EDP Sciences
Free access
Volume 483, Number 3, June I 2008
Page(s) 847 - 855
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20078677
Published online 11 March 2008

A&A 483, 847-855 (2008)
DOI: 10.1051/0004-6361:20078677

Early emission of rising optical afterglows: the case of GRB 060904B and GRB 070420

A. Klotz1, 2, B. Gendre3, 4, G. Stratta5, A. Galli3, 6, A. Corsi3, B. Preger5, S. Cutini5, 7, A. Pélangeon8, J. L. Atteia8, M. Boër1, and L. Piro3

1  Observatoire de Haute-Provence, 04870 Saint Michel l'Observatoire, France
2  CESR, Observatoire Midi-Pyrénées, CNRS, Université de Toulouse, BP 4346, 31028 Toulouse Cedex 04, France
    e-mail: klotz@cesr.fr
3  Istituto di Astrofisica Spaziale e Fisica Cosmica, Sede di Roma, INAF, via Fosso del Cavaliere 100, 00133 Roma, Italy
4  University degli Studi di Milano-Bicocca, Piazza dell'Ateneo Nuovo, 20126, Milano, Italy
5  ASI Science Data Center (ASDC), via G. Galilei, 00044 Frascati, Italy (INAF personnel resident at ASDC under ASI contract 1/024/05/0.)
6  INFN-Trieste, Padriciano 99, 34012 Trieste, Italy
7  Università di Perugia, Dipartimento di fisica, viale A. Pascoli, 06123 Perugia, Italy
8  LATT, Observatoire Midi-Pyrénées, CNRS, Université de Toulouse, 14 avenue E. Belin, 31400 Toulouse, France

(Received 14 September 2007 / Accepted 8 February 2008)

Aims. We present the time-resolved optical emission of gamma-ray bursts GRB 060904B and GRB 070420 during their prompt and early afterglow phases.
Methods. We used time resolved photometry from optical data taken by the TAROT telescope and time resolved spectroscopy at high energies from the Swift spacecraft instrument.
Results. The optical emissions of both GRBs are found to increase from the end of the prompt phase, passing to a maximum of brightness at $t_{\rm peak}=9.2$ min and 3.3 min for GRB 060904B and GRB 070420 respectively and then decrease. GRB 060904B presents a large optical plateau and a very large X-ray flare. We argue that the very large X-flare occurring near $t_{\rm peak}$ is produced by an extended internal engine activity and is only a coincidence with the optical emission. GRB 070420 observations would support this idea because there was no X-flare during the optical peak. The nature of the optical plateau of GRB 060904B is less clear and might be related to the late energy injection.

Key words: gamma ray : bursts

© ESO 2008