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.

References

It remains to be seen how robust these limits are, given that the selection criteria used to calculate detection efficiency permitted a substantial number of false positives and included some manual, subjective cuts.

— Eppur non si trovano Vol. 2: No Planetary-mass Primordial Black Holes toward the Andromeda Galaxy  (2604.00111 - Mróz et al., 31 Mar 2026) in Conclusions

Potential observational effects, at this stage, seem inconclusive: in part because they involve large uncertainties, with whether or not the assumptions made in calculating these constraints [...] are reliable (being) a key question'' \parencite[p. 11]{Green2025NonParticleDarkMatter}, and in part becausethere may be other potential explanations of these phenomena, and a PBH interpretation may be inconsistent with exclusion limits from other observations” (op.cit., p. 9).

— Dark Matters of Principle: Principles in the Dark Matter Paradigm  (2608.25954 - Duerr et al., 26 Aug 2026) in Section 4, subsection “Primordial black holes,” paragraph “Regulative principles,” item “Robustness”