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K.E. McGowan (LANL), P.A. Charles (University of Southampton)
The light curves of Be X-ray binaries display long-term modulations which are thought to be due to non-orbital changes in the structure of the Be star's equatorial disc. Due to the timescales involved (tens of days to years) it is difficult to study these modulations using normal observing programs. The 8 years of LMC data collected by the MACHO project therefore presents an ideal opportunity to investigate such systems. Here we present observations of two LMC Be X-ray transients, EXO0531-66 and H0544-665, the MACHO light curves of which show substantial (0.5 mag) long-term variations. However, our analysis shows little evidence for any periodic phenomena in the light curves of either source. We find upper limits for detection of a short (1-100 d) periodicity in the V- and R-band light curves of EXO0531-66 of 0.041 mag and 0.047 mag semi-amplitude, respectively. The upper limits for the V- and R-band data of H0544-665 are 0.054 mag and 0.075 mag semi-amplitude, respectively. We find that both sources become redder as they brighten. This indicates that the variations in the light curves are likely to be due to variations in the structure of the equatorial disc around the Be star. We also present spectra of the sources, both of which show H\alpha emission. We find that the emission in the spectra of EXO0531-66 varies over time, and we confirm that the optical companion of the source is the northern component of a close double.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.