# Two Peculiar Fast Transients in a Strongly Lensed Host Galaxy

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 $$\sim 10^{41}$$ erg s$${}^{-1}$$ ($$M_{AB}<-14$$ mag) 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 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.

## Introduction

\label{sec:Introduction}

The Spock transient events—separately designated HFF14Spo-NW and HFF14Spo-SE—appeared in Hubble Space Telescope (HST) imaging collected in January and August of 2014, respectively (Figure \ref{fig:SpockDetectionImages}). These observations were centered on the galaxy cluster MACS J0416.1-2403 (hereafter, MACS0416) and were collected as part of the Hubble Frontier Fields (HFF) survey (HST-PID:13496, PI:Lotz), a multi-cycle program for deep imaging of 6 massive galaxy clusters and associated “blank sky” fields observed in parallel (Methods \ref{sec:Discovery}).

Combining the HST imaging and lens models of the MACS0416 gravitational lens leads to three key observables for the HFF14Spo events: (1) they are both more luminous than a classical nova, but less luminous than almost all supernovae (SNe), reaching a peak luminosity of roughly $$10^{41}$$ erg s$${}^{-1}$$ ($$M_{V}=−14$$); (2) both transients exhibited fast light curves, with rise and decline timescales of $$\sim$$2–5 days in the rest frame; and (3) it is possible that both events arose from the same physical location but highly unlikely that they were coincident in time—they were probably separated by 3-5 months in the rest frame. These transients thus present a puzzle: they are broadly consistent with the expected behavior of stellar explosions (they each exhibit a single isolated rise and decline in brightness), but they can not be trivially classified into any common category of explosive astrophysical transients.

The HFF survey was not designed with the discovery of peculiar extragalactic transients as a core objective, but it has unintentionally opened an effective window of discovery for such events. Very faint sources at relatively high redshift ($$z\gtrsim 1$$) in these fields are made detectable by the substantial gravitational lensing magnification from the foreground galaxy clusters. Very rapidly evolving sources are also more likely to be found, due to the necessity of a rapid cadence for repeat imaging in the HFF program. These unusual characteristics for an HST survey enabled a precise measurement of the lensing magnification for SN Tomas, a high-redshift thermonuclear (Type Ia) SN (Rodney et al., 2015). The HFF imaging program also contributed to the detection and characterization of SN Refsdal, a high-redshift core-collapse SN and the first strongly-lensed SN observed with multiple resolved images (Kelly et al., 2015). The long-term monitoring campaign of SN Refsdal then led to the discovery of the peculiar lensed transient Icarus, which is posited to be a stellar caustic crossing event (Kelly et al., 2017). Even among this collection of rare transients, the HFF14Spo events appear to be unique. No single astrophysical model is clearly sufficient to explain all of the available observational data, and the best available models require either an extreme stellar source or a very unusual gravitational lensing configuration.