Evidence for supernova signatures in the spectrum of the late-time bump of the optical afterglow of GRB 021211

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 ∼ 10 days past maximum light. Particularly we have measured a strong, broad absorption feature at 3770 Å, which we have identified with CaII blueshifted by ∼14400 km s-1, 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.