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 and pulse-phase-resolved energy spectra of two high-luminosity accretion-powered X-ray pulsars SMC X-1 and LMC X-4 made with ASCA. The phase-averaged energy spectra definitely show the presence of a soft excess in both sources. If the soft excess is modeled as a separate blackbody- or thermal-bremsstrahlung-type component, pulse-phase-resolved spectroscopy of SMC X-1 shows that the soft component also has a pulsating nature. The same may be true for LMC X-4, although a very small pulse fraction limits the statistical significance. The pulsating soft component is found to have a nearly sinusoidal profile, dissimilar to the complex profile seen at higher energies, which can be an effect of smearing. Due to the very high luminosity of these sources, the size of the emission zone required for the soft component is large (radius ~300-400 km). We show that the pulsating nature of the soft component is difficult to explain if a thermal origin is assumed for it. We further investigate with alternate models, such as an inversely broken power law or two different power-law components, and find that these models can also be used to explain the excess at low energy. A soft power-law component may be a common feature of accreting X-ray pulsars, which is difficult to detect because most high-mass X-ray binary pulsars are in the Galactic plane and experience large interstellar absorption. In LMC X-4, we have also measured two additional mideclipse times, which confirm the known orbital decay.