255 research outputs found
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
Suzaku X-Ray Observation of the Dwarf Nova Z Camelopardalis at the Onset of an Optical Outburst
We present the result of a Suzaku X-ray spectroscopic observation of the
dwarf nova Z Camelopardalis, which was conducted by chance at the onset of an
optical outburst. We used the X-ray Imaging Spectrometer (a 38 ks exposure) and
the Hard X-ray Detector (34 ks) to obtain a 0.35-40 keV spectrum
simultaneously. Spectral characteristics suggest that the source was in the
X-ray quiescent state despite being in the rising phase of an outburst in the
optical band. The spectrum shows a clear signature of circumstellar absorption
in excess of interstellar absorption and the reprocessed emission features of
Fe fluorescence and Compton scattering. The extra absorption is explained due
to partial coverage by either neutral or ionized matter. We found a spectral
change during the observation, which is attributable only to the change in the
circumstellar absorption. Such an X-ray spectral variation is reported for the
first time in dwarf novae. We speculate that the variation in the circumstellar
absorption is interpreted as a time-varying disk wind or geometrically flaring
disk around the white dwarf during the propagation of a heat wave inward along
the accretion disk at the beginning of the outburst, in which optical outburst
and X-ray quiescent states co-exist.Comment: 10 pages, 9 figures, accepted for publication in PAS
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
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