HEAD Division Meeting 1999, April 1999
Session 15. X-Ray Binaries
Poster, Tuesday, April 13, 1999, 8:30am-6:02pm, Gold Room

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[15.17] LMC~X--2: The Search for the Orbital Period

S. Wachter (CTIO), J. Greene (Yale/CTIO), A. P. Smale (NASA/GSFC)

LMC~X--2 is the most X-ray luminous low mass X-ray binary (LMXB) known and its X-ray characteristics, such as X-ray flaring activity and the lack of X-ray bursts, resemble those of the bright Galactic Z sources. In addition, its location in the LMC provides an accurately known distance and extinction, which are difficult parameters to determine for most Galactic LMXBs. LMC~X--2 also offers the unique opportunity to study accretion physics in a low metallicity environment in connection with a comparison to Galactic sources.

Considering its brightness, it is surprising how little is known about LMC~X--2. Its V ~18.5 counterpart shows a fairly typical LMXB spectrum, but its orbital period, the most basic quantity of any binary system, has not yet been determined despite extensive observations. 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). If LMC~X--2 is found to be a Z source, evolutionary scenarios call for an evolved mass donor and accordingly long orbital period.

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~hour each night for 44 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 ~.1~mag variability during the nightly observations. The search for the orbital period is greatly hindered by the strong periodic signal introduced by the sampling window. Using the CLEAN and PDM algorithms we find some evidence for periods at 8.18~hrs, 12.4~hrs and ~4~days. Large scatter is apparent in the light curves when the data is folded on any of these periods. Due to the large random photometric variability, radial velocity observations are urgently needed to confirm a possible 8~hour orbital period.

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