A study of the populations of X-ray sources in the Small Magellanic Cloud with ASCA

The Advanced Satellite for Cosmology and Astrophysics (ASCA) has made multi ple observations of the Small Magellanic Cloud (SMC). X-ray mosaic images i n the soft (0.7-2.0 keV) and hard (2.0-7.0 keV) bands are separately constr ucted, and the latter provides the first hard X-ray view of the SMC. We ext ract 39 sources from the two-band images with a criterion of S/N > 5 and co nduct timing and spectral analyses for all of these sources. Coherent pulsa tions are detected from 12 X-ray sources, five of which are new discoveries . Most of the 12 X-ray pulsars are found to exhibit long-term flux variabil ities; hence they are likely to be X-ray binary pulsars (XBPs). On the othe r hand, we classify four supernova remnants (SNRs) as thermal SNRs, because their spectra exhibit emission lines from highly ionized atoms. We find th at XBPs and thermal SNRs in the SMC can be clearly separated by their hardn ess ratio (the ratio of the count rate between the hard and soft bands). Us ing this empirical grouping, we find many XBP candidates in the SMC, althou gh no pulsations have yet been detected from these sources. Possible implic ations on the star formation history and evolution of the SMC are presented by a comparison of the source populations in the SMC and our Galaxy.