SN 2008am: A SUPER-LUMINOUS TYPE IIn SUPERNOVA

We present observations and interpretation of the Type IIn supernova SN2008am discovered by the ROTSE Supernova Verification Project (RSVP). SN2008am peaked at approximately -22.3 mag at a redshift of z = 0.2338, giving it a peak luminosity of 3 × 1044ergs-1 and making it one of the most luminous supernovae ever observed. The total radiated energy is ≃2 × 1051erg. The host galaxy appears to be an SB1 of normal luminosity ( -20) with metallicity Z 0.4 Z. ROTSE upper limits and detections constrain the rise time to be 34days in the rest frame, significantly shorter than similar events, SN2006gy and SN2006tf. Photometric observations in the ultraviolet, optical, and infrared bands (J, H, Ks ) constrain the spectral energy distribution evolution. We obtained six optical spectra of the supernova, five on the early decline from maximum light and a sixth nearly a year later plus a very late time spectrum (2yr) of the host galaxy. The spectra show no evidence for broad supernova photospheric features in either absorption or emission at any phase. The spectra of SN2008am show strong Balmer-line and He I λ5876 emission with intermediate widths (25) in the first 40days after optical maximum. The width formally corresponds to a velocity of 1000kms -1. We examine a variety of models for the line wings and conclude that multiple scattering is most likely, implying that our spectra contain no specific information on the bulk flow velocity. We examine a variety of models for the ROTSE light curve subject to the rise time and the nature of the spectra, including radioactive decay, shocks in optically thick and optically thin circumstellar media (CSMs) and a magnetar. The most successful model is one for which the CSM is optically thick and in which diffusion of forward shock-deposited luminosity gives rise to the observed light curve. The model suggests strong mass loss and a greater contribution from the interaction of the forward shock with optically thick CSM than from the reverse shock. Diffusion of the shock-deposited energy from the forward shock is found to be important in accounting for the rising part of the light curve. Although there are differences in detail, SN2008am appears to be closely related to other super-luminous Type IIn supernovae, SN2006gy, SN2006tf, and perhaps SN2008iy, that may represent the deaths of very massive luminous-blue-variable-type progenitors and for which the luminosity is powered by the interaction of the ejecta with a dense CSM. © 2011. The American Astronomical Society. All rights reserved.