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\begin{abstract} In January and August of 2014, two unusual transient events were observed in a strongly lensed galaxy at z=1.0054$\pm$0.0002. Discovered by the FrontierSN team in Hubble Space Telescope (HST) (\HST) observations from the Hubble Frontier Fields (HFF) program, these events are designated \spockone and \spocktwo, and collectively nicknamed ``Spock''. Both transient episodes were faster and fainter

rose to a peak absolute optical/ultraviolet luminosity of $M\sim-14$ mag ($10^{41}$ erg s$^{-1}$) in only $\lesssim$5 rest-frame days, and then faded away below detectability in roughly the same amount of time. These The \spock events appeared in two adjacent arcs of a strongly lensed galaxy that is multiply-imaged into at least 3 distinct images by the gravitational potential of the galaxy cluster \MACS0416 (z=0.396). Using five independent lens models of this cluster, we find it is entirely plausible that the two events are {\it spatially} coincident on the source plane, but very unlikely that they were also {\it temporally} coincident. We compare these events to Comparing the observational constraints against existing categories of astrophysical transients and transients, we find thatnone of them can readily account for all characteristics of the \spock events. The light curves could be plausibly explained most plausible classification is either asoptical/UV emission from a neutron star merger (a kilonova), a white dwarf He shell explosion (a .Ia supernova), eruptive episodes from recurrent nova (RN) or a luminous blue variable (LBV), or H explosions from an extremely luminous nova. Among these, the nova model is the least disfavored, as it allows for a rapid recurrence period with little or no intervening variability. This (LBV). The RN model would imply that the \spock system has the fastest known recurrence timescale of any nova (3 be strained to 5 months) and that its physical limits to accommodate the \spock is about 2 orders of magnitude more luminous than an average nova. This then suggests observations. It would require that the\spock system's primary star is a white dwarf very close to the Chandrasekhar mass limit, and that it is drawing mass from the secondary star its companion at an extremely efficient rate ($>10^{-7}$ \Msun yr$^{-1}$), making yr$^{-1}$). The LBV model is the most compatible, as it a potential Type Ia Supernova progenitor candidate. We conclude with suggestions allows for modeling efforts a short recurrence period, relatively high luminosity, and observational tests rapid light curve rise and decline timescales. This model would imply thatcould help to clarify the nature \spock system will most likely exhibit more eruptions in the near future. A high-cadence imaging campaign could catch these future eruptions, allowing a clear test of this unusual transient. classification and providing an opportunity for a very precise measurement of the gravitational lensing time delay. \end{abstract}

decline time is not unheard of. For example, the bright nova M31N-2007-11d had $t_2 = 9.5$ days \citep{Shafter:2009}. The extremely luminous nova SN 2010U had $t_2 = 3.5 \pm 0.3$ \citep{Czekala:2013}. The nova L91 required at least 4 days to rise to maximum \citep{Shafter:2009}, and then declined with $t_2 = 6 \pm 1$ days \citep{DellaValle:1991,Schwarz:2001, Williams:1994, Schwarz:2001}.The rise to maximum of L91 is also quite long, measure at least 4 days. \citet{Shafter:2009}. Another reason to consider the RN model is that it provides a natural explanation for having two separate explosions that are

a viable physical model to explain these events. Rapid transient episodes in LBVs such as SN 2002kg and SN 2009ip may best be explained by a sudden ejection of an optically thick shell \citep[e.g.,]{Smith:2010,Smith:2011b}, \citep[e.g.,][]{Smith:2010, Smith:2011b}, or by some form of S Dor-type variability \citep{Weis:2005,VanDyk:2006,Foley:2011}, \citep{Weis:2005, VanDyk:2006, Foley:2011}, which may be driven by stellar pulsation rather than mass ejection \citep{VanGenderen:1997,VanGenderen:2001}. \citep{VanGenderen:1997, VanGenderen:2001}. For massive stars such as \etacar at its great eruption and the rapidly varying SN 2009ip, the effective photospheric radius during eruption must have been comparable to the orbit of Saturn \citep[$10^{14}$ cm;][]{Davidson:1997,Smith:2011,Foley:2011}. cm;][]{Davidson:1997, Smith:2011b, Foley:2011}. With observed photospheric velocities of order 500 km s$^{-1}$ for such events, the dynamical timescale of the extended photosphere is on the order of tens to hundreds of days. Thus, if the very rapid light curves of both \spock events are indeed LBV eruptions, then they will be near the extreme limits of physical models for massive stellar eruptions. %To examine the temperature and total energy output, we first make a %set of (admittedly unfounded) assumptions: (1) the two outbursts had a

\enddata \tablecomments{Time delays give the predicted delay relative to an appearance in the NW host image, 11.2. Positive (negative) values indicate the NW image is the leading (trailing) image of the pair.} pair. \todo{Need to update with latest Jauzac models!} } \end{deluxetable} %\renewcommand{\arraystretch}{1.}

%% This BibTeX bibliography file was created using BibDesk. %% http://bibdesk.sourceforge.net/ %% Created for rodney at 2016-08-17 16:14:28 2016-08-23 09:15:55 -0400 %% Saved with string encoding Unicode (UTF-8) @article{Shafter:2009, Author = {{Shafter}, A.~W. and {Rau}, A. and {Quimby}, R.~M. and {Kasliwal}, M.~M. and {Bode}, M.~F. and {Darnley}, M.~J. and {Misselt}, K.~A.}, Journal = {\apj}, Month = jan, Pages = {1148-1157}, Title = {{M31N 2007-11d: A Slowly Rising, Luminous Nova in M31}}, Volume = 690, Year = 2009} @article{Williams:1994, Author = {{Williams}, R.~E. and {Phillips}, M.~M. and {Hamuy}, M.}, Journal = {\apjs}, Month = jan, Pages = {297-316}, Title = {{The Tololo nova survey: Spectra of recent novae}}, Volume = 90, Year = 1994} @article{Schwarz:2001, Author = {{Schwarz}, G.~J. and {Shore}, S.~N. and {Starrfield}, S. and {Hauschildt}, P.~H. and {Della Valle}, M. and {Baron}, E.}, Journal = {\mnras}, Month = jan, Pages = {103-123}, Title = {{Multiwavelength analyses of the extraordinary nova LMC 1991$^{*}$}}, Volume = 320, Year = 2001} @article{DellaValle:1991, Author = {{della Valle}, M.}, Journal = {\aap}, Month = dec, Pages = {L9-L12}, Title = {{Nova LMC 1991 - Evidence for a super-bright nova population}}, Volume = 252, Year = 1991} @article{VanGenderen:1997, Author = {{van Genderen}, A.~M. and {de Groot}, M. and {Sterken}, C.}, Journal = {\aaps},

\label{fig:PeakLuminosityDeclineTime} Peak luminosity vs. decline time for \spock and other rapidly declining recurrent transients. Constraints for \spockone and \spocktwo are plotted as overlapping colored bands, as in Figure~\ref{fig:PeakLuminosityDeclineTimeWide}.Two .Ia candidates are shown as stars \citep{Kasliwal:2010,Poznanski:2010}, and arrows indicate lower limits for two kilonova candidates \citep{Perley:2009,Tanvir:2013}. Grey bands show the MMRD relation for classical novae, as in Figure~\ref{fig:PeakLuminosityDeclineTimeWide}. Circles mark the observed peak luminosities and decline times for classical novae from the Milky Way \citep{Downes:2000}, M31 \citep{Shafter:2011}, and the local group \citep{Kasliwal:2011b}. Black '+' symbols mark the 7 rapidly declining Recurrent Novae from our own galaxy \citep{Schaefer:2010}, and the large cross labeled at the bottom shows the rapid recurrence nova M31N 2008-12a \citep{Tang:2014,Darnley:2015}. Each orange diamond marks a separate short transient event from the two rapid LBV outburst systems, SN 2009ip \citep{Pastorello:2013} and NGC3432-LBV1 \citep[a.k.a. SN 2000ch][]{Pastorello:2010}. These LBV events provide only upper limits on the decline time due to limited photometric sampling.