Searching for Highly Magnified Stars at Cosmological Distances in Archival Hubble Galaxy-Cluster Imaging

The detection of gravitational waves emitted from ~10-100 solar-mass black-hole mergers has revived the search for dark matter formed by primordial black holes (PBHs). If present in foreground galaxy-cluster lenses, PBHs should cause microlensing peaks that extremely magnify well-aligned background stars, allowing the high-redshift stars to be detected with the Hubble Space Telescope (HST). Since the microlensing event rate should be proportional to the density of microlenses times the magnification of the macromodel, searching for lensed stars behind clusters provides one of few methods to probe stellar-mass PBHs. Only four highly magnified stars have been discovered to date, and all were found behind powerful Hubble Frontier Field (HFF) galaxy clusters. Here we report the most recently discovered highly magnified star at redshift z= 0.94 in a strongly lensed arc behind a HFF cluster, MACS J0416.1-2403, discovered as part of a systematic archival search. In this ongoing search, we have found >30 bright transient candidates in the HFF fields that include known lensed stars and supernovae. Twenty-four of them, however, have not yet been publicly reported and need to be further identified. This search for transients is being extended to lower signal-to-noise microlensing candidates using machine-learning methods. We expect the study will improve constraints on the abundance of PBHs in the ~10-100 solar-mass regime.