Astronomy and Astrophysics, volume 448, 971-982 (2006/3-4)
The weak INTEGRAL bursts GRB 040223 and GRB 040624: an emerging population of dark afterglows.
FILLIATRE P., COVINO S., D'AVANZO P., DE LUCA A., GOETZ D., McGLYNN S., McBREEN S., FUGAZZA D., ANTONELLI A., CAMPANA S., CHINCARINI G., CUCCHIARA A., DELLA VALLE M., FOLEY S., GOLDONI P., HANLON L., ISRAEL G., McBREEN B., MEREGHETTI S., STELLA L. and TAGLIAFERRI G.
Abstract (from CDS):
We report here γ-ray, X-ray and near-infrared observations of GRB040223
along with γ-ray and optical observations of GRB040624
was detected by INTEGRAL close to the Galactic plane and GRB040624
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. GRB040223
is among the weakest and longest of INTEGRAL GRBs. The X-ray afterglow of this burst was detected 10h 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 17h 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 GRB040624
, we carried out optical observations 13h after the burst with FORS1 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.
gamma-rays: bursts - gamma-rays: observations
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