Letter to the Editor

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

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 . 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 (∼1 Gyr), and that ongoing star formation is present at a level of  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.