Multicolor observations of the afterglow of the short/hard GRB 050724 ^*

¹ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries vej 30, 2100 København Ø, Denmark e-mail: malesani@astro.ku.dk
² INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
³ Università dell'Insubria, Dipartimento di Fisica e Matematica, via Valleggio 11, 22100 Como, Italy
⁴ INAF, Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Roma), Italy
⁵ Università degli Studi di Milano-Bicocca, Dipartimento di Fisica, piazza delle Scienze 3, 20126 Milano, Italy
⁶ European Space Astronomy Centre, European Space Agency (ESA), Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁷ ASI Science Data Center, via G. Galilei, 00044 Frascati (Roma), Italy
⁸ INAF, Osservatorio Astrofisico di Arcetri, largo E. Fermi 5, 50125 Firenze, Italy
⁹ Kavli Institute for Theoretical Physics, University of California, Santa Barbara, CA 93106, USA
¹⁰ Laboratoire Astroparticule et Cosmologie, 10 rue A. Domon et L. Duquet, 75205 Paris Cedex 13, France
¹¹ Service d'Astrophysique, DSM/DAPNIA/SAp, CEA-Saclay, 91191 Gif-sur-Yvette, France
¹² JILA, University of Colorado, Boulder, CO 80309-0440, USA
¹³ Astrophysics Research Institute, Liverpool JMU, Twelve Quays House, Egerton Wharfs, Birkenhead CH41 1LD, UK
¹⁴ Instituto de Astronomía y Física del Espacio (CONICET/UBA), Casilla de Correos 67, Suc. 28 (1428) Buenos Aires, Argentina
¹⁵ Dunsink Observatory, DIAS, Dunsink lane, Dublin 15, Ireland
¹⁶ School of Physical Sciences and NCPST, Dublin City University, Dublin 9, Ireland

Received: 14 May 2007
Accepted: 23 June 2007

Abstract

Context.New information on short/hard gamma-ray bursts (GRBs) is being gathered thanks to the discovery of their optical and X-ray afterglows. However, some key aspects are still poorly understood, including the collimation level of the outflow, the duration of the central engine activity, and the properties of the progenitor systems.

Aims.We want to constrain the physical properties of the short GRB 050724 and of its host galaxy, and make some inferences on the global short GRB population.

Methods.We present optical observations of the afterglow of GRB 050724 and of its host galaxy, significantly expanding the existing dataset for this event. We compare our results with models, complementing them with available measurements from the literature. We study the afterglow light curve and spectrum including X-ray data. We also present observations of the host galaxy.

Results.The observed optical emission was likely related to the large flare observed in the X-ray light curve. The apparent steep decay was therefore not due to the jet effect. Available data are indeed consistent with low collimation, in turn implying a large energy release, comparable to that of long GRBs. The flare properties also constrain the internal shock mechanism, requiring a large Lorentz factor contrast between the colliding shells. This implies that the central engine was active at late times, rather than ejecting all shells simultaneously. The host galaxy has red colors and no ongoing star formation, consistent with previous findings on this GRB. However, it is not a pure elliptical, and has some faint spiral structure.

Conclusions.GRB 050724 provides the most compelling case for association between a short burst and a galaxy with old stellar population. It thus plays a pivotal role in constraining progenitors models, which should allow for long delays between birth and explosion.