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Individual highly magnified stars have been recently discovered at lookbacktimes of more than half the age of the Universe, in lensed galaxies thatstraddle the critical curves of massive galaxy clusters. Having confirmed theirdetectability, it is now important to carry out systematic searches for them inorder to establish their frequency, and in turn learn about the statisticalproperties of high-redshift stars and of the granularity of matter in theforeground deflector. Here we report the discovery of a highly magnified starat redshift $z = 0.94$ in a strongly lensed arc behind a Hubble Frontier Fieldgalaxy cluster, MACS J0416.1-2403, discovered as part of a systematic archivalsearch. The bright transient (dubbed "Warhol") was discovered in Hubble SpaceTelescope data taken on 2014 September 15 and 16. This single image faded overa period of two weeks, and observations taken on 2014 September 1 show that theduration of the microlensing event was at most four weeks in total. The lightcurve may also exhibit slow changes over a period of years consistent with thelevel of microlensing expected from stars responsible for the intraclusterlight (ICL) of the cluster. Optical and infrared observations taken near peakbrightness can be fit by a stellar spectrum with moderate host-galaxyextinction. A blue supergiant matches the measured spectral energy distributionnear peak, implying a temporary magnification of at least several thousand.While the spectrum of an O-type star would also fit the transient's spectralenergy distribution, extremely luminous O-type stars are much less common thanblue supergiants. The short timescale of the event and the estimated effectivetemperature indicate that the lensed source is an extremely magnified star.

astrophysical journal/the astrophysical journal

Searching for Highly Magnified Stars at Cosmological Distances:  Discovery of a Redshift 0.94 Blue Supergiant in Archival Images of the Galaxy  Cluster MACS J0416.1-2403