(Proposal ID) S17B-076

(PI) Takada, Masahiro

(Proposal Title)
Constraining primordial black holes with HSC observation of M31 -- Part II

(Abstract)
We carried an unprecedented dense-cadence (2 min sampling) r-band observation 
of M31 with HSC to search for microlensing (ML) events of stars in M31 due to 
primordial black holes (PBHs) in the Milky Way (MW) halo, which are viable 
dark matter (DM) candidates (Niikura, Takada et al. 2017). We found one 
possible candidate of PBH ML that cannot be otherwise ruled out with the 
currently available data, and derived the tightest upper bound on the abundance
of PBHs over a wide range of mass scales, M_{PBH}~ [10^{-13},10^{-6}]M_sun. 
Here we propose to carry out additional cadence r-band observation of M31, 
over 3 nights in total that are split into 6 half nights over different months.
Our goals are: (1) To test the nature of the previous candidate; whether the 
candidate shows a signature of repeated time variability (it should not, if it 
is a real PBH ML). (2) To search for new ML events and obtain new constraints 
in order to "close" an allowed window of PBH mass, where a single mass-scale 
PBH makes up all DM.  We will also use the constraint to test other exotic DM 
scenario such as axion DM model. (3) To search for a longer timescale ML event 
in order to constrain heavier PBHs, towards stellar mass-scale PBH that are 
potential progenitors of LIGO gravitational wave sources.