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

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 E₀ = 56 ± 2 keV and E_peak = 41 ± 3 keV. The peak flux is 1.8 × 10^-7 erg cm^-2 s^-1 and fluence 3.4 × 10^-7 erg cm^-2 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.