With galactic microlensing events being sensitive to the distribution of mass among the lens objects and the surface brightness of the source star, the same event can provide valuable new insight into different fields at the current forefront of science. In fact, the binarity of the lens (including the presence of planets around the lens star) is probed along with stellar atmosphere models that predict a brightness profile for the source star.

As reported in PLANET anomaly alert 2005 #10 (issued 22-Jul, 6:20 UT), data collected with the Danish 1.54m at ESO LaSilla (Chile) show a slowly-varying magnification at a level of ~1.5 mag above previous single-lens point-source models. A likely interpretation of this is the finite source passing over a caustic created by a binary lens, where the latter was entered roughly between HJD=2453571.8 and HJD=2453572.3 (20-Jul, 7:00 to 19:00 UT). Independently, the OGLE collaboration alerted on the same behaviour seen in their data.

In PLANET anomaly update 2005 #10-1 (issued 23-Jul, 8:20 UT), we reported that PLANET data taken with the SAAO 1.0m at Sutherland (South Africa) and the Danish 1.54m at ESO LaSilla (Chile) show that the light curve of OGLE 2005-BLG-327 remained roughly flat between HJD=2453574.252 (22-Jul, 18:03 UT) and HJD=2453574.695 (23-Jul, 4:41 UT) at a level about 0.1 mag fainter than the points collected the previous night, which triggered our anomaly alert.

After the high-magnification level was retained for more than 3 days, PLANET anomaly update 2005 #10-2 (issued 26-Jul, 8:55 UT) reported a rise by about 0.15 mag over 8 hrs seen in PLANET data taken during the most recent night with the Danish 1.54m at ESO LaSilla (Chile). This could be a sign of the source approaching a caustic exit.

As pointed out in PLANET anomaly update 2005 #10-3 (issued 28-Jul, 18:05 UT), the observed rise steepened significantly over the previous 1.5 days, reaching about 0.7 mag/day, and latest Tasmanian data from the most recent night initially show a continuation of this trend, followed by a flattening. This suggests the suspected caustic exit being in progress and the associated peak being reached. A first exploration of parameter space (by D. Kubas) revealed the possibility that the observed anomaly results from a jovian planet orbiting the lens star, while there is a roughly similar probability for a more moderate mass ratio between the components of the considered binary lens.

In PLANET anomaly update 2005 #10-4 (issued 29-Jul, 8:15 UT), we reported that we succeeded in observing OGLE 2005-BLG-327 during most of the second half of the caustic exit of OGLE 2005-BLG-327 with the Danish 1.54m at ESO LaSilla (Chile), where a typical sampling interval of 3 min provides a quasi-continuous coverage. Moreover, prior to these data, about 40 points have been taken with the SAAO 1.0m at Sutherland (South Africa). Independently, the OGLE team reported that their data show a fading by more than 2 mag during the recent night. With the last Danish 1.54m data point taken at HJD=2453580.8146 (29-Jul, 7:33 UT), the end of the caustic exit was not reached yet.

As announced in PLANET anomaly update 2005 #10-5 (issued 30-Jul, 8:35 UT), PLANET data taken with the Danish 1.54m at ESO LaSilla (Chile) during the night starting 29-Jul indicate a decrease of OGLE 2005-BLG-327 by less than 0.2 mag/day. Our observations show that the source is expected to have left the caustic between HJD=2453580.815 (29-Jul, 7:33 UT) and HJD=2453580.820 (29-Jul, 7:41 UT) after it took between 20 and 21.5 hrs to cross it.

Our recent data is in agreement with previous binary-lens models with moderate mass ratios, but appear to be in conflict with our planetary models. However, the dense observations towards the end of the caustic exit provide a good opportunity for studies about the atmosphere of the source star.

The observation of another caustic entry was reported in PLANET anomaly update 2005 #10-6 (issued 15-Aug, 7:40 UT) after OGLE 2005-BLG-327 rose slightly during the preceding two weeks. PLANET data taken with the Danish 1.54m at ESO LaSilla (Chile) indicate an increase by about 2.8 mag from HJD=2453596.6225 (14-Aug, 2:56 UT) and HJD=2453597.7503 (15-Aug, 6:00 UT), where the source has likely entered a caustic between the former epoch and HJD=2453596.7259 (14-Aug, 5:25 UT).

Continued dense monitoring will provide powerful constraints on the nature of the lens system, including the potential assessment of annual parallax effects and the orbital lens motion. Moreover, caustic passages under different angles enhance the opportunities for measuring the brightness profile of the source star.

Data obtained on this event are shown on our light curve.