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J. Greene, S. Wachter (CTIO), A.P. Smale (NASA/GSFC)
LMC X-2 is the most X-ray luminous low mass X-ray binary (LMXB) known, as well as one of the five brightest X-ray sources in the LMC. It has been tentatively classified as a Z source based on its high luminosity and X-ray flaring activity. Its location in the LMC provides a known distance and low line of sight absorption, neither of which are easy to determine for Galactic LMXBs. In addition, the low metallicity of the LMC offers the opportunity to compare the physical properties of LMXBs in different environments. However, despite extensive observation, no orbital period has yet been established for this system. Orbital period searches have produced contradictory results, with evidence for photometric and/or spectroscopic periods of 6.4 hrs (Bonnet-Bidaud et al. 1989), 8.15 hrs (Callanan et al. 1990) and 12.5 days (Crampton et al. 1990).
We present synoptic V band photometry of LMC X-2 obtained with the YALO 1m telescope at CTIO. LMC X-2 was observed for 1 hr each night for 45 nights from 9 Nov 1998 to 8 Jan 1999. We find LMC X-2 to be variable over a range of V=17.9-18.9. Our observed light curve is very complex, exhibiting modulations of up to 0.5 mag in the brightness level from night to night in addition to ~0.1 mag variability during the nightly observations. Using the CLEAN and PDM algorithms we find the strongest periodic signal at 8.18 hrs. If LMC X-2 is indeed a Z source with an ~ 8 hr period, this would challenge current evolutionary scenarios in which Z sources contain an evolved mass donor and thus a long orbital period. Due to the random variability component apparent in our lightcurve, additional photometry is unlikely to yield a more accurate determination of the orbital period. Therefore, radial velocity observations of LMC X-2 are urgently needed to confirm a probable 8 hr orbital period.
This research was carried out as part of the 1999 Research Experiences for Undergraduates program at CTIO, funded by the National Science Foundation.
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