Monitoring of fold-caustic exit in OGLE 2005-BLG-128

Monitoring of fold-caustic exit in OGLE 2005-BLG-128

First posted on homepage: 19-Jun-2005
Moved to news archive: 6-Oct-2005

After OGLE data indicated that a fold caustic, created by a binary lens, had been entered by the source star between 25-Apr and 4-May, and a rise to the corresponding caustic exit peak had been observed, the OGLE team predicted a subsequent caustic exit to occur on 18-Jun. As reported in PLANET anomaly alert 2005 #4 (issued 19-Jun, 6:40 UT), PLANET data collected with the Danish 1.54m at LaSilla between HJD=2453540.6105 (19-Jun, 2:39 UT) and HJD=2453540.6701 (19-Jun, 4:05 UT) show a decrease by about 0.5 mag/hour and, together with earlier OGLE data, indicate the trailing limb of the caustic exit to occur at HJD=2453540.99 -0.11 +0.44 (19-Jun, 11:50 UT) after a caustic passage time of (18 -3 +11) hrs. While the data yield a reasonable picture, these conclusions crucially depend on each of just two measurements, which might be affected by systematic offsets that cannot be properly assessed at the moment.

In PLANET anomaly update 2005 #4-1 (issued 21-Jun, 12:50 UT), we reported that the suspected early decrease in OGLE 2005-BLG-128 on 19-Jun has been contradicted by OGLE data collected later that night, which show that the event was still rising. While a single point from OGLE at HJD=2453541.6211 (20-Jun, 2:54 UT) indicates that the caustic peak has been passed, the flat behaviour of the light curve after HJD=2453541.7849 (20-Jun, 6:50 UT), revealed by PLANET data taken with the Danish 1.54m at ESO LaSilla, clearly shows that the source has exited the caustic. Another data point from the same site at HJD=2453542.7089 (21-Jun, 5:01 UT) suggests that the light curve remained flat since. OGLE and PLANET data collected on this event constrain the caustic exit to have lasted < 18 h.

Further monitoring of the event will help to constrain the binary nature of the lens.

Our regularly updated light curve shows current data along with a model.

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