Microlensing optical depth, event rate, and limits on compact objects in dark matter based on 20 yr of OGLE observations of the Small Magellanic Cloud

TL;DR

This study analyzes 20 years of OGLE data on the Small Magellanic Cloud to measure microlensing optical depth and event rate, setting limits on the contribution of primordial black holes and other compact objects to dark matter.

Contribution

It provides new long-duration microlensing event measurements and tighter constraints on the role of massive compact objects in dark matter, extending previous OGLE results.

Findings

Detected six long-timescale microlensing events, including three new ones.

Measured the optical depth as (0.32 ± 0.18) × 10^{-7}.

Set limits excluding primordial black holes as dominant dark matter components.

Abstract

Some previous studies have suggested that massive and intermediate-mass primordial black holes (PBHs) could comprise a substantial fraction of dark matter in the Universe. Such black holes, if they existed in the Milky Way halo, would give rise to long-duration microlensing events that may potentially last for years. However, earlier searches were not sufficiently sensitive to detect such events. Here, we present the results of searches for long-timescale gravitational microlensing events toward the Small Magellanic Cloud (SMC) using nearly 20 years of photometric observations collected by the Optical Gravitational Lensing Experiment (OGLE) from 2001 to 2020. We found six events, three of which are new discoveries. We use a sample of five events to measure the microlensing optical depth toward the SMC $\tau = (0.32 \pm 0.18) \times 10^{-7}$ and the event rate $\Gamma = (1.18 \pm 0.57) \times 10^{-7}\,\mathrm{yr}^{-1}\,\mathrm{star}^{-1}$. The properties of the detected events are consistent with lenses originating from known stellar populations within the SMC or in the Milky Way disk. No events with timescales longer than 1 yr were detected, which provides competitive limits on the fraction of massive compact objects, including PBHs, in the Milky Way dark matter halo. Together with the earlier OGLE studies of microlensing events in the direction of the Large Magellanic Cloud, these observations rule out PBHs and other compact objects with masses ranging from $10^{-8}$ to $10^3\,M_{\odot}$ as dominant components of dark matter.