GALACTIC VERSUS EXTRAGALACTIC PIXEL LENSING EVENTS TOWARD M31

A new type of gravitational microlensing experiment toward a held wher e stars are not resolved is being developed observationally and theore tically: pixel lensing. When the experiment is carried out toward the M31 bulge area, events may be produced both by massive compact halo ob jects (MACHOs) in our Galactic halo and by lenses in M31. We estimate that similar to 10%-15% of the total events are caused by Galactic hal o MACHOs assuming an all-MACHO halo. If these Galactic events could be identified, they would provide us with an important constraint on the shape of the halo. We test various observables that can be used for t he separation of Galactic halo/M31 events. These observables include t he Einstein timescale, the effective duration of an event, and the flu x at the maximum amplification, but they cannot be used to separate th e two populations of events. However, we find that most high maximum-f lux Galactic halo events can be isolated through a satellite-based mea surement of the flux difference caused by the parallax effect. For the detection of the flux difference, it is required to monitor events wi th an exposure time of similar to 20 minutes by a 0.5 m telescope moun ted on a satellite. Such observations could be carried out as a minor component of a mission aimed primarily at events seen toward the Galac tic bulge and the Large Magellanic Cloud. In addition, proper motion c an be used to isolate Galactic halo/M31 events, but for only similar t o 5% of high signal-to-noise ratio M31 events and only 1% of Galactic halo events.