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