Two Peculiar Fast Transients in a Strongly Lensed Host Galaxy

TL;DR

This paper reports two unusual, fast, luminous transient events in a strongly lensed galaxy at redshift 1, which are brighter than classical novae but fainter than supernovae, and explores possible astrophysical explanations.

Contribution

It presents the discovery and analysis of two peculiar transients in a lensed galaxy, proposing multiple hypotheses for their nature and emphasizing the need for further observations.

Findings

Transients reached peak luminosity of ~10^41 erg/s in 5 days

Events are faster and fainter than supernovae, brighter than classical novae

Possible explanations include LBV, recurrent nova, or stellar microlensing

Abstract

A massive galaxy cluster can serve as a magnifying glass for distant stellar populations, with strong gravitational lensing exposing details in the lensed background galaxies that would otherwise be undetectable. The MACS J0416.1-2403 cluster (hereafter MACS0416) is one of the most efficient lenses in the sky, and in 2014 it was observed with high-cadence imaging from the Hubble Space Telescope (HST). Here we describe two unusual transient events that appeared behind MACS0416 in a strongly lensed galaxy at redshift $z = 1.0054 \pm 0.0002$. These transients---designated HFF14Spo-NW and HFF14Spo-SE and collectively nicknamed "Spock"---were faster and fainter than any supernova (SN), but significantly more luminous than a classical nova. They reached peak luminosities of $\sim10^{41}$ erg s$^{-1}$ ($M_{\rm AB} < -14$ mag) in 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 HFF14Spo can be explained as a luminous blue variable (LBV), a recurrent nova (RN), 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.