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Session 43 - Source Surveys.
Display session, Wednesday, June 11
South Main Hall,
A microlensing survey of M31 has the advantages of sensitivity to lensing by Machos in the halos of both the Galaxy and M31, with a high optical depth, \tau_M31 up to 10^-5 (c.f. \tau_Gal of 10^-6 assuming all Macho spherical halos [Crotts 1992, ApJ, 399, L43]). In contrast to single sight-line surveys through the Galactic halo towards the LMC (MACHO, Alcock et al. 1997a ApJ in press [astroph/9606165]; EROS, Ansari et al., 1996, Aamp;A, 314, 94) and the Galactic Bulge (OGLE, Udalski et al. 1994, Act. Ast., 44, 165; MACHO, Alcock et al. 1997b ApJ in press), M31 offers many sight lines in a small area of sky. Furthermore, due to the high inclination of M31 \tau_M31 is strongly modulated from the far to near side of the galaxy, implying that a measure of \tau across the face of the galaxy could constrain the shape of its halo. Given these motivations we have been engaged in a survey since 1994 to search for microlensing in M31 on various timescales, primarily with the KPNO 4-m and the VATT 1.8-m telescopes. We use an image subtraction technique to isolate and photometer individual variables among M31's unresolved stars (Tomaney amp; Crotts 1996, AJ, 112, 2872). The survey has been successful at detecting many variables: in particular a 11'\times11' far-side field in the bulge/inner disk has yielded over 2200 variables. From these we have reported six candidates that are consistent with microlensing from objects with minimum masses around 0.5 M_ødot within the constraints of our data (Crotts amp; Tomaney 1996, ApJ, 473, 87). We present data taken in 1996 which extends these light curves and eliminates one candidate. We also discuss how Miras with periods around 240^d could mimick these light curves given our present sampling.
