) Pergamon www.elsevier.com/locate/asr Adv. Space Res. Vol. 28, Nos 2-3, pp. 399-403, 2001 © 2001 COSPAR. Published by Elsevier Science Ltd. All fights reserved Printed in Great Britain 0273-1177/01 $20.00 + 0.00 PII: $0273-1177(01)00438-0 DETECTION OF PULSATING SOFT X-RAY EXCESS IN THE MAGELLANIC CLOUD PULSARS WITH ASCA B. Paul 1, J. Yokogawa 2, M. Nishiuchi 2, T. Endo 3, T. Dotani 3, and F. Nagase 3 1 Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India 2Department of Physics, Kyoto University, Kyoto 606-8502, Japan 3Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa, 229-8510, Japan 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. © 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved. INTRODUCTION The X-ray continuum spectra of accreting pulsars are often described to be a broken power-law type or a power-law type with exponential cutoff. The break in the spectrum is in the range of 10-20 keV and power-law photon index bellow the break energy is in the range of 1-2 (White, Nagase and Parmar 1995). Some 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 modeled as a blackbody and/or thermal bremsstrahlung emission or an inversely broken power-law. The soft component in 4U 1626- 67 when modeled as a black body emission requires the size of emission region to be comparable to that of the neutron star because the intrinsic luminosity of this source is of the order of 1035 erg s -1 (Orlandini et al. 1998). Similarly, the soft component in Her X-1 can be modeled as a black-body which is reprocessed emission from the innermost part of the accretion disk (Endo et al. 2000). However, for the bright X-ray pulsars in the Magellanic Clouds for which the distance is of the order of 50--60 kpc and the luminosity is close to the Eddington limit, a thermal model for the soft component requires an emission region which is much larger than the size of a neutron star. Therefore, a pulsating nature of the soft component that has been observed in the high luminosity X-ray pulsars LMC X-4 (Woo et al. 1996, present work), SMC X-1 (present work) and XTE J0111.2-7317 (Yokogawa et al. 2000b) is difficult to explain if a thermal origin is assumed for the soft excess. LMC X-4 and SMC X-1 are two bright, eclipsing, accreting binary X-ray pulsars with spin periods of ~13.5 and ~0.7 s and binary periods of -v1.4 and ,-~3.9 day respectively. The companions of both the pulsars are high mass main sequence stars and the binary orbits are nearly circular in both the systems. The orbital period of the two binaries are found to decay with time scale of 106 yr in LMC X-4 and 3 × 105 yr in SMC X-1. Another one striking similarity between these two sources is a long-period of 30.5 and 50-60 days respectively, that is known to be stable in LMC X-4 and quasi-stable in SMC X-1. The long-period 399