The weakINTEGRALbursts GRB 040223 and GRB 040624: an emerging population of dark afterglows

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.

Global characteristics of GRBs observed with INTEGRAL and the inferred large population of low-luminosity GRBs

“…Almost 70% (31/46) of the GRBs observed by INTEGRAL do not have a detected optical counterpart, including 9 of the 11 long-lag GRBs. The optical observations revealed faint afterglows for GRB 040323 and GRB 040827, and near-IR afterglows for GRB 040223 and GRB 040624 (Filliatre et al 2006). A non-spectroscopic redshift in the range 0.5 < z < 1.7 was obtained for GRB 040827 (de Luca et al 2005).…”

“…GRB 040223 and GRB 040624 (Filliatre et al 2006) provide good examples of GRBs with dark or faint optical afterglows. GRB 040223 was observed close to the galactic plane, so NIR observations were carried out to overcome the high dust obscuration.…”

“…INTEGRAL has detected 46 longduration GRBs (T 90 > ∼ 2 s) and 1 short-duration GRB (T 90 < ∼ 2 s) between October 2002 and July 2007. INTEGRAL bursts of particular interest include the low-luminosity GRB 031203 (Sazonov et al 2004), the very intense GRB 041219a (McBreen et al 2006), a number of X-ray rich GRBs such as GRB 040223 (McGlynn et al 2005;Filliatre et al 2006), GRB 040403 ) and GRB 040624 (Filliatre et al 2006), and the short-duration GRB 070707 (McGlynn et al 2008a). In addition, Marcinkowski et al (2006) have detected a bright, hard GRB outside the field of view using the ISGRI Compton mode.…”

Probing the neutral fraction of the IGM with GRBs during the epoch of reionization

“…Interestingly, approximately one‐half of Swift bursts are ‘dark bursts’– bursts that have detected X‐ray afterglows, but that have no measurable optical emission (e.g. Filliatre et al 2006). While it is probable that most dark bursts originate from low‐redshift, dust‐rich galaxies, a fraction of dark bursts may originate from z > 6 and are ‘dark’ because Lyα absorption from the high‐redshift IGM absorbs the optical emission (e.g.…”

The dark nature of GRB 051022 and its host galaxy

“…Dark GRBs seem to constitute a significant fraction of the GRB population (e.g. Filliatre et al 2006). Although in these cases where no optical/near infrared (nIR) afterglows have been detected, transient X-ray and radio emission has pinpointed the parent galaxies where the bursts occurred.…”