Near-IR Search for Lensed Supernovae Behind Galaxy Clusters - II. First Detection and Future Prospects

Powerful gravitational telescopes in the form of massive galaxy clusters can be used to enhance the light collecting power over a limited field of view by about an order of magnitude in flux. This remarkable effect is exploited here to increase the depth of a survey for lensed supernovae at near-IR wavelengths. A pilot supernova search programme conducted with the ISAAC camera at VLT is presented. Lensed galaxies behind the massive clusters A1689, A1835 and AC114 were observed for a total of 20 hours split into 2, 3 and 4 epochs respectively, separated by approximately one month to a limiting magnitude $J\lsim 24$ (Vega). Image subtractions including another 20 hours worth of archival ISAAC/VLT data were used to search for transients with lightcurve properties consistent with redshifted supernovae, both in the new and reference data. The feasibility of finding lensed supernovae in our survey was investigated using synthetic lightcurves of supernovae and several models of the volumetric Type Ia and core-collapse supernova rates as a function of redshift. We also estimate the number of supernova discoveries expected from the inferred star formation rate in the observed galaxies. The methods consistently predict a Poisson mean value for the expected number of supernovae in the survey between N$_{\rm SN}$=0.8 and 1.6 for all supernova types, evenly distributed between core collapse and Type Ia supernovae. One transient object was found behind A1689, 0.5" from a galaxy with photometric redshift z_gal=0.6 +- 0.15. The lightcurve and colors of the transient are consistent with being a reddened Type IIP supernova at z_SN=0.62. The lensing model predicts 1.7 mag of amplification at the location of the transient, without which this object would have been hard to detected with a ground based search (abridged)