# Microlensing constraints on primordial black holes with the Subaru/HSC Andromeda observation

## TL;DR

This study uses high-cadence observations of Andromeda with Subaru to search for microlensing events caused by primordial black holes, setting new constraints on their abundance in a previously unexplored mass range.

## Contribution

It provides the first dense-cadence, long-duration microlensing survey of M31 to constrain primordial black holes in the mass range $10^{-14}$ to $10^{-9} M_\,od$, establishing the most stringent limits to date.

## Key findings

- Only one candidate microlensing event detected.
- Set the tightest upper bounds on PBH abundance in the mass range $10^{-11}$ to $10^{-6} M_\,od$.
- Demonstrated the effectiveness of dense-cadence, wide-field observations for dark matter constraints.

## Abstract

Primordial black holes (PBHs) have long been suggested as a viable candidate for the elusive dark matter (DM). The abundance of such PBHs has been constrained using a number of astrophysical observations, except for a hitherto unexplored mass window of $M_{\rm PBH}=[10^{-14},10^{-9}]M_\odot$. Here we carry out a dense-cadence (2~min sampling rate), 7 hour-long observation of the Andromeda galaxy (M31) with the Subaru Hyper Suprime-Cam to search for microlensing of stars in M31 by PBHs lying in the halo regions of the Milky Way (MW) and M31. Given our simultaneous monitoring of tens of millions of stars in M31, if such light PBHs make up a significant fraction of DM, we expect to find many microlensing events for the PBH DM scenario. However, we identify only a single candidate event, which translates into the most stringent upper bounds on the abundance of PBHs in the mass range $M_{\rm PBH}\simeq [10^{-11}, 10^{-6}]M_\odot$.