Near-IR search for lensed supernovae behind galaxy clusters ^*

II. First detection and future prospects

¹ Department of Physics, Stockholm University, Albanova University Center, 106 91 Stockholm, Sweden e-mail: ariel@physto.se
² The Oskar Klein Center, Stockholm University, 106 91 Stockholm, Sweden
³ CENTRA - Centro Multidisciplinar de Astrofísica, Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal
⁴ Space Telescope Science Institute, Baltimore, MD 21218, USA
⁵ University of Oxford Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁶ Laboratoire d'Astrophysique de Marseille, OAMP, CNRS-Université Aix-Marseille, 38, rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
⁷ ESO, Vitacura, Alonso de Cordova, 3107, Casilla 19001, Santiago, Chile
⁸ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁹ Department of Astronomy, California Institute of Technology, 105-24, Pasadena, CA 91125, USA
¹⁰ Department of Astronomy, Stockholm University, Albanova University Center, 106 91 Stockholm, Sweden

Received: 30 October 2008
Accepted: 7 April 2009

Abstract

Aims. 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 effect is exploited here to increase the depth of a survey for lensed supernovae at near-IR wavelengths.

Methods. We present a pilot supernova search programme conducted with the ISAAC camera at VLT. Lensed galaxies behind the massive clusters A1689, A1835, and AC114 were observed for a total of 20 h divided into 2, 3, and 4 epochs respectively, separated by approximately one month to a limiting magnitude J ≲ 24 (Vega). Image subtractions including another 20 h 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.

Results. 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 of between N_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 = 0.59. The lensing model predicts 1.4 mag of magnification at the location of the transient, without which this object would not have been detected in the near-IR ground-based search described in this paper (unlensed magnitude J ~ 25). We perform a feasibility study of the potential for lensed supernovae discoveries with larger and deeper surveys and conclude that the use of gravitational telescopes is a very exciting path for new discoveries. For example, a monthly rolling supernova search of a single very massive cluster with the HAWK-I camera at VLT would yield ≳ 10 lensed supernova lightcurves per year, where type Ia supernovae would constitute about half of the expected sample.