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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%  % MAIN DOCUMENT TEXT  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%  \shorttitle{A Type Ia SN Behind Abell 2744}  \shortauthors{Rodney et al.}  \begin{document}  \title{Illuminating a Dark Lens : A Type Ia Supernova Magnified by \\ the Frontier Fields Galaxy Cluster Abell 2744}  \input{authors}  \begin{abstract}  {  SN \tomas\ is In January and August of 2014, two unusual transient events were observed in  a Type Ia Supernova (SN) discovered strongly lensed galaxy  at $z=1.3457\pm0.0001$ behind z=1.0054$\pm$0.0002.   Discovered by  the galaxy cluster Abell 2744 ($z=0.308$).  In a cosmology-independent analysis, we find that \tomas\ is  $0.77\pm0.15$ magnitudes brighter FrontierSN team in Hubble Space Telescope (HST) observations from the Hubble Frontier Fields (HFF) program, these events are designated HFF14Spo-SE and HFF14Spo-NW, and collectively nicknamed ``Spock''. Both transient episodes were faster and fainter  than unlensed Type Ia SNe at similar  redshift, implying any normal supernova: they rose to  a lensing magnification peak absolute optical/ultraviolet luminosity of $M\sim-14$ mag ($10^{40}$ ergs/sec) in only $\sim$3 rest-frame days, and then faded away below detectability in roughly the same amount of time. These events appeared in two adjacent arcs  of$\mu_{\rm  obs}=2.03\pm0.29$. This observed magnification provides a rare  opportunity for  a direct empirical test strongly lensed galaxy that is multiply-imaged into at least 3 distinct images by the gravitational potential  of the  galaxy cluster MACSJ0416 (z=0.396). Using four independent  lensmodels.  Here we test 17 lens models, 13 of which were generated before the SN  magnification was known, qualifying as pure ``blind tests''. The  modelsare collectively fairly accurate: 8  of the models deliver  median magnifications that are consistent with the measured $\mu$ to  within 1$\sigma$. However, there this cluster, we find it  is a subtle systematic bias: entirely plausible that  the significant disagreements all involve models NW and SE events are  {\it overpredicting} spatially} coincident on  the magnification. source plane, but very unlikely that they were also {\it temporally} coincident.  We evaluate possible causes several physical models  for this mild bias, these events,  and find no single physical or methodological that the least disfavored  explanation to account for  it. We do find is that we have observed two distinct outbursts from a single extraordinary recurrent nova. This model would imply  that the HFF14Spo system has the fastest known recurrence timescale of any nova ($\sim$8 months). Further, if our estimate for the gravitational lensing magnification is correct, then HFF14Spo is about 2 orders of magnitude more luminous than an average nova. This  model accuracy can be improved would therefore require that the HFF14Spo system's primary star is a white dwarf very close  to some extent  with stringent quality cuts on multiply-imaged systems, such as  requiring the Chandrasekhar mass limit, and that it is drawing mass from the secondary at an extremely efficient rate (??? $>10{-8}$ \Msun/yr ???). This in turn suggests  that this system is  a large fraction have spectroscopic redshifts. In  addition strong candidate  to testing model accuracies eventually explode  as we have done here, a  Type Ia SN  magnifications could also be used as inputs for future lens models of  Abell 2744 and other clusters, providing valuable constraints in  regions where traditional strong- and weak-lensing information is  unavailable.} Supernova.  \end{abstract}  \keywords{ supernovae: general, supernovae: individual: HFF14Tom,   galaxies: clusters: general, galaxies: clusters: individual: Abell 2744,   gravitational lensing: strong, gravitational lensing: weak }  \section{Introduction}  \label{sec:Introduction}  Galaxy clusters can be used as cosmic telescopes to magnify distant  background objects through gravitational lensing, which can  substantially increase the reach of deep imaging surveys. The lensing  magnification enables the study of objects that would otherwise be  unobservable because they are either intrinsically  faint \citep[e.g.][]{Schenker:2012,Alavi:2014} or extremely  distant \citep[e.g.][]{Franx:1997,Ellis:2001,Hu:2002,Kneib:2004,Richard:2006,Richard:2008,Bouwens:2009a,Maizy:2010,Zheng:2012,Coe:2013,Bouwens:2014,Zitrin:2014b}.  Background galaxies are also {\it spatially} magnified, allowing for  studies of the internal structure of galaxies in the early universe  with resolutions of $\sim$100  pc \citep[e.g.][]{Stark:2008,Jones:2010,Yuan:2011,Wuyts:2014,Livermore:2015}.