Limits on planetary-mass primordial black holes from the OGLE high-cadence survey of the Magellanic Clouds

TL;DR

This study used high-cadence microlensing observations of the Magellanic Clouds to place stringent limits on planetary-mass primordial black holes, ruling out their significant contribution to dark matter and challenging their role in short-timescale microlensing events.

Contribution

The paper provides the first strong observational constraints on planetary-mass primordial black holes as dark matter candidates using the OGLE survey data.

Findings

No short-timescale microlensing events detected in the Magellanic Clouds.

Planetary-mass PBHs constitute at most 1% of dark matter.

Short-timescale events toward the Galactic bulge are likely caused by free-floating planets.

Abstract

Observations of the Galactic bulge revealed an excess of short-timescale gravitational microlensing events that are generally attributed to a large population of free-floating or wide-orbit exoplanets. However, in recent years, some authors suggested that planetary-mass primordial black holes (PBHs) comprising a substantial fraction (1%-10%) of the dark matter in the Milky Way may be responsible for these events. If that was the case, a large number of short-timescale microlensing events should also be seen toward the Magellanic Clouds. Here, we report the results of a high-cadence survey of the Magellanic Clouds carried out from 2022 October through 2024 May as part of the Optical Gravitational Lensing Experiment. We observed almost 35 million source stars located in the central regions of the Large and Small Magellanic Clouds and found only one long-timescale microlensing event candidate. No short-timescale events were detected despite high sensitivity to such events. That allows us to infer the strongest available limits on the frequency of planetary-mass PBHs in dark matter. We find that PBHs and other compact objects with masses from $1.4 \times 10^{-8}\,M_{\odot}$ (half of the Moon mass) to $0.013\,M_{\odot}$ (planet/brown dwarf boundary) may comprise at most 1% of dark matter. That rules out the PBH origin hypothesis for the short-timescale events detected toward the Galactic bulge and indicates they are caused by the population of free-floating or wide-orbit planets.