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 $\\Gamma_{\\rm b}\\sim (7-15)f_{\\rm b}(N/50)$ per season, where $f_{\\rm b}$ is the fraction of binaries with projected separations of $10^{-3} {\\rm AU}<\\tilde{d}<10^3 {\\rm AU}$ 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 $q\\gtrsim 0.1$ but with separations populated within a narrow range of $1 {\\rm AU}\\lesssim \\tilde{d}\\lesssim 5 {\\rm AU}$. 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.

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