Early Warning System

The "Early Warning System" (EWS) is a new feature implemented in the OGLE data system. It allows the detection and identification of microlensing events in real time. Thus, the system makes it possible to reschedule observations to obtain the best coverage of a microlensing event while it unfolds, as well as to arrange follow-up photometric and/or spectroscopic observations from other observatories. The system was tested in April 1994 during the first subrun of OGLE 1994 season, and also on the archive data from 1992 and 1993.

Currently OGLE monitors about two milion non-variable stars in the Galactic Bulge every night with the Swope 1 meter telescope at the Las Campanas Observatory. These are stars that have shown no brightness variations exceeding photometric errors over a period of two seasons (1992 and 1993, about 1,100,000 stars in 13 fields) or one season (1993 only, about 900,000 stars in 7 fields). The fields are identified in two papers: Udalski et al., 1992, Acta Astron., 42, 253, and 1994, Acta Astron., 44, 1. Our 1994 observing season is from April 21 to September 15 in the following subruns:
Apr 21 - Apr 26, Jun 7 - Jun 21, Jun 30 - Jul 16, Jul 24 - Aug 6, Aug 19 - Aug 28, Sep 5 - Sep 15.

All new CCD frames are reduced within 24 hours, and the current magnitude of a non-variable object is compared with the mean magnitude from previous season(s). If the object is brighter then a pre-assigned threshold a flag is set in the EWS database. When 5 sequential observations over the threshold are detected the EWS sends an alarm message to the operator.

Following the EWS alarm message the following procedure is performed. First, the full light curve from the current season is inspected to check whether the candidate star behaved in a reasonable way over the entire season: are the light variations smooth and do they resemble the rising branch of a microlensing light curve. Second, the most recent frame is inspected to check whether the star is located in a good frame region, its measurements are not contaminated by chip defects etc, and its brightness really increases. Third, for at least three further nights the candidate is observed twice per night and if the smooth brightness increase continues the candidate is marked as the "prime microlensing candidate", and the message is e-mailed to the interested observers. It includes all the available photometric data for the star, its coordinates, and a finding chart in a form of a PostScript subframe around the candidate: 150×150 pixels of 0.44'' (66''×66'').

* What is the probablility that the "prime microlensing candidate" is indeed a microlensing event?

The EWS system as described was tested extensively on the archive observing data from 1993 and 1992 seasons. All microlensing events reported to date by the OGLE (Udalski et al., 1993, Acta Astron., 43, 289; 1994, ApJ, 426, L69) were easily detected and identified in "real-time", while no other object met the "prime microlensing candidate" criteria. For 1994 observing season the non-variable object database is even more secure as for 13 out of 20 fields the database of non-variable stars is based on two observing seasons. The tests performed in the real envinronment in April 1994 showed no microlensing candidates and no spurious alarms. Thus, we estimate that the "hit rate" should be above 90%. However, we cannot exclude the possibility that some "prime microlensing candidates" might turn out to be non-microlensing events as the selection is made on the basis of the beginning of the rising branch only. Nevertheless, any object increasing its brightness after remaining constant for up to two years is interesting and worth observing, anyway. On the basis of our 1992/1993 experience we expect that between 2 and 4 events with a characteristic time scale in excess of 10 days will be found by the OGLE in 1994.

* What kind of follow-up observations are important?
Any photometric observations will improve light curve coverage of the event. Photometric multicolor observations of the candidate are important to check achromaticity of the event. One should, however, be aware that the Galactic Bulge fields are very crowded so a good seeing site is necessary to obtain precise photometry. Spectroscopic observations may also help in establishing the microlensing nature of the event and help to identify the lensing object when the event is over. The most important are frequent observations near the maximum, as they may reveal the finite size of the source (Nemiroff and Wickramasinghe, 1994, ApJ, 424, L21) or the presence of planets around the lens (Mao and Paczynski, 1991, ApJ, 374, L37). Unfortunately, it is not possible to predict the time of maximum light with the early rising branch of the light curve on which the "prime microlensing candidate" is based, hence catching the peak intensity is possible only by means of frequent observations.

* Referencing
Information about the on-going microlensing events will be distributed to all interested observers via e-mail. This information will be placed in "public domain" by making it acessible over anonymous ftp from
host: (
directory: /ogle
file: ogle94.ews

Please feel free to redistribute it to others, but reference the source of information, i.e., the OGLE project. It is expected that the OGLE project will be acknowledged in any publication that will result from using the OGLE EWS information.
If you want to be on the OGLE EWS e-mailing list, please send your request to and describe briefly what kind of follow-up observations you intend to make (please specify the site(s), instrument(s), expected ability to get (use) the observing time). If you would like to accept a larger risk of a false alarm and prefer to receive the information at the time the EWS sends its alarm to the operator (i.e., 3 nights earlier than standard) - please state so clearly. Please note, however that in this case a fraction of false alarms may be substantial.
Andrzej Udalski and Michal Szymanski
Warsaw University Observatory
Warsaw, Poland