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Issue A&A
Volume 447, Number 2, February IV 2006
Page(s) L5 - L8
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:200500228



A&A 447, L5-L8 (2006)
DOI: 10.1051/0004-6361:200500228

Letter

Optical emission from GRB 050709: a short/hard GRB in a star-forming galaxy

S. Covino1, D. Malesani2, G. L. Israel3, P. D'Avanzo1, 4, L. A. Antonelli3, G. Chincarini1, 5, D. Fugazza1, M. L. Conciatore3, M. Della Valle6, F. Fiore3, D. Guetta3, K. Hurley7, D. Lazzati8, L. Stella3, G. Tagliaferri1, M. Vietri9, S. Campana1, D. N. Burrows10, V. D'Elia3, P. Filliatre11, 12, N. Gehrels13, P. Goldoni11, 12, A. Melandri3, 14, S. Mereghetti15, I. F. Mirabel16, A. Moretti1, J. Nousek10, P. T. O'Brien17, L. J. Pellizza13, R. Perna8, S. Piranomonte3, P. Romano1 and F. M. Zerbi1

1  INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
    e-mail: covino@mi.astro.it
2  International School for Advanced Studies (SISSA-ISAS), via Beirut 2-4, 34014 Trieste, Italy
3  INAF, Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio Catone (Roma), Italy
4  Dipartimento di Fisica e Matematica, Università dell'Insubria, via Valleggio 11, 22100 Como, Italy
5  Università degli studi di Milano-Bicocca, Dipartimento di Fisica, piazza delle Scienze 3, 20126 Milano, Italy
6  INAF, Osservatorio Astrofisico di Arcetri, largo E. Fermi 5, 50125 Firenze, Italy
7  University of California, Berkeley, Space Sciences Laboratory, Berkeley, CA 94720-7450, USA
8  JILA, University of Colorado, 440 UCB, Boulder CO 80309-0440, USA
9  Scuola Normale Superiore, piazza dei Cavalieri 7, 56126 Pisa, Italy
10  Department of Astronomy & Astrophysics, Pennsylvania State University, State College, PA 16801, USA
11  Laboratoire Astroparticule et Cosmologie, UMR 7164, 11 Place Marcelin Berthelot, 75231 Paris Cedex 05, France
12  Service d'Astrophysique, DSM/DAPNA, CEA Saclay, 91911 Gif-sur-Yvette Cedex, France
13  NASA, Goddard Space Flight Center, Greenbelt, MD 20771, USA
14  Università degli Studi di Cagliari, Dipartimento di Fisica, 09042 Monserrato (Ca), Italy
15  INAF/IASF Milano "G. Occhialini", via E. Bassini 15, 20133 Milano, Italy
16  European Southern Observatory - Vitacura, Casilla 19001, Santiago 19, Chile
17  X-Ray & Observational Astronomy Group, Dept. of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK

(Received 7 September 2005 / Accepted 21 December 2005 )

Abstract
We present optical observations of the short/hard gamma-ray burst GRB 050709, the first such event with an identified optical counterpart. The object is coincident with a weak X-ray source and is located inside a galaxy at redshift $z = 0.1606 \pm 0.0002$. Multiband photometry allowed us to study the broad-band spectral energy distribution.

Late-time monitoring places strong limits on any supernova simultaneous with the GRB. The host galaxy is not of early type. Spectra show that the dominant stellar population is relatively young ($\sim $1 Gyr), and that ongoing star formation is present at a level of $2\mbox{--}3\,L/L_*~M_\odot$ yr-1. This is at least 2 orders of magnitude larger than that observed in the elliptical hosts of the short GRB 050509B and GRB 050724. This shows that at least some short GRBs originate in a young population. Short/hard GRB models based on the merger of a binary degenerate system are compatible with the host galaxy characteristics, although there is still the possibility of a connection between young stars and at least a fraction of such events.


Key words: radiation mechanisms: non-thermal -- gamma rays: bursts -- gamma rays: individual GRB 050709

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