GRB 090313: X-shooter's first shot at a gamma-ray burst^*

¹ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Lc, Italy e-mail: antonio.deugarte@brera.inaf.it
² European Southern Observatory, Casilla 19001, Santiago 19, Chile
³ APC – UMR 7164, 10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex, France
⁴ Service d'Astrophysique, DSM/IRFU/SAp, CEA-Saclay, 91191 Gif-sur-Yvette, France
⁵ INAF – Osservatorio Astronomico di Roma, Via Frascati, 33, 00040, Monteporzio Catone (Rome), Italy
⁶ ASI – Science Data Centre, Via Galileo Galilei, 00044 Frascati (Rome), Italy
⁷ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
⁸ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92195 Meudon, France
⁹ Astronomical Institute, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
¹⁰ European Southern Observatory, K. Schwarzschild Str. 2, 85748 Garching, Germany
¹¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹² Department of Astrophysics, IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹³ INAF – IASF di Bologna, via Gobetti 101, 40129 Bologna, Italy
¹⁴ INAF – Osservatorio Astronomico di Trieste, via Tiepolo, 11, 34131 Trieste, Italy
¹⁵ Scuola Normale Superiore di Pisa, Piazza dei Cavalieri 7, 56126 Pisa, Italy

Received: 12 August 2009
Accepted: 20 January 2010

Abstract

Context. X-shooter is the first second-generation instrument to become operative at the ESO Very Large Telescope (VLT). It is a broad-band medium-resolution spectrograph designed with gamma-ray burst (GRB) afterglow spectroscopy as one of its main science drivers.

Aims. During the first commissioning night on sky with the instrument fully assembled, X-shooter observed the afterglow of GRB 090313 as a demonstration of the instrument's capabilities.

Methods. GRB 090313 was observed almost two days after the burst onset, when the object had already faded to R ~ 21.6. Furthermore, the 90% illuminated Moon was just 30 degrees away from the field. In spite of the adverse conditions, we obtained a spectrum that, for the first time in GRB research, simultaneously covers the range from 5700 to 23 000 Å.

Results. The spectrum shows multiple absorption features at a redshift of 3.3736, which we identify as the redshift of the GRB. These features are composed of 3 components with different ionisation levels and velocities. Some of the features have never been observed before in a GRB at such a high redshift. Furthermore, we detect two intervening systems at redshifts of 1.8005 and 1.9597.

Conclusions. These results demonstrate the potential of X-shooter in the GRB field, because it was capable of observing a GRB down to a magnitude limit that would include 72% of long GRB afterglows 2 h after the burst onset. Coupled with the rapid response mode available at VLT, allowing reaction times of just a few minutes, X-shooter constitutes an significant leap forward on medium resolution spectroscopic studies of GRBs, their host galaxies and intervening systems, probing the early history of the Universe.