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Title: Are ultrashort microlensing events caused by planets or primordial black holes?
Plain: Surveys watching millions of stars see a few brightenings lasting less than a day, caused by passing objects about as heavy as the Earth. They are usually attributed to planets drifting free of any star, but primordial black holes would produce identical events.
Precise: OGLE bulge data contain 6 ultrashort events that Niikura et al. (2019, arXiv:1901.07120) reproduce with Earth-mass PBHs forming about a percent of dark matter, while MOA (Sumi et al. 2023, arXiv:2303.08280) attributes such events to about 21 free-floating planets per star. Determine $f_{\mathrm{PBH}} = \Omega_{\mathrm{PBH}} / \Omega_{c}$ for $M = 1e-7\ \text{to}\ 1e-5\ M_{\mathrm{sun}}$ ($2e26\ \text{to}\ 2e28\ \mathrm{g}$) by separating lens populations through event-rate dependence on direction (bulge, disk, halo, lines of sight to the LMC and M31) and through lens-distance information.
Settled by: Nancy Grace Roman Space Telescope microlensing statistics with parallax for short events, compared between the bulge and halo-dominated lines of sight.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: open
Status note: Mroz et al. (2024, Nature 632, 749) bound $f_{\mathrm{PBH}} < 0.1$ for $1.3e-5 \text{ to } 860\,M_{\mathrm{sun}}$ toward the LMC, leaving the Earth-mass range less constrained.
Posed since: 2019