Finite-source peak anomaly in OGLE 2005-BLG-226

Finite-source peak anomaly in OGLE 2005-BLG-226

First posted on homepage: 22-Jul-2005
Moved to news archive: 6-Oct-2005

The observed rising part of the light curve of event OGLE 2005-BLG-226 let us expect a large peak magnification of A_0 ~ 20 or even more, resulting in a large sensitivity for picking up lens binarity, including the presence of planets around the lens star, or finite-source effects. As announced in PLANET anomaly alert 2005 #9 (issued 22-Jul, 3:25 UT), just in the vicinity of the expected peak, data collected with the Danish 1.54m at ESO LaSilla (Chile) show a rise by more than 1 mag with respect to a best-fitting single-lens point-source model, in agreement with a caustic entry near HJD=2453573.2 (21-Jul, 17:00 UT). A similar rise has been inpependently observed by OGLE, which triggered their early early-warning system (EEWS), resulting in a secondary alert being circulated.

In PLANET anomaly update 2005 #9-1 (issued 22-Jul, 12:20 UT), we reported that PLANET data collected with the Danish 1.54m at ESO LaSilla (Chile) between HJD=2453573.523 (22-Jul, 0:33 UT) and HJD=2453573.807 (22-Jul, 7:22 UT), OGLE data extending this range to earlier epochs including HJD=2453573.473 (21-Jul, 23:21 UT), and PLANET data from the Perth 0.6m near Bickley (Western Australia) taken between HJD=2453573.939 (22-Jul, 10:32 UT) and HJD=2453574.006 (22-Jul, 12:08 UT) favour the finite source passing over several caustic lines at once (e.g. near a cusp) rather than the light curve being due a the caustic entering a single fold line. By estimating the source to have taken 9-15 hours from the caustic entry to the observed peak near HJD=2453573.75 (22-Jul, 6:00 UT), assuming rough symmetry in agreement with the most recent data, a caustic exit is expected between HJD=2453574.12 (22-Jul, 14:50 UT) and HJD=2453574.38 (22-Jul, 21:10 UT).

As stated in PLANET anomaly update 2005 #9-2 (issued 23-Jul, 7:50 UT), PLANET data collected with the Canopus 1.0m near Hobart (Tasmania) and the SAAO 1.0m at Sutherland (South Africa) show that there is a slope discontinuity in the light curve of OGLE 2005-BLG-226 near HJD=2453574.26 (22-Jul, 18:15 UT), where a steep decrease changes to a rather flat behaviour, indicating a caustic exit along our predictions.

Recent data are shown on our light curve.

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