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Issue A&A
Volume 443, Number 1, November III 2005
Page(s) L1 - L5
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:200500196



A&A 443, L1-L5 (2005)
DOI: 10.1051/0004-6361:200500196

Letter

GRB 050904 at redshift 6.3: observations of the oldest cosmic explosion after the Big Bang

G. Tagliaferri1, L. A. Antonelli2, G. Chincarini1, 3, A. Fernández-Soto4, D. Malesani5, M. Della Valle6, P. D'Avanzo1, 7, A. Grazian2, V. Testa2, S. Campana1, S. Covino1, F. Fiore2, L. Stella2, A. J. Castro-Tirado8, J. Gorosabel8, D. N. Burrows9, M. Capalbi10, G. Cusumano11, M. L. Conciatore2, V. D'Elia2, P. Filliatre12, 13, D. Fugazza1, N. Gehrels14, P. Goldoni12, 13, D. Guetta2, S. Guziy8, E. V. Held15, K. Hurley16, G. L. Israel2, M. Jelínek8, D. Lazzati17, A. López-Echarri18, A. Melandri2, 19, I. F. Mirabel20, M. Moles8, A. Moretti1, K. O. Mason21, J. Nousek9, J. Osborne22, L. J. Pellizza23, R. Perna17, S. Piranomonte2, L. Piro24, A. de Ugarte Postigo8 and P. Romano1

1  INAF-Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (Lc), Italy
    e-mail: tagliaferri@merate.mi.astro.it
2  INAF-Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio Catone (Roma), Italy
3  Università degli studi di Milano-Bicocca, Dipartimento di Fisica, piazza delle Scienze 3, 20126 Milano, Italy
4  Observatori Astronomic, Universitat de Valencia, Aptdo. Correos 22085, Valencia, 46071, Spain
5  International School for Advanced Studies (SISSA-ISAS), via Beirut 2-4, 34014 Trieste, Italy
6  INAF-Osservatorio Astrofisico di Arcetri, largo E. Fermi 5, 50125 Firenze, Italy
7  Dipartimento di Fisica e Matematica, Università dell'Insubria, via Valleggio 11, 22100 Como, Italy
8  Instítuto de Astrofísica de Andalucía (IAA-CSIC), PO Box 03004, 18080 Granada, Spain
9  Department of Astronomy & Astrophysics, Pennsylvania State University, State College, PA 16802, USA
10  ASI Science Data Center, via G. Galilei 5, 00044 Frascati (Roma), Italy
11  INAF-IASF, sezione di Palermo, via U. La Malfa 153, 90146 Palermo, Italy
12  Laboratoire Astroparticule et Cosmologie, UMR 7164, 11 place Marcelin Berthelot, 75231 Paris Cedex 05, France
13  Service d'Astrophysique, DSM/DAPNIA, CEA Saclay, 91911 Gif-sur-Yvette Cedex, France
14  NASA, Goddard Space Flight Center, Greenbelt, MD 20771, USA
15  INAF-Osservatorio Astronomico di Padova, vicolo dell'Osservatorio 5, 35122 Padova, Italy
16  University of California, Berkeley, Space Sciences Laboratory, Berkeley, CA 94720-7450, USA
17  JILA, University of Colorado, 440 UCB, Boulder CO 80309-0440, USA
18  Instítuto de Astrofísica de Canarias, C/ Vía Láctea s/n, 38200 La Laguna (Tenerife), Spain
19  Università degli Studi di Cagliari, Dipartimento di Fisica, 09042 Monserrato (Ca), Italy
20  European Southern Observatory - Vitacura, Casilla 19001, Santiago 19, Chile
21  MSSL, University College London, Holmbury St. Mary, Dorking, RH5 6NT Surrey, UK
22  X-Ray & Observational Astronomy Group, Dept. of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK
23  AIM (UMR 7158 CEA/CNRS/Université Paris 7), Service d'Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette, France
24  INAF-IASF, sezione di Roma, via Fosso del Cavaliere 100, 00133 Roma, Italy

(Received 26 September 2005 / Accepted 4 October 2005)

Abstract
We present optical and near-infrared observations of the afterglow of the gamma-ray burst GRB 050904. We derive a photometric redshift z = 6.3, estimated from the presence of the Lyman break falling between the I and J filters. This is by far the most distant GRB known to date. Its isotropic-equivalent energy is $3.4 \times 10^{53}$ erg in the rest-frame 110-1100 keV energy band. Despite the high redshift, both the prompt and the afterglow emission are not peculiar with respect to other GRBs. We find a break in the J-band light curve at $t_{\rm b} = 2.6 \pm 1.0$ d (observer frame). If we assume this is the jet break, we derive a beaming-corrected energy $E_\gamma \sim (4 \div 12) \times 10^{51}$ erg. This limit shows that GRB 050904 is consistent with the Amati and Ghirlanda relations. This detection is consistent with the expected number of GRBs at z > 6 and shows that GRBs are a powerful tool to study the star formation history up to very high redshift.


Key words: cosmology: observations -- early Universe -- gamma rays: bursts -- gamma rays: individual: GRB 050904

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