Letter to the Editor

GRB 050223: a dark GRB in a dusty starburst galaxy

¹ Laboratoire AIM (UMR 7158), Service d'Astrophysique, CEA Saclay, L'Orme des Merisiers, Bât. 709, 91191 Gif-sur-Yvette, France
² Instituto de Astronomía y Física del Espacio (CONICET/UBA) C.C. 67, Suc. 28, 1428 Buenos Aires, Argentina e-mail: pellizza@iafe.uba.ar
³ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
⁴ Chercheur Associé, Observatoire de Paris, 75014 Paris, France
⁵ European Southern Observatory, Alonso de Córdova 3107, Santiago 19, Chile (on leave from CEA Saclay, France)
⁶ INAF, Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio Catone (Roma), Italy
⁷ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
⁸ Università degli studi di Milano-Bicocca, Dipartimento di Fisica, piazza delle Scienze 3, 20126 Milano, Italy
⁹ INAF, Osservatorio Astrofisico di Arcetri, largo E. Fermi 5, 50125 Firenze, Italy
¹⁰ Agenzia Spaziale Italiana, Science Data Center, via di Villa Grazioli, 00198 Roma, Italy
¹¹ Laboratoire Astroparticule et Cosmologie, UMR 7164, 11 Place Marcelin Berthelot, 75231 Paris Cedex 05, France
¹² INAF-IASF, Sezione di Roma, via Fosso del Cavaliere 100, 00133 Roma, Italy
¹³ Scuola Normale Superiore, piazza dei Cavalieri 7, 56126 Pisa, Italy

Received: 11 July 2006
Accepted: 30 August 2006

Abstract

Aims.We aim at detecting and determining the properties of the host galaxy of the dark GRB 050223.

Methods.We use VLT optical/NIR images coupled to Swift X-ray positioning, and optical spectra of the host galaxy to measure its properties.

Results.We find a single galaxy within the Swift error box of GRB 050223. It is located at and its luminosity is L^*. Emission lines in the galaxy spectrum imply an intrinsic yr^-1, and a large extinction mag within it. We also detect absorption lines, which reveal an underlying stellar population with an age between 40 Myr and 1.5 Gyr.

Conclusions.The identification of a host galaxy with atypical properties using only the X-ray transient suggests that a bias may be present in the former sample of host galaxies. Dust obscuration together with intrinsic faintness are the most probable causes for the darkness of this burst.