Discovery of an X-Ray Pulsar in the SMC: AX J0058-7203

We report on the discovery and analysis of an X-ray pulsar, AX J0058-7203, in the Small Magellanic Cloud. This pulsar exhibits coherent pulsations at P= 280.3 s with a double-peak structure. The X-ray spectrum is well fitted with a simple power-law model of photon index ~ 0.7. No significant change of the pulsation period over the observation was found. A comparison with ROSAT observations in the same field reveals that AX J0058-7203 is highly variable, and is most likely a Be star binary pulsar.Comment: 5 pages, 4 figures, to be published in PAS

ASCA Discovery of a Be X-Ray Pulsar in the SMC: AX J0051-733

ASCA observed the central region of the Small Magellanic Cloud, and found a hard X-ray source, AX J0051-733, at the position of the ROSAT source RX J0050.8-7316, which has an optical counterpart of a Be star. Coherent X-ray pulsations of 323.1 +/- 0.3 s were discovered from AX J0051-733. The pulse profile shows several sub-peaks in the soft (0.7-2.0 keV) X-ray band, but becomes nearly sinusoidal in the harder (2.0-7.0 keV) X-ray band. The X-ray spectrum was found to be hard, and is well fitted by a power-law model with a photon index of 1.0 +/- 0.4. The long-term flux history was examined with the archival data of Einstein observatory and ROSAT; a flux variability with a factor > 10 was found.Comment: 6 pages, 5 figures, accepted for publication in PAS

Discovery of the Slowest X-Ray Pulsar in the SMC, AX J0049.5-7323, with ASCA

The discovery of coherent pulsations with ASCA from an X-ray source, AX J0049.5-7323, is reported. The barycentric period was determined to be 755.5 +/- 0.6 s, which is the longest among X-ray pulsators in the SMC. The X-ray spectrum has been found to be unchanged through ASCA observations, with a photon index of ~0.8 and a luminosity of ~5e35 erg s^-1 (0.7-10 keV). Archival data of the Einstein and the ROSAT satellites indicate that the flux has been < 1e-12 erg s^-1 cm^-2 (< 5e35 erg s^-1) for over 20 years and exhibits a variability with a factor of >10. We argue that AX J0049.5-7323 is an X-ray pulsar with a Be star companion.Comment: 4 pages, 4 figures, to be published in PASJ. Also available at http://www-cr.scphys.kyoto-u.ac.jp/member/jun/job

Discovery of 101-s Pulsations from AX J0057.4-7325 in the SMC with ASCA

The results from two ASCA observations of AX J0057.4-7325 = RX J0057.3-7325 are presented. Coherent pulsations with a barycentric period of 101.45 +/- 0.07 s were discovered in the second observation. The X-ray spectrum was found to be hard (photon index ~ 0.9) and unchanged through these observations, except for the flux. The ROSAT archival data show that AX J0057.4-7325 exhibits a flux variation with a factor >~ 10. A discussion on a possible optical counterpart is given.Comment: 4 pages, 4 figures, to be published in PASJ. Also available at http://www-cr.scphys.kyoto-u.ac.jp/member/jun/job

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

We made 22 observations on the Small Magellanic Cloud (SMC) and covered full regions by the end of the ASCA mission. We detected 106 discrete sources with a criterion of S/N > 5 and performed systematic analyses on all of the sources. We determined the source positions with an ~40'' error radius (90% confidence) for sources detected in the central 20' 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 logN--logS relation of extragalactic sources. Based on these ASCA results and further information from other sattelites, 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 ~2e7yr. 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 1e7yr ago. We also discuss the recent change of the star-forming rate in the SMC.Comment: 61 pages, 19 figures, to be published in PASJ. Also available at http://www-cr.scphys.kyoto-u.ac.jp/member/jun/job/ (with high-resolution images