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Issue A&A
Volume 406, Number 2, August I 2003
Page(s) L33 - L37
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20030855



A&A 406, L33-L37 (2003)
DOI: 10.1051/0004-6361:20030855

Letter

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

M. Della Valle1, D. Malesani2, S. Benetti3, V. Testa4, M. Hamuy5, 1, L. A. Antonelli4, G. Chincarini6, 7, G. Cocozza4, 8, S. Covino6, P. D'Avanzo6, D. Fugazza9, G. Ghisellini6, R. Gilmozzi10, D. Lazzati11, E. Mason10, P. Mazzali12 and L. Stella4

1  INAF, Osservatorio Astrofisico di Arcetri, largo E. Fermi 5, 50125 Firenze, Italy
2  International School for Advanced Studies (SISSA/ISAS), via Beirut 2-4, 34016 Trieste, Italy
3  INAF, Osservatorio Astronomico di Padova, vicolo dell'Osservatorio 5, 35122 Padova, Italy
4  INAF, Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Roma), Italy
5  Carnegie Observatories, 813 Santa Barbara Street, Pasadena, California 91101, USA
6  INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (Lc), Italy
7  University of Milano-Bicocca, Department of Physics, Piazza delle Scienze 3, 20126 Milano, Italy
8  University of Roma "Tor Vergata", Department of Physics, via della Ricerca Scientifica 1, 00133 Roma, Italy
9  INAF, Telescopio Nazionale Galileo, Roque de Los Muchachos, PO box 565, 38700 Santa Cruz de La Palma, Spain
10  ESO, Alonso de Cordova 3107, Casilla 19001, Vitacura, Santiago, Chile
11  Institute of Astrophysics, University of Cambridge, Madingley Road, CB3 0HA Cambridge, UK
12  INAF, Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy

(Received 25 April 2003 / Accepted 9 June 2003)

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 Ca II blueshifted by ~ $14\,400$ 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.


Key words: gamma rays: bursts -- supernovae

Offprint request: M. Della Valle, massimo@arcetri.astro.it

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© ESO 2003


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