Predicted multiple caustic-passage binary-lens event OGLE 2005-BLG-018

Predicted multiple caustic-passage binary-lens event OGLE 2005-BLG-018

First posted on homepage: 21-May-2005
Moved to news archive: 6-Oct-2005

Observations by OGLE that revealed a first peak around HJD=2453461 (31-Mar), with a half-maximum width of about 8 d, and a subsequent rise observed both by OGLE and PLANET/RoboNet with the Faulkes North 2.0m led to models of the event that predicted at least one pair of fold-caustic passages.

In PLANET anomaly alert 2005 #0 (issued 9-May, 23:15 UT), we reported that we expect the lens to to be a stellar binary with a mass ratio in the range q ∼ 0.7–1.0, where this is however quite uncertain. If this interpretation is approximately correct, a fold-caustic entry is expected within the subsequent 10 days.

As pointed out in PLANET anomaly update 2005 #0-1 (issued 18-May, 18:10 UT), data collected after 9-May by OGLE and PLANET/Robonet with the Danish 1.54m at ESO LaSilla and the Faulkes North 2.0m has been used to refine our models of this event. We now believe that it is likely that the source is on the approach to a cusp until HJD=2453513.9 (23-May, 9:30 UT) before it enters a fold caustic at around on HJD=453526.66 (5-June, 3:50 UT), which is exited about 3 days later. This prediction is based on the assumption of a point-like source.

In PLANET anomaly update 2005 #0-2 (issued 21-May, 8:10 UT), we reported that PLANET data collected with the Canopus 1.0m near Hobart show that the expected cusp passage started around HJD=2453511.1 (20-May, 14:30 UT). Data obtained with the Danish 1.54m at ESO LaSilla taken from HJD=2453511.628 (21-May, 3:05 UT) confirm this interpretation and show that the passage is in progress, where it still might take up to 24 hrs until the peak is reached, roughly in agreement with the prediction of our latest model for a point-like source calculated before the caustic passage started. Prior to our update, OGLE had issued a secondary alert reporting the anomaly being in progress.

As announced in PLANET anomaly update 2005 #0-3 (issued 21-May, 13:45 UT), PLANET data taken with the Canopus 1.0m near Hobart between HJD=2453512.016 (21-May, 12:23 UT) and HJD=2453512.023 (21-May, 12:33 UT) suggest a flattening in the light curve compared to the rise observed in earlier data collected by PLANET with the Danish 1.54m at ESO LaSilla and by OGLE, where the OGLE data was particularly useful for us for properly aligning the Danish data in magnitude. The flattening suggests that the event was around peak at the time when the anomaly update was circulated. If this interpretation is correct, it would imply a passage duration of ∼ 1.8 days, so that the caustic will be exited at about HJD=2453513.95 (23-May, 11:00 UT).

It has turned out that the previously suggested flattening resulted from a misalignment between observing sites and that the event continued to rise instead. In PLANET anomaly update 2005 #0-4 (issued 23-May, 16:25 UT), we reported that a model arising from just placing a finite source on top of our most recent point-source model provides a reasonable fit of the caustic entry as well as all data collected by PLANET with the Canopus 1.0m near Hobart and the Danish 1.54m at ESO LaSilla and by OGLE during the caustic passage between HJD=2453511.1937 (20-May, 16:39 UT) and HJD=3512.9325 (22-May, 10:23 UT), where the first data point arises from the Canopus 1.0m and the most recent one from OGLE, who also took the last data point before the caustic entry. Despite the success of our model in the region of the caustic passage, it fails on reproducing the first peak, which could be attributed to the negligence of both the orbital motion of the binary lens and the annual parallax caused by the revolution of the Earth. While we will try to resolve this, we expect that it will affect the light curve over the next 10-14 days much less than it did since the first peak (that occured around 31-Mar, 7.5 weeks ago). Unfortunately, we currently lack of more recent data that could confirm the model, contradict it, or yield further constraints. If our model is right, the caustic peak is strongly skewed, hanging towards later epochs, and the maximum was reached at HJD=2453512.85 (22-May, 8:25 UT) with 3.9 mag above baseline. We expect a current magnification of 3.3 mag and a decrease with the rather constant rate of 0.45 mag/day until about HJD=245315.5 (25-May, 0:00 UT), where the light curve starts flattening until reaching a minimum near HJD=2453518.2 (27-May, 17:00 UT) at 2.1 mag above baseline.

PLANET data taken with the Perth 0.6m from HJD=2453513.0986 (22-May, 14:22 UT) to HJD=2453513.2982 (22-May, 19:09 UT) and from HJD=2453514.1307 (23-May, 15:08 UT) to HJD=2453514.3640 (23-May, 20:44 UT) reveal a stronger decrease in magnitude than our simple model predicted. However, as pointed out in PLANET anomaly update 2005 #0-5 (issued 25-May, 10:20 UT), a parallax-included model provides a reasonable fit to both the caustic peak region and the first peak. Although Perth data were not included (in a first fit), they match the revised model prediction almost perfectly. This model predicts the source to exit the caustic near HJD=2453516.12 (25-May, 14:55 UT) and re-enter near HJD=2453520.73 (30-May, 5:30 UT) before finally exiting around HJD=2453530.56 (9-Jun, 1:26 UT). With the source moving under a small angle with respect to the caustic tangent, the epoch of caustic re-entry is difficult to estimate and therefore subject to refinement as further data has been obtained. A small shortcoming around the first peak should be resolved by lens orbital motion which has been found to play a smaller role than parallax from the data. However, this might also affect the future predicted behaviour a bit. The data point taken at the Danish 1.54m in the night of 24-May is strongly doubtful, thanks to the bright sky due to the moon. If this measurement is correct however, it would point to an earlier caustic exit, which could have already taken place.

We reported in PLANET anomaly update 2005 #0-6 (issued 1-Jun, 9:15 UT) that PLANET data taken with the Danish 1.54m at ESO LaSilla, the Perth 0.6m and the Canopus 1.0m near Hobart, in agreement with OGLE data indicate that the source has exited the caustic before HJD=2453516.664 (26-May, 3:57 UT) and remained outside until that time. For a proper interpretation of the light curve, both the annual parallax resulting from the revolution of the Earth around the Sun, and the orbital motion of the stellar binary comprising the lens have to be taken into account. Since both of these effects can locally cause similar deviations, it is not easy to find their true combination, resulting in some uncertainty for the prediction of the expected re-entry of the source into the caustic. However, our models reveal a preferred time range for the caustic re-entry of HJD=2453523.4 +/- 0.1 (1-Jun, 19:00 - 24:00 UT). A precise determination of this epoch by collected data would provide strong constraints for the model parameters, while observations shortly after the caustic entry are highly sensitive to the brightness profile of the stellar limb.

In PLANET anomaly update 2005 #0-7 (issued 3-Jun, 01:00 UT) we reported the re-entry of the source into the caustic soon after HJD=2453523.8 (2-Jun, 07:12 UT), based on PLANET data from the Canopus 1.0m and the Danish 1.5m telescopes, as well as OGLE data. The event is now rising at around 0.3 mag per day, as shown by additional Canopus, Danish, and OGLE data since the alert.

With the second caustic passage being in progress, PLANET anomaly update 2005 #0-8 (issued 5-Jun, 14:30 UT) reported that an updated model predicts the light curve to reach a peak around HJD=2453528.93 (7-Jun, 10:20 UT) and then fall at an increasing rate until the trailing limb of the source exits the caustic at about HJD=2453531.88 (10-Jun, 9:05 UT). Observations around an expected inflection point near HJD=2453527.90 (6-Jun, 9:35 UT) can be even more valuable for measuring the stellar brightness profile near the limb than those in the vicinity of caustic entries or exits.

In PLANET anomaly update 2005 #0-9 (issued 13-Jun, 11:10 UT), we reported that the final caustic exit likely occured between HJD=2453531.90 (10-Jun, 9:30 UT) and HJD=2453532.00 (10-Jun, 12:00 UT). Our models predict the light curve to decrease monotonically subsequently until reaching baseline. No further rise is expected.

With its long-lasting caustic passage of up to about a week duration, OGLE 2005-BLG-018 provides an unprecedented opportunity for resolving a stellar atmosphere. The huge potential is linked to several favourable properties, namely the long event time-scale, the large angular source size as compared to the angular Einstein radius, the brightness of the source, the variety of caustic crossing angles, and the predictability. Photometric and spectroscopic follow-up observations are strongly encouraged.

A good coverage of all the distinctive features in this event will moreover allow a reliable determination of the characteristics of the underlying binary lens.

Current data and model are shown on our regularly updated light curve.

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