Detection of M31 Binaries via High-Cadence Pixel-lensing Surveys

The Angstrom Project is using a distributed network of 2 m class telescopes to conduct a high-cadence pixel-lensing survey of the bulge of the Andromeda galaxy (M31). In this paper we estimate the detection rate of binary-lens events expected from high-cadence pixel-lensing surveys toward M31 such as the Angstrom Project based on detailed simulation of events and application of realistic observational conditions. Under the conservative detection criteria that only high signal-to-noise ratio caustic-crossing events with long enough durations between caustic crossings can be firmly identified as binary-lens events, we estimate that the rate would be Γb ~ (7 - 15)fb(N/50) per season, where fb is the fraction of binaries with projected separations of 10-3 AU < < 103 AU out of all lenses and N is the rate of stellar pixel-lensing events. We find that detected binaries would have mass ratios distributed over a wide range down to q ≳ 0.1 but with separations populated within a narrow range of 1 AU ≲ ≲ 5 AU. Implementation of an alert system and subsequent follow-up observations would be important not only for the increase of the binary-lens event rate but also for the characterization of the lens matter.