OGLE-III EARLY WARNING SYSTEM - 2004 SEASON

• 41 events in 1998,
• 48 events in 1999,
• 78 events in 2000.

In April/May 2002 the new EWS system was implemented for the third phase of the OGLE project - OGLE-III. In the 2004 Galactic bulge season about 120 million stars are regularly monitored and analyzed by the EWS system. Currently observed fields are shown as blue squares in this map of the Galactic center. 40 fields in the Small Magellanic Cloud (13 square degrees) and 116 fields in the Large Magellanic Cloud (38 square degrees) are also regularly monitored for microlensing events.

During the previous observing seasons of OGLE-III the EWS system discovered the following number of microlensing event candidates:

• 2002 season - about 350.
• 2003 season - about 450.

Another real time system designed for detection of transient objects in the OGLE-III fields - the NOOS system - was implemented in 2003. Many objects detected by NOOS can also be microlensing events of faint stars - fainter than the OGLE-III detection limit when unmagnified. See the NOOS system page for details and light curves of detected transients.

The Early Warning System implementation for OGLE-III is much different than during the previous phases. The old OGLE photometric data pipeline was replaced by the new one, based on Difference Image Analysis (DIA) photometry (Alard and Lupton 1998, ApJ, 503, 325, Alard 2000, Aston. Astrophys. Suppl. Ser., 144, 363, Wozniak 2000, Acta Astron., 50, 421). DIA photometry ensures better quality photometry in dense stellar fields compared to the classical approach with the profile fitting photometry.

The EWS analyzes photometry of all stars in the field and flags a star when it is detected in the difference images three times in a row. For microlensing event detection only objects increasing brightness are marked. Initial filtering masks the objects which were detected in difference images during the previous observing seasons (3-10% of all stars in the frame) to avoid false signals by variable stars.

The light curves of flagged stars are inspected visually, their positions on the recent CCD frames are checked for bad pixels, bleeding columns, very bright neighboring stars etc. If a star passes all these checks, it is announced as a microlensing candidate.

One has to remember that we cannot guarantee that all alerted stars are indeed true microlensing events. However, our experience from the previous years ensures that the number of false alerts should be relatively small.

So far, the EWS system has detected 608 candidate microlensing events in the 2004 observing season. The table given below provides interactive WWW access to the finding charts, photometry data and other information about all events. Currently ongoing events are marked with . Particularly interesting events are marked with .

Microlensing events data can also be found on Data download site or on Anonymous FTP.

There is also a mailing list providing on-line notification on the detected events. Astronomers interested in follow-up observations and willing to receive e-mail notification, should fill out subscribe/unsubscribe form. If you have any questions or comments, send us a message.

The following table contains basic parameters of the candidate events as well as the links to the WWW pages with finding charts, light curve plots and photometric data. The microlensing light curve model is a simple point mass approximation with blending to give a preliminary information on the event in progress. Blending is included only when (a) f_bl + three times the formal error is less than 1.00 and (b) f_bl is larger than its formal error. Please note that some of the fit parameters may change considerably when the event develops. No parameters are given for a possible binary microlensing. Microlensing light curve model fit parameters have following meaning:

• T_max - Heliocentric Julian Date at maximum brightness (also given as UT date)
• tau - event time scale (time in which the lens moves by the length of the Einstein ring radius)
• u_min - minimum 'u' (at maximum amplification)
• A_max - maximum amplification (intensity units)
• D_mag - maximum amplification (magnitudes)
• f_bl - blending ratio - fraction of the lens light to the total blend light. 1.000 means no blending
• I_bl - total base I-band magnitude
• I₀ - base (minimum) I-band magnitude for lens.

The OGLE EWS photometry is provided here for informative purposes only to facilitate the microlensing community monitoring the on-going events and follow up observations. Please contact us if you want to use the data for publication. We may require to be co-authors when our photometry is important for analysis.

When referring to the OGLE EWS system, please cite the paper by Udalski 2003, Acta Astron., 53, 291 and provide the WWW address of the EWS system or the OGLE WWW home page.