the presence of a soft excess in both the sources. Ifthe soft excess is modeled as a separate black-body or thermal bremsstrahlungtype component, pulse phase resolved spectroscopy of SMC X-1 shows that thesoft component also has a pulsating nature. Same may be true for LMC X-4,though a very small pulse fraction limits the statistical significance. Thepulsating soft component is found to have a nearly sinusoidal profile,dissimilar to the complex profile seen at higher energies, which can be aneffect of smearing. Due to very high luminosity of these sources, the size ofthe emission zone required for the soft component is large (radius ~300-400km). We show that the pulsating nature of the soft component is difficult toexplain if a thermal origin is assumed for it. We further investigated withalternate models, like inversely broken power-law or two different power-lawcomponents and found that these models can also be used to explain the excessat low energy. A soft power-law component may be a common feature of theaccreting X-ray pulsars, which is difficult to detect because most of the HMXBpulsars are in the Galactic plane and experience large interstellar absorption.In LMC X-4, we have also measured two additonal mid-eclipse times, whichconfirm the known orbital decay.Comment: 28 pages (AAS preprint), 10 Postscript figures, ApJ accepte