Triggers:Microlensing
Standard events
Amplitude: can be many magnitudes, in range 0.1-10 mag.
Time-scale: is defined as Einstein Ring crossing time and is close to FWHM of the bell-like light curve of the event.
The light curves are usually symmetric with slow rise and slow decline. In case of high magnification, the rise in magnitude speeds up closer to the peak.
Sources: can be located anywhere, however, the further source, the higher chance of it being microlensed.
Exotic events
It is believed, around 10% of all events (at least towards the bulge) exhibit some kind of anomalous behaviour. In most cases it due to binary companion either to the source or the lens. The light curves are then usually asymmetric, with high amplification regions and sharp rises in magnitude in case of binary lenses.
Figure from Jaroszynski et al. 2004.
OGLE's EWS
OGLE Early Warning System (EWS) is detecting in real-time about 600 events per year towards the Galactic bulge.
Distribution of time scales corrected for detection efficiency for OGLE-III Bulge events.
All-sky events
99% of events are currently being detected towards the Galactic Centre, due to its largest optical depth. There is couple of events reported towards the Magellanic Cloud.
Serendipitous detection of a microlensing event located not in the crowded field: Gaudi et al. 2008
GSC 3656-1328
Detection efficiency for Gaia
Over 5 years of operation of Gaia, there should be around 3000 microlensing events occurring in the Bulge. Assuming mean time-scale of 30 days and mean detection efficiency for events before the peak of 30%, it means about a 1000 events towards the Bulge should be detected by Gaia. This number, however, might be affected by crowding effects in the Bulge. On the other hand, the satellite is planned to be slowed down a bit to observe Baade's Window (towards the Bulge) more frequently, what can increase the efficiency of detection.
Over entire sky there is something like 750 events expected to happen during the mission. If the mean time-scale is about 15 days, the mean efficiency is about 20%, thus there should be additional 150 events detected all over the sky before they reach the peak brightness.
