GRB010222: afterglow emission in a rapidly decelerating jet

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  • 19 March 2001
Abstract
The GRB010222 optical and near-infrared afterglow was monitored at the TNG and other Italian telescopes starting ~1 day after the high-energy prompt event. The optical UBVRI light curves are continuously steepening, and can be described by two power laws, f(t) = t^(-alpha), of indices alpha ~ 0.6 and alpha ~ 1.3 before and after a break occurring at about 0.5 days after the GRB start time, respectively. The temporal break appears to be achromatic in the optical. A low-resolution optical spectrum has also been taken with TNG. In the optical spectrum we found three absorption systems at different redshifts (0.927, 1.155 and 1.475), the highest of which represents a lower limit to the redshift of this GRB. The broad-band optical spectral energy distributions do not appear to vary with time, consistently with the achromatic behaviour of the light curves. The behaviour of the K-band light curve appears instead peculiar, being practically constant on the two nights of observation. This could be interpreted either as produced by the passing of a spectral break through the K band between the two observational epochs or considering the interaction between the GRB ejecta and a dense and dusty circumburst environment. We evaluate various interpretations of the light and spectrum temporal evolution, and find that our measurements favor a transition from relativistic to non-relativistic regime in the conditions of the shock propagation.

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