Finite Source Sizes and the Information Content of MACHO-Type Lens Search Light Curves

If the dark halo matter is primarily composed of MACHOs toward the lower end of the possible detection range ($< 10^{-3}$ $M_{\odot}$) a fraction of the lens detection events should involve the lens crossing directly in front of the disk of the background star. Previously, Nemiroff (1987) has shown that each crossing would create an inflection point in the light curve of the MACHO event. Such inflection points would allow a measure of the time it took for the gravitational lens to cross the stellar disk. Given an independent estimate of the stellar radius by other methods, one could then obtain a more accurate estimate of the velocity of the lens. This velocity could then, in turn, be used to obtain a more accurate estimate of the mass range for the MACHO or disk star doing the lensing.Comment: in press: ApJ (Lett.), 10 pages in Plain TeX version 3.0, 1 figure available by FA