Eppur non si trovano Vol. 2: No Planetary-mass Primordial Black Holes toward the Andromeda Galaxy

Abstract

A recent preprint by Sugiyama et al. reported the discovery of twelve candidates for short-timescale (less than one day) gravitational microlensing events based on high-cadence photometric observations of the Andromeda Galaxy (M31) using the Subaru Hyper Suprime-Cam. These detections were attributed to a large population of planetary-mass primordial black holes (PBHs) that could account for the entirety of the dark matter in the Milky Way and M31 halos. However, these results are in clear tension with previous searches for short-timescale microlensing events toward the Magellanic Clouds, such as those by the OGLE survey. In addition, both the temporal and spatial distributions of the Subaru candidates are inconsistent with expectations for microlensing events. Here, we reanalyze the Subaru data using an independent difference image analysis photometric pipeline. We find that all twelve candidates identified by Sugiyama et al. exhibit asymmetric light curves and/or variability on multiple nights of Subaru observations. Our analysis reveals that among them ten objects are RR Lyrae stars, one is an eclipsing binary, and one is an unclassified variable star. We find no compelling evidence for short-timescale microlensing events among the candidates identified in the Subaru data set, nor for a significant population of planetary-mass PBHs as dark matter components. Our results underscore the necessity of robust variable-star rejection in high-cadence microlensing searches using large telescopes.

Figures (18)

• Figure 1: Full Subaru HSC light curve of the candidate microlensing event identified by niikura2019 reveals significant variability on three nights (2014-11-24, 2017-09-20, and 2020-11-20), consistent with an RR Lyrae star rather than a microlensing event.
• Figure 2: Light curve of the candidate microlensing event identified by niikura2019 folded with four possible pulsation periods.
• Figure 3: Finding charts for the microlensing event candidates found by sugiyama2026. Each image is $30" \times 30"$, North is up and East is to the left.
• Figure 4: Stacks of $\approx 15$ difference images centered on the peaks of the purported microlensing events. The position of each variable is marked by the white tick marks. Images are $30" \times 30"$, North is up and East is to the left.
• Figure 5: Full Subaru HSC light curve of the candidate microlensing event #01 found by sugiyama2026. Each panel shows data from a separate night.