AMPM II. A Lunar-Mass Primordial Black Hole Microlensing Candidate in the Milky Way Halo

TL;DR

This paper reports the discovery of a lunar-mass primordial black hole candidate, named Phoebe, through a microlensing event, suggesting a population of such objects in the Milky Way's dark matter halo.

Contribution

It presents the first detection of a lunar-mass PBH candidate via microlensing, providing evidence for a population of compact dark matter objects in the Milky Way.

Findings

Detected an hour-long microlensing event with a timescale of about 60 minutes.

Bayesian analysis indicates the lens is likely a lunar-mass primordial black hole.

Supports the existence of a population of lunar-mass objects in the Milky Way's dark matter halo.

Abstract

Primordial Black Holes (PBH) are hypothesised to form during inflation and have long been considered a candidate for compact dark matter. Gravitational microlensing is known as a productive method for exoplanet discovery and characterisation, but also provides an experimental avenue to constrain the PBH abundance in the mass regime from $\sim 10^{-11}\ M_{\odot}$ to $\sim 10^5\ M_{\odot}$. We performed a high-cadence, optical microlensing survey with DECam over five nights towards the Large Magellanic Cloud, sensitive to microlensing timescales from minutes to days. Here, we report the discovery of an hour-long microlensing event. An optical depth probabilistic analysis indicates that the lensing object, which we refer to as Phoebe, is 5 orders of magnitude more likely to be part of the Milky Way's dark matter halo than part of the stellar content of the Milky Way and Large Magellanic Cloud. No matter the location of Phoebe, it is among the fastest and lowest mass microlensing signals ever detected, with an Einstein timescale of approximately 60 minutes. Using Bayesian modelling, we interpret Phoebe as a PBH with mass $0.032^{+0.227}_{-0.027} M_{\oplus}$, or approximately 3 lunar masses. Phoebe suggests a population of compact, lunar-mass objects associated with the dark matter distribution of the Milky Way, and potentially opens a new window to the physics of inflation.