Journal Article

Orbital period determinations for four SMC Be/X-ray binaries

M. P. E. Schurch, M. J. Coe, V. A. McBride, L. J. Townsend, A. Udalski, F. Haberl and R. H. D. Corbet

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 412, issue 1, pages 391-400
Published in print March 2011 | ISSN: 0035-8711
Published online March 2011 | e-ISSN: 1365-2966 | DOI:
Orbital period determinations for four SMC Be/X-ray binaries

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We present an optical and X-ray study of four Be/X-ray binaries located in the Small Magellanic Cloud (SMC). OGLE I-band data of up to 11 years of semicontinuous monitoring has been analysed for SMC X-2, SXP172 and SXP202B, providing both a measurement of the orbital period (Porb= 18.62, 68.90 and 229.9 d for the pulsars, respectively) and a detailed optical orbital profile for each pulsar. For SXP172 this has allowed a direct comparison of the optical and X-ray emission seen through regular RXTE monitoring, revealing that the X-ray outbursts precede the optical by around 7 d. Recent X-ray studies by XMM–Newton have identified a new source in the vicinity of SXP15.3 raising doubt on the identification of the optical counterpart to this X-ray pulsar. Here we present a discussion of the observations that led to the proposal of the original counterpart and a detailed optical analysis of the counterpart to the new X-ray source, identifying a 21.7 d periodicity in the OGLE I-band data. The optical characteristics of this star are consistent with that of a SMC Be/X-ray binary. However, this star was rejected as the counterpart to SXP15.3 in previous studies due to the lack of Hα emission.

Keywords: stars: emission-line, Be; Magellanic Clouds; X-rays: binaries

Journal Article.  6991 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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