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 the sources. If
the soft excess is modeled as a separate black-body or thermal bremsstrahlung
type component, pulse phase resolved spectroscopy of SMC X-1 shows that the
soft component also has a pulsating nature. Same may be true for LMC X-4,
though 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 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 investigated with
alternate models, like inversely broken power-law or two different power-law
components and found 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 the
accreting X-ray pulsars, which is difficult to detect because most of the HMXB
pulsars are in the Galactic plane and experience large interstellar absorption.
In LMC X-4, we have also measured two additonal mid-eclipse times, which
confirm the known orbital decay.Comment: 28 pages (AAS preprint), 10 Postscript figures, ApJ accepte
