After data collected by OGLE, MOA, PLANET/RoboNet and MicroFUN indicated that event OGLE 2005-BLG-158 / MOA 2005-BLG-17 would peak at a magnification in excess of 50, assuming a point-like source, the monitoring of this event has been intensified, since finite-source effects as well as lens binarity could be expected to show up. While the observation of finite-source effects provides the opportunity to resolve the stellar atmosphere, events where the angular separation between lenses and sources becomes very small, resulting in very large peak magnifications, harbour a large potential for planets around the lens star to be revealed.

As reported in PLANET anomaly alert 2005 #2 (issued 20-May, 7:15 UT), PLANET succeeded in obtaining data with the Danish 1.54m telescope between HJD=2453510.7526 (20-May, 6:04 UT) and HJD=2453510.7963 (20-May, 7:07 UT), despite bad weather at the site. These data show that the event was near peak at a magnification of about 90, while further monitoring still being in progress.

After the observations at the Danish 1.54m terminated for the night at HJD=2453510.9321 (20-May, 10:22 UT), we reported in PLANET anomaly update 2005 #2-1 (issued 20-May, 14:40 UT) that the light curve near the peak showed a flattening that indicates finite-source effects. The same behaviour is independently confirmed by OGLE data covering a similar time range. From a simple finite-source model (that needs some refinement), we estimate that the source moves by its radius in about 9 hours relative to the lens. With the source being a clump giant with angular radius ~ 5.7 μas (A. Gould, private communication), this implies a relative proper motion of ~ 15 μas/day. More information about the nature of the event will result from PLANET data taken before the peak with the Canopus 1.0m near Hobart.

The finite-source model is displayed on our light curve.