SN 2006oz: rise of a super-luminous supernova observed by the SDSS-II SN Survey

¹ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark
e-mail: giorgos@dark-cosmology.dk
² Department of Astronomy, University of Texas at Austin, Austin, TX, USA
³ Las Cumbres Observatory Global Telescope Network, Goleta, CA 93117, USA
⁴ Department of Physics, University of California, Santa Barbara, Broida Hall, Santa Barbara, CA 93106-9530, USA
⁵ Instituto de Astrofísica de Andalucía (IAA-CSIC), Granada, Spain
⁶ Department of Optics and Quantum Electronics, University of Szeged, Szeged, Hungary
⁷ African Institute for Mathematical sciences, Muizenberg, Cape Town, South Africa
⁸ South African Astronomical Observatory, Cape Town, South Africa
⁹ Mathematics Department, University of Cape Town, Cape Town, South Africa
¹⁰ University of Pennsylvania, 209 South 33rd Street, PA 19104, USA
¹¹ Department of Astronomy and Astrophysics, The University of Chicago, Chicago, IL 60637, USA
¹² Kavli Institute for Cosmological Physics, The University of Chicago, Chicago, IL 60637, USA
¹³ Center for Particle Astrophysics, Fermi National Accelerator Laboratory, Batavia, IL 60510, USA
¹⁴ The Oskar Klein Centre, Stockholm University, Albanova University Centre, 10691 Stockholm, Sweden
¹⁵ Department of Physics, Stockholm University, Albanova University Centre, 10691 Stockholm, Sweden
¹⁶ Institute of Cosmology and Gravitation, Dennis Sciama Building, University of Portsmouth, Portsmouth, PO1 3FX, UK
¹⁷ Space Sciences Laboratory, University of California Berkeley, Berkeley, CA 94720, USA
¹⁸ E. O. Lawrence Berkeley National Lab, 1 Cyclotron Rd., Berkeley, CA 94720, USA
¹⁹ Institut de Física d’Altes Energies, Universitat Autonoma de Barcelona, 08193 Bellaterra (Barcelona), Spain
²⁰ Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802, USA
²¹ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
²² Astrophysics, Cosmology and Gravity Centre, Department of Mathematics and Applied Mathematics, University of Cape Town, 7700 Rondebosch, South Africa
²³ Department of Astronomy, Stockholm University, Albanova University Centre, 10691 Stockholm, Sweden
²⁴ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark

Received: 22 November 2011
Accepted: 29 February 2012

Abstract

Context. A new class of super-luminous transients has recently been identified. These objects reach absolute luminosities of M_u  <   −21, lack hydrogen in their spectra, and are exclusively discovered by non-targeted surveys because they are associated with very faint galaxies.

Aims. We aim to contribute to a better understanding of these objects by studying SN 2006oz, a newly-recognized member of this class.

Methods. We present multi-color light curves of SN 2006oz from the SDSS-II SN Survey that cover its rise time, as well as an optical spectrum that shows that the explosion occurred at z ~ 0.376. We fitted black-body functions to estimate the temperature and radius evolution of the photosphere and used the parametrized code SYNOW to model the spectrum. We constructed a bolometric light curve and compared it with explosion models. In addition, we conducted a deep search for the host galaxy with the 10 m GTC telescope.

Results. The very early light curves show a dip in the g- and r-bands and a possible initial cooling phase in the u-band before rising to maximum light. The bolometric light curve shows a precursor plateau with a duration of 6–10 days in the rest-frame. A lower limit of M_u <  − 21.5 can be placed on the absolute peak luminosity of the SN, while the rise time is constrained to be at least 29 days. During our observations, the emitting sphere doubled its radius to  ~2 × 10¹⁵ cm, while the temperature remained hot at  ~15 000 K. As for other similar SNe, the spectrum is best modeled with elements including O ii and Mg ii, while we tentatively suggest that Fe iii might be present. The host galaxy is detected in gri with 25.74  ±  0.19, 24.43  ±  0.06, and 24.14  ±  0.12, respectively. It is a faint dwarf galaxy with M_g =  −16.9.

Conclusions. We suggest that the precursor plateau might be related to a recombination wave in a circumstellar medium (CSM) and discuss whether this is a common property of all similar explosions. The subsequent rise can be equally well described by input from a magnetar or by ejecta-CSM interaction, but the models are not well constrained owing to the lack of post-maximum observations, and CSM interaction has difficulties accounting for the precursor plateau self-consistently. Radioactive decay is less likely to be the mechanism that powers the luminosity. The host is a moderately young and star-forming, but not a starburst, galaxy.