Open AccessMACHO photometry of two Large Magellanic Cloud Be X‐ray transients, EXO 0531–66 and H 0544–665
Author(s) -
McGowan K. E.,
Charles P. A.
Publication year - 2002
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.2002.05675.x
Subject(s) - physics , astrophysics , photometry (optics) , light curve , amplitude , astronomy , large magellanic cloud , galaxy , stars , optics
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 time‐scales involved are so long (tens of days to years) that systematic studies have been very restricted. The ∼8‐yr 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 Hα emission; for EXO 0531–66 we find the emission varies over time, thereby confirming its optical identification.