Review of Discrete X-Ray Sources in the Small Magellanic Cloud: Summary of the ASCA Results and Implication on the Recent Star-Forming Activity

Abstract

Abstract We made 22 observations on the Small Magellanic Cloud (SMC) and covered full regions of the main body and the eastern wing by the end of the ASCA mission. We detected 106 discrete sources with a criterion of $\mathrm{S/N} \gt 5$$\mathrm{S/N} \gt 5$ and performed systematic analyses on all of the sources. We determined the source positions with an $\sim {40{}^{\mathrm {\prime \prime }}}$ error radius (90% confidence) for sources detected in the central ${20{}^{\mathrm {\prime }}}$ radius of the GIS. We detected coherent pulsations from 17 sources. Among them, eight were newly discovered during this study. We classified most of these pulsars as X-ray binary pulsars (XBPs) based on their properties, such as the flux variability and the existence of an optical counterpart. We detected X-ray emission from eight supernova remnants (SNRs). Among them, five SNRs showed emission lines in their spectra, hence we regarded the five as thermal SNRs. We found that XBPs and thermal SNRs in the SMC can be clearly separated by their spectral hardness ratio. Applying this empirical law to faint (thus unclassified) sources, we found 19 XBP candidates and four thermal SNR candidates. We also found several tens of candidates for active galactic nuclei, both from the hardness ratio and the $\log N \hbox{--}\log S$ relation of extragalactic sources. Based on these ASCA results and further information from ROSAT, SAX, RXTE, CGRO, Chandra, and XMM-Newton, we compiled comprehensive catalogues of discrete X-ray sources in the Small Magellanic Cloud. Using the catalogues, we derived the spatial distributions of XBPs and SNRs. XBPs and SNRs were found to be concentrated in the main body and eastern wing, which resembles the distribution of young stars with ages of $\sim 2 \times 10^7 \,\mathrm{yr}$. By comparing the source populations in the SMC and our Galaxy, we suggest that the star-forming rate (per unit mass) in the SMC was much higher than the Galaxy $\sim 10^7 \,\mathrm{yr}$ ago. We also discuss the recent change of the star-forming rate in the SMC.