The weak INTEGRAL bursts GRB 040223 and GRB 040624: an emerging population of dark afterglows

¹ Laboratoire Astroparticule et Cosmologie, UMR 7164, 11 place Marcelin Berthelot, 75231 Paris Cedex 05, France e-mail: filliatr@yahoo.fr
² Service d'Astrophysique, CEA/DSM/DAPNIA/SAp, CE-Saclay, Orme des Merisiers, Bât. 709, 91191 Gif-sur-Yvette Cedex, France
³ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
⁴ Universita` Insubria, Dipartimento di Fisica e Matematica, via Valleggio 11, 22100 Como, Italy
⁵ INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, via E. Bassini 15, 20133 Milano, Italy
⁶ School of Physics, University College Dublin, Dublin 4, Ireland
⁷ Astrophysics Missions Division, Research Scientific Support Department of ESA, ESTEC, Noordwijk, The Netherlands
⁸ INAF, Osservatorio Astronomico di Roma, via Frascati 33, Monteporzio Catone, 00040 Rome, Italy
⁹ Università degli Studi di Milano-Bicocca, piazza dell'Ateneo Nuovo 1, 20126 Milano, Italy
¹⁰ INAF, Osservatorio Astrofisico di Arcetri, largo E. Fermi 5, 50125 Firenze, Italy

Received: 19 August 2005
Accepted: 17 November 2005

Abstract

We report here γ-ray, X-ray and near-infrared observations of GRB 040223 along with γ-ray and optical observations of GRB 040624. GRB 040223 was detected by INTEGRAL close to the Galactic plane and GRB 040624 at high Galactic latitude. Analyses of the prompt emission detected by the IBIS instrument on INTEGRAL are presented for both bursts. The two GRBs have long durations, slow pulses and are weak. The γ-ray spectra of both bursts are best fit with steep power-laws, implying they are X-ray rich. GRB 040223 is among the weakest and longest of INTEGRAL GRBs. The X-ray afterglow of this burst was detected 10 h after the prompt event by XMM-Newton. The measured spectral properties are consistent with a column density much higher than that expected from the Galaxy, indicating strong intrinsic absorption. We carried out near-infrared observations 17 h after the burst with the NTT of ESO, which yielded upper limits. Given the intrinsic absorption, we find that these limits are compatible with a simple extrapolation of the X-ray afterglow properties. For GRB 040624, we carried out optical observations 13 h after the burst with FORS 1 and 2 at the VLT, and DOLoRes at the TNG, again obtaining upper limits. We compare these limits with the magnitudes of a compilation of promptly observed counterparts of previous GRBs and show that they lie at the very faint end of the distribution. These two bursts are good examples of a population of bursts with dark or faint afterglows that are being unveiled through the increasing usage of large diameter telescopes engaged in comprehensive observational programmes.