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Session 104 - Accretion-Powered X-Ray Sources.
Display session, Thursday, January 18
North Banquet Hall, Convention Center

[104.07] Photometry of Two Intense Low Mass X-Ray Binaries

S. Wachter, B. Margon, S. Anderson (U. of Washington)

The intense galactic X-ray source GX349+2 (Sco X-2) belongs to the class of persistently bright low-mass X-ray binaries called Z-sources. GX349+2 has only recently been optically identified with a 19th mag star. Of the six known Z-sources, only two (\mboxSco X-1 and Cyg X-2) have been studied in the optical. It has been suggested that Z-sources as a group are characterized by evolved companions and correspondingly long orbital periods (\mboxSco X-1, P=0.8\,d; \mboxCyg X-2, P=9.8\,d). Recently Southwell et al. have presented spectroscopic observations of GX349+2 suggesting a 14\,d orbital period. We have obtained broadband photometry of the system on six consecutive nights in May 1995, and find evidence for a 21.7 \pm 0.3\,hr period of 0.14 mag half-amplitude, superposed on erratic flickering typical of Sco X-1 type objects. As with other Z-sources, caution will be needed to insure that the variations are truly periodic, and not simply due to chaotic variability observed over a relatively short time span. If our period is confirmed, then the nature of the 14\,d spectroscopic variation found by Southwell et al. is unclear. GX13+1 is a bright X-ray burst source, located in the galactic bulge. Due to heavy obscuration, no optical counterpart brighter than R\sim 22 has been detected, but an infrared counterpart (K=12) has recently been identified by Naylor et al. (1991) based on spatial coincidence with an accurate radio position. GX13+1 is unusual as there is a disagreement over its classification. Studies of the X-ray time variability place it among the Atoll-sources. However, there is some evidence that the system contains a giant companion (Garcia et al. 1992) which would place it among the Z-sources. In an attempt to determine the period of the system, we observed GX13+1 for 9 days in May -- July 1995. Preliminary photometry confirms variability of \sim0.4 mag on a timescale of several days, as previously discovered by Charles amp; Naylor (1992). If GX13+1 is found to have a large orbital period, it would be indicative of a giant companion, and thus challenge the distinction between Atoll- and Z-sources on the basis of orbital and evolutionary characteristics.

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