The highly energetic expansion of SN 2010bh associated with GRB 100316D

Abstract

We present the spectroscopic and photometric evolution of the nearby (z = 0.059) spectroscopically confirmed Type Ic supernova, SN2010bh, associated with the soft, long-duration gamma-ray burst (X-ray flash) GRB100316D. Intensive follow-up observations of SN2010bh were performed at the ESO Very Large Telescope (VLT) using the X-shooter and FORS2 instruments. Thanks to the detailed temporal coverage and the extended wavelength range (3000-24800Å), we obtained an unprecedentedly rich spectral sequence among the hypernovae, making SN2010bh one of the best studied representatives of this SN class. We find that SN2010bh has a more rapid rise to maximum brightness (8.0 ± 1.0 rest-frame days) and a fainter absolute peak luminosity (L bol ≈ 3 × 1042 erg s-1) than previously observed SN events associated with GRBs. Our estimate of the ejected 56Ni mass is 0.12 ± 0.02 M o. From the broad spectral features, we measure expansion velocities up to 47,000kms-1, higher than those of SNe 1998bw (GRB980425) and 2006aj (GRB060218). Helium absorption lines He I λ5876 and He I 1.083 μm, blueshifted by 20,000-30,000kms-1 and 28,000-38,000kms-1, respectively, may be present in the optical spectra. However, the lack of coverage of the He I 2.058 μm line prevents us from confirming such identifications. The nebular spectrum, taken at 186days after the explosion, shows a broad but faint [O I] emission at 6340Å. The light curve shape and photospheric expansion velocities of SN2010bh suggest that we witnessed a highly energetic explosion with a small ejected mass (E k ≈ 1052 erg and M ej ≈ 3 Mo). The observed properties of SN2010bh further extend the heterogeneity of the class of GRB SNe. © © 2012. The American Astronomical Society. All rights reserved.