MACHO Photometry of Two LMC Be X-ray Transients, EXO 0531-66 and H 0544-665

Abstract

Long-term variations are well-known in Be X-ray binaries, and are attributed to non-orbital changes in the structure of the Be circumstellar (equatorial) disc. However, the timescales involved are so long (tens of days, to years) that systematic studies have been very restricted. The ~8 year MACHO monitoring of the Large Magellanic Cloud (LMC) therefore presents an ideal opportunity to undertake such studies of Be X-ray systems that lie within the monitored fields. Here we present MACHO observations of two LMC Be X-ray transients, EXO 0531-66 and H 0544-665, the 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 an upper limit for detection of a short (1-100 d) periodicity in the V- and R-band light curves of EXO 0531-66 of 0.041 mag and 0.047 mag semi-amplitude, respectively. The upper limits for the V- and R-band data of H 0544-665 are 0.054 mag and 0.075 mag semi-amplitude, respectively. Both EXO 0531-66 and H 0544-665 become redder as they brighten, possibly due to variations in the structure of the equatorial disc around the Be star. Spectra of both sources show H$\alpha$ emission; for EXO 0531-66 we find the emission varies over time, thereby confirming its optical identification.

8 pages, 7 figures, accepted for publication in MNRAS

References in corpus (4)

• Calibration of the MACHO Photometry Database
• EXPERIMENTAL LIMITS ON THE DARK MATTER HALO OF THE GALAXY FROM GRAVITATIONAL MICROLENSING.
• Probable Gravitational Microlensing towards the Galatic Bulge
• A 421 d Activity Cycle in the BeX Recurrent Transient A0538-66 from MACHO monitoring

Cited by in corpus (5)

• Optical and X-ray variability of two Small Magellanic Cloud X-ray binary pulsars - SXP46.6 and SXP6.85
• The XMM-Newton survey of the Small Magellanic Cloud: XMMUJ010633.1-731543 and XMMUJ010743.1-715953, two new Be/X-ray binary systems
• Periodic Optical Outbursts from the Be/Neutron Star Binary AX J0049.4-7323
• Populations of highly variable X-ray sources in the XMM$-$Newton slew survey
• Timing and spectral changes of the Be X-ray transient EXO 0531-6609.2 through high and low state