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

PBHs and other compact objects as potential dark matter are constrained by a long-term microlensing search toward the SMC using ~20 years of OGLE data. The analysis detects six events (three new) and yields a microlensing optical depth of $\tau = (0.32 \pm 0.18) \times 10^{-7}$ and an event rate of $\Gamma = (1.18 \pm 0.57) \times 10^{-7} \mathrm{yr}^{-1}$ from five events; no events with timescale $t_E > 1$ year were found. The detected events are consistent with self-lensing from known stellar populations, and the non-detection of very long events provides competitive limits on the abundance of massive compact objects in the Milky Way halo. Together with previous OGLE LMC results, the study rules out PBHs and other compact objects with masses in the range $10^{-8}$ to $10^{3} M_{\odot}$ as dominant dark matter components. This strengthens microlensing-based constraints on nonbaryonic DM and helps delineate the viable parameter space for PBHs.

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 $τ= (0.32 \pm 0.18) \times 10^{-7}$ and the event rate $Γ= (1.18 \pm 0.57) \times 10^{-7}\,\mathrm{yr}^{-1}\,\mathrm{star}^{-1}$. 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.

Figures (1)

• Figure 1: OGLE-IV fields toward the Small Magellanic Cloud (SMC) are shown with white polygons. The numbers in the center of each polygon in the right panel indicate the field number. The yellow lines mark the regions that were observed during the OGLE-III phase. Detected microlensing events are marked with black circles. The background image of the SMC was generated with bsrender written by Kevin Loch, using data from the ESA/Gaia database.