A critical analysis of the recent OGLE limits on stellar mass primordial black holes in the halo of the Milky Way

TL;DR

The paper challenges the OGLE-based claim that massive black holes cannot constitute dark matter in the Milky Way halo by dissecting OGLE's methodology across photometric passbands, light-curve quality, event selection, detection efficiency, and halo modeling. It argues that OGLE's I-band, single-color data, sparse cadence, and potentially flawed efficiency calibration inflate uncertainties and can mimic self-lensing or non-PBH explanations, making the OGLE results unable to exclude a PBH-dominated halo. By contrasting with MACHO's detections (e.g., $ au$ values around $1.2\times10^{-7}$) and noting that OGLE-IV's $ au\approx 0.121\times10^{-7}$ is not decisively diagnostic, the authors contend that the reported 13 events could be consistent with PBHs under a Thermal History mass function and with a significant PBH halo fraction. Overall, they argue that OGLE's conclusions are not robust constraints on PBHs and that PBHs remain a plausible component of dark matter, warranting more reliable cross-survey analyses and halo-rotation-curve considerations.

Abstract

This paper is a response to recent claims that a population of primordial black holes in the Galactic halo has been ruled out by the OGLE collaboration. This claim was based on the latest results from the OGLE microlensing survey towards the Large Magellanic Cloud which failed to detect even the number of events expected from known stellar populations. In particular, their results are completely inconsistent with the results of the MACHO survey which detected a population of compact bodies in the Galactic halo which could not be accounted for by any known stellar population. The discrepancy between the results of these two groups has a long history, and includes problems such as different choice of photometric passbands, quality of light curves, microlensing event selection, detection efficiency, self lensing and halo models. In this paper it is demonstrated that these issues not only account for the discrepancy between the OGLE and MACHO results, but imply that the OGLE observations can put no meaningful constraints on a population of primordial black holes in the Galactic halo.

Figures (4)

• Figure 1: Hess diagram for LMC stars adapted from Alcock et al. (1999). The red circles show the positions of the microlensing candidates from Figure 7 of Alcock et al. (2000). The yellow crosses show the positions of the microlensing candidates from Figure 5 of Mróz et al. (2024), but with the $I$ and $(V-I)$ colours transformed to $V$ and $(V-R)$ to enable comparison of the two datasets.
• Figure 2: Simulated light curves for a point source (left), and source radius a tenth (centre) and a third (right) of the Einstein radius of the lens.
• Figure 3: Light curves in the MACHO blue and red passbands adapted from Figure 2 of Alcock et al. (1993) for a candidate microlensing event.
• Figure 4: Light curves in the $I$-band adapted from Figure 4 of Mróz et al. (2024b) for two candidate microlensing events.