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Evidence for Supernova Signatures in the Spectrum of the Late-time Bump of the Optical Afterglow of GRB 021211

M. Della Valle·D. Malesani·S. Benetti·V. Testa·M. Hamuy·L. A. Antonelli·G. Chincarini·G. Cocozza·S. Covino·P. D'Avanzo·D. Fugazza·G. Ghisellini·R. Gilmozzi·D. Lazzati·E. Mason·P. Mazzali·L. Stella
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Abstract

We present photometric and spectroscopic observations of the gamma-ray burst GRB 021211 obtained during the late stages of its afterglow. The light curve shows a rebrightening occurring ~25 days after the GRB. The analysis of a VLT spectrum obtained during the bump (27 days after the GRB) reveals a suggestive resemblance with the spectrum of the prototypical type-Ic SN 1994I, obtained about ~10 days past maximum light. Particularly we have measured a strong, broad absorption feature at 3770 A, which we have identified with Ca II blueshifted by ~14400 km/s, thus indicating that a supernova (SN) component is indeed powering the `bump' in the afterglow decay. Assuming SN 1994I as a template, the spectroscopic and photometric data together indicate that the SN and GRB explosions were at most separated by a few days. Our results suggest that GRBs might be associated also to standard type-Ic supernovae.

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M. Della Valle, D. Malesani, S. Benetti, V. Testa, M. Hamuy, L. A. Antonelli, G. Chincarini, G. Cocozza, S. Covino, P. D'Avanzo, D. Fugazza, G. Ghisellini, R. Gilmozzi, D. Lazzati, E. Mason, P. Mazzali, L. Stella. 2003-06-16. Evidence for Supernova Signatures in the Spectrum of the Late-time Bump of the Optical Afterglow of GRB 021211. https://doi.org/10.1051/0004-6361%3A20030855

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