Out of the darkness: the infrared afterglow of the INTEGRAL burst GRB 040422 observed with the VLT

¹ Laboratoire Astroparticule et Cosmologie, UMR 7164, 11 place Marcelin Berthelot, 75231 Paris Cedex 05, France e-mail: filliatr@cea.fr
² Service d'Astrophysique, CEA/DSM/DAPNIA/SAp, CE-Saclay, Orme des Merisiers, Bât. 709, 91191 Gif-sur-Yvette Cedex, France
³ Università dell'Insubria, Dipartimento di Fisica e Matematica, via Vallegio, 11, 22100 Como, Italy
⁴ Università degli studi di Milano-Bicocca, Dipartimento di Fisica, Piazza delle Scienze 3, 20126 Milano, Italy
⁵ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
⁶ International school for advanced studies (SISSA/ISAS), via Beirut 2-4, 34014 Trieste, Italy
⁷ Department of Experimental Physics, University College, Dublin 4, Ireland
⁸ Department of Physics & Astronomy, University of Southampton, Southampton, SO17 1BJ, UK
⁹ INAF, Osservatorio Astronomico di Roma, via Frascati 33, Monteporzio Catone, 00040 Rome, Italy
¹⁰ INAF - Osservatorio Astrofisico di Arcetri, largo E. Fermi 5, 50125 Firenze, Italy
¹¹ NASA Goddard Space Flight Center, Code 661, Greenbelt, MD 20771, USA
¹² Astrophysics Missions Division, Research Scientific Support Department of ESA, ESTEC, Noordwijk, The Netherlands
¹³ Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802, USA
¹⁴ Department of Astrophysical and Planetary Sciences, University of Colorado at Boulder, 440 UCB, Boulder, CO, 80309, USA

Received: 23 December 2004
Accepted: 31 March 2005

Abstract

GRB 040422 was detected by the INTEGRAL satellite at an angle of only 3 degrees from the Galactic plane. Analysis of the prompt emission observed with the SPI and IBIS instruments on INTEGRAL are presented. The IBIS spectrum is well fit by the Band model with a break energy of and . The peak flux is and fluence in the range 20-200 keV. We then present the observations of the afterglow of GRB 040422, obtained with the ISAAC and FORS 2 instruments at the VLT less than 2 h after the burst. We report the discovery of its near-infrared afterglow, for which we give the astrometry and photometry. No detection could have been obtained in the R and I bands, partly due to the large extinction in the Milky Way. We imaged the position of the afterglow again two months later in the K_s band, and detected a likely bright host galaxy. We compare the magnitude of the afterglow with those of a compilation of promptly observed counterparts of previous GRBs, and show that the afterglow of GRB 040422 lies at the very faint end of the distribution, brighter only than that of GRB 021211, singled out later and in the optical bands, and GRB 040924 after accounting for Milky Way extinction. This observation suggests that the proportion of dark GRBs can be lowered significantly by a more systematic use of 8-m class telescopes in the infrared in the very early hours after the burst.