Discovery of 16.6 and 25.5 s Pulsations from the Small Magellanic Cloud

We report the serendipitous detection of two previously unreported pulsars from the direction of the Small Magellanic Cloud, with periods of 16.6 and 25.5 seconds. The detections are based on archival PCA data from the Rossi X-ray Timing Explorer (RXTE). The observation leading to these detections occurred in September 2000 extending over 2.1 days with an exposure of 121 ks. A possible identification of the 16.6 s pulsar with an X-ray source RX J0051.8-7310 seen by both ROSAT and ASCA imaging X-ray satellites is presented.Comment: 9 pages with 3 figures. Submitted to ApJ Letter

X-ray Pulsars in the Small Magellanic Cloud

XMM-Newton archival data for the Small Magellanic Cloud have been examined for the presence of previously undetected X-ray pulsars. One such pulsar, with a period of 202 s, is detected. Its position is consistent with an early B star in the SMC and we identify it as a high mass X-ray binary (HMXB). In the course of this study we determined the pulse period of the possible AXP CXOU J010043.1-721134 to be 8.0 s, correcting an earlier report (Lamb et al 2002b) of a 5.4 s period for this object. Pulse profiles and spectra for each of these objects are presented as well as for a recently discovered (Haberl & Pietsch 2004) 263 s X-ray pulsar. Properties of an ensemble of 24 optically identified HMXB pulsars from the SMC are investigated. The locations of the pulsars and an additional 22 X-ray pulsars not yet identified as having high mass companions are located predominately in the young (ages $\le 3 \times 10^{7}$ years) star forming regions of the SMC as expected on the basis of binary evolution models. We find no significant difference between the distribution of spin periods for the HMXB pulsars of the SMC compared with that of the Milky Way. For those HMXB pulsars which have Be companions we note an inverse correlation between spin period and maximum X-ray flux density. (This anti-correlation has been previously noted for all X-ray binary pulsars by Stella, White & Rosner 1986). The anti-correlation for the Be binaries may be a reflection of the fact that the spin periods and orbital periods of Be HMXBs are correlated. We note a similar correlation between X-ray luminosity and spin period for the Be HMXB pulsars of the Milky Way and speculate that exploitation of the correlation could serve as a distance indicator.Comment: final version accepted in The Astrophysical Journa

0103-72.6: A New Oxygen-Rich Supernova Remnant in the Small Magellanic Cloud

0103$-$72.6, the second brightest X-ray supernova remnant (SNR) in the Small Magellanic Cloud (SMC), has been observed with the {\it Chandra X-Ray Observatory}. Our {\it Chandra} observation unambiguously resolves the X-ray emission into a nearly complete, remarkably circular shell surrounding bright clumpy emission in the center of the remnant. The observed X-ray spectrum for the central region is evidently dominated by emission from reverse shock-heated metal-rich ejecta. Elemental abundances in this ejecta material are particularly enhanced in oxygen and neon, while less prominent in the heavier elements Si, S, and Fe. We thus propose that 0103$-$72.6 is a new ``oxygen-rich'' SNR, making it only the second member of the class in the SMC. The outer shell is the limb-brightened, soft X-ray emission from the swept-up SMC interstellar medium. The presence of O-rich ejecta and the SNR's location within an H{\small II} region attest to a massive star core-collapse origin for 0103$-$72.6. The elemental abundance ratios derived from the ejecta suggest an $\sim$18 M$_{\odot}$ progenitor star.Comment: 6 pages (ApJ emulator format), including 5 figures and 2 tables. For high quality Figs.1,2, & 3, contact [email protected]. Accepted by the ApJ Letter

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 multiple observations of the Small Magellanic Cloud (SMC). X-ray mosaic images in the soft (0.7--2.0 keV) and hard (2.0--7.0 keV) bands are separately constructed, and the latter provides the first hard X-ray view of the SMC. We extract 39 sources from the two-band images with a criterion of S/N>5, and conduct timing and spectral analyses for all of these sources. Coherent pulsations 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 variabilities, hence they are likely to be X-ray binary pulsars (XBPs). On the other hand, we classify four supernova remnants (SNRs) as thermal SNRs, because their spectra exhibit emission lines from highly ionized atoms. We find that XBPs and thermal SNRs in the SMC can be clearly separated by their hardness ratio (the ratio of the count rate between the hard and soft bands). Using this empirical grouping, we find many XBP candidates in the SMC, although no pulsations have yet been detected from these sources. Possible implications 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.Comment: 11 pages, 39 Figures, to be published in ApJ Supplement. Tables (body and figures also) are available at http://www-cr.scphys.kyoto-u.ac.jp/member/jun/job

An Investigation of Be/X-ray Pulsars with OGLE-III Data

We have studied five seasons of OGLE-III data for eight SMC Be/X-ray pulsars for which no other survey data were available. We have determined orbital periods for four of these binary systems, one of which also shows nonradial pulsations. Optical identification of SMC X-2 is reconsidered, but no periods were found for either of the two possible candidates

Where Are Be/black-hole Binaries?

We apply the tidal truncation model proposed by Negueruela & Okazaki(2001) to arbitrary Be/compact star binaries to study the truncation efficiency dependance on the binary parameters. We find that the viscous decretion disks around the Be stars could be truncated very effectively in narrow systems. Combining this with the population synthesis results of Podsiadlowski, Rappaport and Han (2003) that binary black holes are most likely to be born in systems with orbital periods less than about 30 days, we suggest that most of the Be/black-hole binaries may be transient systems with very long quiescent states. This could explain the lack of observed Be/black-hole X-ray binaries. We also discuss the evolution of the Be/black-hole binaries and their possible observational features.Comment: 14 pages,3 figures, ApJ accepte

AGN in the XMM-Newton first-light image as probes for the interstellar medium in the LMC

The XMM-Newton first-light image revealed X-ray point sources which show heavily absorbed power-law spectra. The spectral indices and the probable identification of a radio counterpart for the brightest source suggest AGN shining through the interstellar gas of the Large Magellanic Cloud (LMC). The column densities derived from the X-ray spectra in combination with HI measurements will allow to draw conclusions on HI to H_2 ratios in the LMC and compare these with values found for the galactic plane.Comment: 4 pages, LaTex, 4 figures, Accepted for publication in A&A Letter

Discovery of a new Transient X-ray Pulsar in the Small Magellanic Cloud

Rossi X-Ray Timing Explorer observations of the Small Magellanic Cloud have revealed a previously unknown transient X-ray pulsar with a pulse period of 95s. Provisionally designated XTE SMC95, the pulsar was detected in three Proportional Counter Array observations during an outburst spanning 4 weeks in March/April 1999. The pulse profile is double peaked reaching a pulse fraction \~0.8. The source is proposed as a Be/neutron star system on the basis of its pulsations, transient nature and characteristically hard X-ray spectrum. The 2-10 keV X-ray luminosity implied by our observations is > 2x10^37 erg/s which is consistent with that of normal outbursts seen in Galactic systems. This discovery adds to the emerging picture of the SMC as containing an extremely dense population of transient high mass X-ray binaries.Comment: Accepted by A&A. 7 pages, 6 figure

ASCA Observation of the New Transient X-ray Pulsar XTE J0111.2-7317 in the Small Magellanic Cloud

The new transient X-ray pulsar XTE J0111.2-7317 was observed with Advanced Satellite for Cosmology and Astrophysics (ASCA) on 1998 November 18, a few days after its discovery with the Proportional Counter Array onboard the Rossi X-ray Timing Explorer. The source was detected at a flux level of 3.6x10^-10 erg cm^-2 s^-1 in the 0.7--10.0 keV band, which corresponds to the X-ray luminosity of 1.8x10^38 erg s^-1, if a distance of 65 kpc for this pulsar in the Small Magellanic Cloud is assumed. Nearly sinusoidal pulsations with a period of 30.9497 +/- 0.0004 s were unambiguously detected during the ASCA observation. The pulsed fraction is low and slightly energy dependent with average value of \~27%. The energy spectrum shows a large soft excess below ~2 keV when fitted to a simple power-law type model. The soft excess is eliminated if the spectrum is fitted to an ``inversely broken power-law'' model, in which photon indices below and above a break energy of 1.5 keV are 2.3 and 0.8, respectively. The soft excess can also be described by a blackbody or a thermal bremsstrahlung when the spectrum above ~2 keV is modeled by a power-law. In these models, however, the thermal soft component requires a very large emission zone, and hence it is difficult to explain the observed pulsations at energies below 2 keV. A bright state of the source enables us to identify a weak iron line feature at 6.4 keV with an equivalent width of 50 +/- 14 eV. Pulse phase resolved spectroscopy revealed a slight hardening of the spectrum and marginal indication of an increase in the iron line strength during the pulse maximum.Comment: 8 pages, 5 Figures, to be published in ApJ. Also available at http://www-cr.scphys.kyoto-u.ac.jp/member/jun/job

Optical spectroscopy of 20 Be/X-ray Binaries in the Small Magellanic Cloud

We present a large sample (20 in total) of optical spectra of Small Magellanic Cloud (SMC) High-Mass X-ray Binaries obtained with the 2dF spectrograph at the Anglo-Australian Telescope. All of these sources are found to be Be/X-ray binaries (Be-XRBs), while for 5 sources we present original classifications. Several statistical tests on this expanded sample support previous findings for similar spectral-type distributions of Be-XRBs and Be field stars in the SMC, and of Be-XRBs in the Large Magellanic Cloud and the Milky Way, although this could be the result of small samples. On the other hand, we find that Be-XRBs follow a different distribution than Be stars in the Galaxy, also in agreement with previous studies. In addition, we find similar Be spectral type distributions between the Magellanic Clouds samples. These results reinforce the relation between the orbital period and the equivalent width of the Halpha line that holds for Be-XRBs. SMC Be stars have larger Halpha equivalent widths when compared to Be-XRBs, supporting the notion of circumstellar disk truncation by the compact object.Comment: 26 pages, 8 figures, accepted for publication in Ap