Very High Column Density and Small Reddening toward GRB 020124 at z=3.20

Hjorth, J., Møller, P., Gorosabel, J., Fynbo, J. P. U., Toft, S., Jaunsen, A. O., Kaas, A. A., Pursimo, T., Torii, K., Kato, T., Yamaoka, H., Yoshida, A., Thomsen, B., Andersen, M. I., Burud, I., Castro Cerón, J. M., Castro-Tirado, A. J., Fruchter, A. S., Kaper, L., Kouveliotou, C., Masetti, N., Palazzi, E., Pedersen, H., Pian, E., Rhoads, J., Rol, E., Tanvir, N. R., Vreeswijk, P. M., Wijers, R. A. M. J., and van den Heuvel, E. P. J.
The Astrophysical Journal, Volume 597, Issue 2, pp. 699-705.


We present optical and near-infrared observations of the dim afterglow of GRB 020124, obtained between 2 and 68 hr after the gamma-ray burst. The burst occurred in a very faint (R>~29.5) damped Lyα absorber (DLA) at a redshift of z=3.198+/-0.004. The derived column density of neutral hydrogen is log(NH I)=21.7+/-0.2, and the rest-frame reddening is constrained to be E(B-V)<0.065, i.e., AV<0.20 for standard extinction laws with RV~3. The resulting dust-to-gas ratio is less than 11% of that found in the Milky Way but consistent with the SMC and high-redshift QSO DLAs, indicating a low metallicity and/or a low dust-to-metal ratio in the burst environment. A gray extinction law (large RV), produced through preferential destruction of small dust grains by the gamma-ray burst, could increase the derived AV and dust-to-gas ratio. The dimness of the afterglow is, however, fully accounted for by the high redshift: if GRB 020124 had been at z=1, it would have been approximately 1.8 mag brighter-in the range of typical bright afterglows.

Based on observations with the Nordic Optical Telescope, which is operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, and on observations collected by the Gamma-Ray Burst Collaboration at ESO (GRACE) at the European Southern Observatory, Paranal, Chile (ESO Large Programme 165.H-0464).