PLANET observations of microlensing event OGLE-1999-BUL-23: limb darkening measurement of the source star
Abstract
We present PLANET observations of OGLE-1999-BUL-23, a binary-lens microlensing event towards the Galactic bulge. PLANET observations in the I and V bands cover the event from just before the first caustic crossing until the end of the event. In particular, a densely-sampled second caustic crossing enables us to derive the linear limb-darkening coefficients of the source star; cV = 0.786+0.080-0.078 and cI = 0.632+0.047-0.037. Combined analysis of the light curve and the color-magnitude diagram suggests that the source star is a G/K subgiant in the Galactic bulge (Teff = 4800 K). The resulting linear limb-darkening coefficient of the source is consistent with theoretical predictions, although it is likely that non-linearity of the stellar surface brightness profile complicates the interpretation, especially for the I band. The global light curve fit to the data indicates that the event is due to a binary lens of a mass ratio q = 0.39 and a projected separation d = 2.42. The lens/source relative proper motion is (22.8 + 1.5) km/s/kpc, typical of bulge/bulge or bulge/disk events.
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