# 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 MACS0416. 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$${}^{-1}$$ (M$${}_{AB}<-14$$) in $$\lesssim$$5 rest-frame days, then faded below detectability in roughly the same time span. Models of the cluster lens suggest that these events may be spatially coincident at the source plane, but are most likely not temporally coincident. We find that Spock can be explained as a luminous blue variable, a recurrent nova, or a pair of stellar microlensing events. To distinguish between these hypotheses will require a clarification of the positions of nearby critical curves, along with high-cadence monitoring of the field that could detect new transient episodes in the host galaxy.