Detection of pulsating soft X-ray excess in the Magellanic Cloud pulsars with ASCA

Abstract

The binary X-ray pulsars which are away from the galactic plane and therefore experience less interstellar absorption, show the presence of a soft component in the spectrum which is often described as a black-body and/or thermal bremsstrahlung emission. A thermal component for the soft excess provides a satisfactory model in the Galactic sources like Her X-1 and 4U 1626-67. But for the pulsars in the Magellanic Clouds for which the distance is of the order of 60 kpc and the luminosity is close to the Eddington limit, a soft component dominating in the lower energy part of the spectrum requires emission region which is a few orders of magnitude larger than the size of a neutron star. Using ASCA observations, we show that a pulsating nature of the soft component in several bright pulsars is difficult to explain if a thermal origin is assumed for the low energy part of the spectrum. We investigate whether a two component power-law with different absorptions or an inversely broken power-law can instead explain the pulsations at low energy. A soft power-law component may also be a common feature of the binary X-ray pulsars, which is difficult to observe because most of the sources are in the galactic plane and experience large interstellar absorption.