Microlensing constraint on Primordial Black Hole abundance with Subaru Hyper Suprime-Cam observations of Andromeda

Abstract: We present updated microlensing analysis results based on high-cadence ($\sim$2~min) Subaru Hyper Suprime-Cam (HSC) observations of the Andromeda Galaxy (M31) in 2014, 2017, and 2020, yielding a total of 39.3 hours of data. We use a point-lens finite-source model for the microlensing light curve model and employ multi-stage selection procedures to identify microlensing candidates. From more than 25,000 variable candidates detected across all nights, we identify 12 microlensing candidates with light-curve timescales shorter than 5~hours, and among them, 4 secure candidates with high-significance detections. We estimate detection efficiencies using light-curve-level simulations that account for observational conditions and finite-source effects. Using a hierarchical Bayesian framework that combines the light-curve fitting information for each candidate with the Poisson statistics of the number of candidates, we derive constraints on parameters that characterize the abundance and mass scale of primordial black hole (PBH) dark matter. First, we derive upper limits on the PBH abundance under the null hypothesis that all events are assumed to be false detections. Next, employing the PBH hypothesis in which all (or only secure) candidates are assumed to be due to PBH microlensing, we derive the allowed region of the PBH parameters; the inferred mass scale is $M_{\rm PBH}\sim10^{-7}$--$10^{-6}M_\odot$, and the PBH abundance to the total dark matter is $f_{\rm PBH}\sim \mathcal{O}(10^{-2}{\rm -}10^{-1})$. Our results demonstrate that HSC-M31 monitoring remains a uniquely powerful probe of PBHs, and highlight the need for further studies for example, using Rubin Observatory LSST observations of the Large Magellanic Cloud.