S18B0093

We propose a 3-year monitoring observation of M31 with HSC in order to search for microlensing events of stars in M31 due to primordial black holes (PBHs) of 10M mass scales in the halo regions of MW and M31, which are candidates of the LIGO GW events. We request about 10min exposure in total (5 visits of 90sec exposure plus 30sec readout) for each of the gr-bands within each HSC run (each month), where the two filter data are needed to confirm the achromatic nature of the microlensing. Since M31 is accessible during June and Feb each year, we can obtain up to 24 epochs worth of M31 images over 3 years. The total exposure including the overhead is about 16 hours, i.e. 3 nights if including the weather factor. Based on the methods and knowledge we have developed for the previous microlensing search due to lunar mass-scale PBHs, we will deliver the tightest constraint on the mass fraction of ∼ 10M PBHs to the dark matter, down to a sub-percent fraction level, which can confirm or falsify the PBH scenario for the LIGO events in Sasaki et al. (2016). The unique dataset of M31 enables us not only to find various types of variable stars, but could also discover unanticipated transients such as direct collapse of supergiant and neutron star merger.


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