Robustness of PBH abundance upper limits from Subaru HSC M31 data

Ascertain the robustness of the upper limits on the fraction of dark matter in planetary-mass primordial black holes derived by Sugiyama et al. (2026) from Subaru Hyper Suprime-Cam high-cadence M31 observations, particularly with respect to detection-efficiency calculations that used event selection criteria permitting false positives and manual, subjective cuts.

Background

The study shows that all twelve candidates identified by Sugiyama et al. (2026) are variable stars, implying that their detections should be treated as false positives. Nevertheless, the corresponding upper limits on PBH abundance—especially at masses below 10^{-8} M☉—appear stronger than those from OGLE constraints, likely due to finite-source effects and the different source-star angular sizes in M31.

Because the original detection-efficiency modeling included manual and subjective cuts and allowed substantial false-positive contamination, evaluating how these choices affect the derived PBH abundance limits remains an unresolved issue.