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\bigskip  {\bf  A massive galaxy cluster can serve as a magnifying glass for distant  stellar populations, with as  strong gravitational lensing exposing details  in the lensed magnifies  background galaxies and exposes details  that would are  otherwisebe  undetectable. The \fullmacs0416 cluster (hereafter \macs0416) is one of the most  efficient lenses in the sky, and in 2014 it was observed with high-cadence In time-domain astronomy,  imaging from the {\it Hubble Space Telescope} (\HST). programs with a short  cadence are able to detect rapidly evolving transients, previously  unseen by surveys designed for slowly evolving supernovae.  Here we describe two unusual transient events that appeared behind \macs0416 discovered  in a strongly lensed Hubble Space  Telescope program that combined these techniques, with high-cadence  imaging on a field with a strong-lensing  galaxy at redshift  $z=1.0054\pm0.0002$. cluster.  These transients---designated  \spockone and \spocktwo and collectively nicknamed ``Spock''---were transients were  faster and fainter than any supernova (SN), supernova,  but significantly more luminous than a classical nova.They reached peak luminosities of $\sim10^{41}$  erg s$^{-1}$ ($M_{AB}<-14$ mag) in $\lesssim5$ rest-frame days, then faded  below detectability in roughly the same time span. Models of the  cluster lens suggest that these events may be {\it spatially}  coincident at the source plane, but are most likely not {\it  temporally} coincident.  We find that \spock they  can be explained as separate eruptions of  a luminous blue variable (LBV), or  a recurrent nova (RN), nova,  or a as an unrelated  pair of stellar microlensing events. To distinguish between these hypotheses will requirea  clarification of the positions of nearby critical curves, cluster lens models,  along with more  high-cadence monitoring imaging  of the field that could detect new related  transient episodes in episodes.  This discovery suggests that  the host galaxy.} intersection of strong lensing with  high-cadence transient surveys may be a fruitful path for future  astrophysical transient studies.  }  % %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%  % BEGIN MAIN TEXT  % Introduction  When a star explodes or a relativistic jet erupts from near the edge  of a black hole, the event can be visible across many billions of  light-years. Such extremely luminous astrophysical The  transients as  supernovae (SNe), gamma-ray bursts, and quasars presented here  are powerful tools for  probing cosmic history designated \spockone  and sampling the matter \spocktwo  and energy content of  the universe. Less energetic transients generated by collectively nicknamed ``Spock.'' As shown in  Figure~\ref{fig:SpockDetectionImages},  the tumultuous  atmospheres \spock events appeared in  Hubble Space Telescope (\HST) imaging collected as part  ofmassive stars or  the interactions of close stellar  binaries are also very valuable Hubble  Frontier Fields (HFF) survey\cite{Lotz:2017}, a multicycle program  for understanding stellar evolution deep imaging of six massive galaxy clusters  and associated ``blank  sky'' fields observed in parallel. One of these clusters was  the physical processes that lead to stellar explosions. However, \fullmacs0416 cluster (hereafter \macs0416), and  the lower luminosity of such two \spock  events makes them accessible only appeared behind \macs0416  in the local universe, and consequently our census separate images  of peculiar transients the same strongly  lensed galaxy  at redshift $z=1.0054\pm0.0002$. \spocktwo appeared 223  days after \spockone. Both transients reached peak luminosities of  $\sim10^{41}$ erg s$^{-1}$ ($M_{AB}<-14$ mag) in $\lesssim5$  rest-frame days, then faded below detectability in roughly  the stellar scale is still highly incomplete. same  time span.  Although recent surveys are beginning to discover progressively more categories of rapidly changing optical transients\cite{Kasliwal:2011a,Drout:2014}, transients  \cite{Kasliwal:2011a,Drout:2014},  most programs remain largely insensitive to transients with peak brightness luminosities  and timescales comparable similar  to the \spock events\cite{Berger:2013b}. events \cite{Berger:2013b}. Such ``peculiar'' transients  may be generated by the tumultuous atmospheres of massive stars or the  interactions of close stellar binaries. These systems are valuable  for understanding extreme outcomes of stellar evolution and the  physical processes that lead to stellar explosions.  Future wide-field observatories such as the Large Synoptic Survey  Telescope\cite{Tyson:2002} will be much more efficient at discovering  such transients, and can be are  expected to reveal many new categories examples  of such  astrophysical transients.As shown in Figure~\ref{fig:SpockDetectionImages}, the \spock events  appeared in \HST imaging collected as part of  the Hubble Frontier Fields (HFF) survey\cite{Lotz:2017}, a multicycle  program for deep imaging of six massive galaxy clusters and associated  ``blank sky'' fields observed in parallel.  \HST is not an efficient wide-field survey telescope, and the HFF survey was not designed with  the discovery of peculiar extragalactic transients as a core  objective. However, but  the HFF program has unintentionally nevertheless  opened an  effective a small  window of discovery for such events. Very faint sources at relatively high redshift ($z\gtrsim1$) in these fields are made detectable bythe substantial  gravitational lensing magnification from the foreground galaxy clusters. Very rapidly evolving sources are  also more likely to be found, owing to the The  necessity of a rapid cadence for repeat imaging in the HFF program. program also made it more likely to catch very rapidly evolving  sources. With this confluence of factors, the HFF survey provides a  glimpse of the potential discovery space available to high-cadence  imaging surveys in the future.  % %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 
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\section{Results}\label{sec:Results}  To evaluate the impact of gravitational lensing from the \MACS0416  cluster on the observed light curves and the timing observable properties  of these the  two \spock  events, we use seven independently constructed cluster mass models. These models indicate that the gravitational time delay between the \spockone location and the \spocktwo location is $<$60 days (Table~\ref{tab:LensModelPredictions}). This falls far short of As shown in  Figure~\ref{fig:SpockDelayPredictions},  the observed 223 day span between the two events, suggesting that  \spocktwo events  isnot a time-delayed image of the \spockone event. As  shown in Figure~\ref{fig:SpockDelayPredictions}, \spockone and  \spocktwo are  inconsistent with these predicted the model-predicted  time delays if one assumes that they are \spocktwo is a gravitationally  delayed images image  of a single event. \spockone.  However, if these were independent events, then a time delay on the order of tens of days between image 11.1 and 11.2 could have resulted in time-delayed events that were missed by the \HST imaging of this field. The models also predict absolute magnification values between about  $\mu=10$ and $\mu=200$ for both events. This wide range is caused 
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from 2009 to 2014, including \Chandra X-ray Space Telescope imaging  that was coeval with the peak of infrared emission from \spocktwo.  Dynamically induced stellar collisions or close interactions in a  dense stellar cluster\citep{Fregeau:2004} could in principle produce a  series of optical transients. Similarly, the collision of a jovian  planet with a main sequence star\cite{Metzger:2012,Yamazaki:2017} or a  terrestrial planet with a white dwarf star\cite{Di-Stefano:2015} could  generate an optical transient with a peak luminosity comparable to  that observed for the \spock events, although it is unclear whether  the UV/optical emission could match the observed \spock light curves.  These scenarios warrant further scrutiny, so that predictions of the  light-curve shape and anticipated rates can be more rigorously  compared to the \spock observations.  Many types of stellar explosions can generate isolated transient  events, and a useful starting point for classification of such objects  is to examine their position in the phase space of peak luminosity 
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highly unlikely to detect two such transients from the same galaxy in  a single year.  Dynamically induced stellar collisions or close interactions in a  dense stellar cluster\citep{Fregeau:2004} could in principle produce a  series of optical transients. Similarly, the collision of a jovian  planet with a main sequence star\cite{Metzger:2012,Yamazaki:2017} or a  terrestrial planet with a white dwarf star\cite{Di-Stefano:2015} could  generate an optical transient with a peak luminosity comparable to  that observed for the \spock events, although it is unclear whether  the UV/optical emission could match the observed \spock light curves.  These scenarios warrant further scrutiny, so that predictions of the  light-curve shape and anticipated rates can be more rigorously  compared to the \spock observations.  Although the two events were most likely not {\it temporally}  coincident, all of our lens models indicate that it is entirely  plausible for the two \spock events to be {\it spatially} coincident: