# Two Peculiar Fast Transients in a Strongly Lensed Host Galaxy

Abstract

Two unusual transient events were observed by the Hubble Space Telescope in 2014, appearing in a galaxy at z=1.0054$$\pm$$0.0002 that is gravitationally lensed by the galaxy cluster MACS J0416.1-2403. These transients—collectively nicknamed “Spock”—were faster and fainter than any supernova, but significantly more luminous than a classical nova: they reached peak luminosities of $$\sim 10^{41}$$ erg s$${}^{-1}$$ in $$\lesssim$$5 rest-frame days, then faded below detectability in roughly the same time span. Lens models of the foreground cluster suggest that it is entirely plausible that the two events are spatially coincident at the source plane, but very unlikely that they were also temporally coincident. We find that Spock can be explained as a luminous blue variable, a recurrent nova, or a stellar caustic crossing event. High-cadence monitoring of the field could distinguish between these hypotheses by detecting new transient episodes at or near the HFF14Spo locations.