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

The Angstrom Project is using a distributed network of two-meter class telescopes to conduct a high cadence pixel-lensing survey of the bulge of the Andromeda Galaxy (M31). With the expansion of global telescope network, the detection efficiency of pixel-lensing surveys is rapidly improving. 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 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 < d̃ < 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 of q & 0.1 but with separations populated within a narrow range of 1 AU . d̃ . 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 lens matter. Subject headings: gravitational lensing – galaxies: individual (M31)