Microlensing constraints on primordial black holes with the Subaru/HSC Andromeda observation (1701.02151v3)

Abstract: Primordial black holes (PBHs) have long been suggested as a viable candidate for the elusive dark matter (DM). The abundance of such PBHs has been constrained using a number of astrophysical observations, except for a hitherto unexplored mass window of $M_{\rm PBH}=[10^{-14},10^{-9}]M_\odot$. Here we carry out a dense-cadence (2~min sampling rate), 7 hour-long observation of the Andromeda galaxy (M31) with the Subaru Hyper Suprime-Cam to search for microlensing of stars in M31 by PBHs lying in the halo regions of the Milky Way (MW) and M31. Given our simultaneous monitoring of tens of millions of stars in M31, if such light PBHs make up a significant fraction of DM, we expect to find many microlensing events for the PBH DM scenario. However, we identify only a single candidate event, which translates into the most stringent upper bounds on the abundance of PBHs in the mass range $M_{\rm PBH}\simeq [10^{-11}, 10^{-6}]M_\odot$.

• The paper employs 2-minute cadence Subaru/HSC observations of M31 to constrain PBH dark matter in the mass range 10⁻¹⁴ to 10⁻⁹ Mₒ.
• It uses image subtraction analysis of light curves to detect microlensing events, identifying one candidate to set tight upper limits on PBH abundance.
• The results narrow the allowed mass window for PBHs as dark matter and highlight high-cadence surveys as a powerful tool for astrophysical research.

Microlensing Constraints on Primordial Black Holes Using Subaru/HSC Andromeda Observations

The paper under review presents a comprehensive investigation into the potential contribution of primordial black holes (PBHs) to the dark matter (DM) content in the universe, employing microlensing techniques with observations made by the Subaru Hyper Suprime-Cam (HSC) targeting the Andromeda galaxy (M31). By focusing on a previously unexplored mass window for PBHs, specifically in the range of $10^{-14}$ to $10^{-9} M_\odot$, the authors aim to place stringent constraints on PBHs as a DM candidate.

Methodology and Observations

The paper utilizes a dense-cadence observation strategy with the Subaru/HSC, targeting M31 with a 2-minute sampling rate over a 7-hour continuous observation period. This approach allows the simultaneous monitoring of tens of millions of stars within M31, enhancing the likelihood of detecting microlensing events if PBHs are abundant within the DM halo of either the Milky Way or M31.

The authors conducted image subtraction to identify variability among the stars, employing a methodology that subtracts a reference image from subsequent target images to identify candidate events. Their analysis focuses on the detection of PBH microlensing by assessing the light curves of these candidate events, looking for signatures that match the expected microlensing profiles.

Results and Numerical Findings

The team's results indicate the detection of a single candidate event, from which they derive constraints on PBH abundance in the mass range of approximately $10^{-11}$ to $10^{-6} M_\odot$. This finding provides the most stringent upper bounds to date on the PBH contribution to DM across these masses. The detection efficiency and confidence levels were rigorously calibrated against the expected microlensing events using both simulated data and empirical observations.

Implications and Future Directions

The implications of these findings are twofold. Firstly, the constraints help narrow the mass window where PBHs could potentially constitute a significant fraction of DM, contributing to the broader understanding of DM composition in the universe. Secondly, the results highlight the utility of high-cadence observations with sophisticated instrumentation like the Subaru/HSC, which can be extended to other astrophysical inquiries regarding DM and cosmic structures.

Theoretical models predicting extended mass functions for PBHs are now further limited by this empirical work. Future research directions could include extending the observation period to cover different temporal scales or employing similar methodologies to probe additional galaxies. An expansion in both spatial coverage and temporal resolution could refine these constraints further.

The constraints suggested by this paper provide valuable benchmarks for DM models and encourage further observational campaigns leveraging high-cadence microlensing surveys. The focus on fainter, unresolved stars in the pixel lensing regime exemplifies innovative approaches to tackling one of astrophysics' most profound questions: the nature of DM in our universe. Future improvements in telescope technology and data analysis techniques could continue to push the boundaries of what such studies can unveil.