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Microlensing constraints on primordial black holes with Subaru/HSC Andromeda observations


Primordial black holes (PBHs) have long been suggested as a viable candidate for the elusive dark matter. The abundance of such PBHs has been constrained using a number of astrophysical observations, except for a hitherto unexplored mass window of MPBH = [10−14, 10−9] solar masses. Here we carry out a dense-cadence, 7-hour-long observation of M31 with the Subaru Hyper Suprime-Cam (HSC) to search for microlensing of stars in M31 by PBHs lying in the halo regions of the Milky Way and M31. Given our simultaneous monitoring of tens of millions of stars in M31, if such light PBHs make up a significant fraction of dark matter, we expect to find many microlensing events. However, we identify only a single candidate event, which translates into stringent upper bounds on the abundance of PBHs in the mass range MPBH [10−11, 10−6] solar masses.

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Fig. 1: The background shows the HSC image of M31 as seen by the 104 CCD chips of the Subaru/HSC camera.
Fig. 2: The expected number of PBH microlensing events per logarithmic interval of the FWHM microlensing timescale tFWHM for a single star in M31.
Fig. 3: Example of the image subtraction technique used for the analysis.
Fig. 4: The single remaining candidate that passed all the criteria imposed to select microlensing event candidates.
Fig. 5: Comparison with other observational constraints on the abundance of PBHs on different mass scales.

Data availability

The catalogue of variability star candidates including the candidates shown in this paper is available from the corresponding author upon reasonable request.


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We thank S. Blinnikov, A. Gould, B. Jain, M. Kawasaki, A. Kusenko, C.-H. Lee, H. Murayama, D. Spergel and M. Tanaka for discussion. We thank N. Kaiser and M. Sasaki for pointing out the importance of wave optics effect in our microlensing constraints when M.T. gave a talk at the seminar of YITP, Kyoto University. This work was supported by the World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan, by the FIRST programme ‘Subaru Measurements of Images and Redshifts (SuMIRe)’, CSTP, Japan, Grant-in-Aid for Scientific Research from the JSPS Promotion of Science (23340061, 26610058 and 15H03654), MEXT Grant-in-Aid for Scientific Research on Innovative Areas (15H05887, 15H05892, 15H05893 and 15K21733) and the JSPS Program for Advancing Strategic International Networks to Accelerate the Circulation of Talented Researchers. The HSC collaboration includes the astronomical communities of Japan and Taiwan, and Princeton University. The HSC instrumentation and software were developed by the National Astronomical Observatory of Japan (NAOJ), the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), the University of Tokyo, the High Energy Accelerator Research Organization (KEK), the Academia Sinica Institute for Astronomy and Astrophysics in Taiwan (ASIAA), and Princeton University. Funding was contributed by the FIRST programme from Japanese Cabinet Office, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), the Japan Society for the Promotion of Science (JSPS), Japan Science and Technology Agency (JST), the Toray Science Foundation, NAOJ, Kavli IPMU, KEK, ASIAA and Princeton University. The Pan-STARRS1 Surveys (PS1) were made possible through contributions of the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, Queen’s University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, the National Aeronautics and Space Administration under grant no. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation under grant no. AST-1238877, the University of Maryland and Eötvös Loránd University (ELTE). Results are based in part on data collected at the Subaru Telescope and retrieved from the HSC data archive system, which is operated by Subaru Telescope and Astronomy Data Center at NAOJ. This paper makes use of software developed for the LSST. We thank the LSST Project for making their code available as free software at

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All the authors discussed the results and commented on the manuscript. M.T., H.N. and S.M. wrote the paper. H.N. performed most of the data analysis, the calculation of microlensing event rates and the model fitting. M.T. proposed the idea. M.T and T.S. prepared the observation plan and strategy for the HSC/Subaru observation of M31. N.Y. and R.H.L. provided advice about the use of the HSC data analysis pipeline, especially the image difference method. T.K. and S.S. carefully estimated the effect of finite-source size and the wave optics effect on microlensing event rates for PBH at 10−9M, and we were able to obtain a more accurate estimation of the upper bounds on the abundance of such PBHs. All the authors commented on the draft text.

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Correspondence to Hiroko Niikura or Masahiro Takada.

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Supplementary Figures 1–19, Supplementary Tables 1–2, Supplementary References 1–14

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Niikura, H., Takada, M., Yasuda, N. et al. Microlensing constraints on primordial black holes with Subaru/HSC Andromeda observations. Nat Astron 3, 524–534 (2019).

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