OGLE‐2003‐BLG‐262: Finite‐Source Effects from a Point‐Mass Lens

We analyze OGLE-2003-BLG-262, a relatively short ( t E = 12.5 ± 0.1 day) microlensing event generated by a point-mass lens transiting the face of a K giant source in the Galactic bulge. We use the resulting finite-source effects to measure the angular Einstein radius, θ E = 195 ± 17 μas, and so constrain the lens mass to the FWHM interval 0.08 < M / M ☉ < 0.54. The lens-source relative proper motion is μ rel = 27 ± 2 km s -1 kpc -1 . Both values are typical of what is expected for lenses detected toward the bulge. Despite the short duration of the event, we detect marginal evidence for a "parallax asymmetry" but argue that this is more likely to be induced by acceleration of the source, a binary lens, or possibly by statistical fluctuations. Although OGLE-2003-BLG-262 is only the second published event to date in which the lens transits the source, such events will become more common with the new OGLE-III survey in place. We therefore give a detailed account of the analysis of this event to facilitate the study of future events of this type.