Optical and radio observations of the bright GRB 010222 afterglow: evidence for rapid synchrotron cooling ?

Abstract

We report photometric observations of the optical afterglow of GRB 010222 in V, R and I passbands carried out between 22-27 February 2001 at Nainital. We determine the CCD Johnson BV and Cousins RI photometric magnitudes for 31 stars in the field of GRB 010222 and use them to calibrate our measurements as well as other published BV RI photometric magnitudes of the GRB 010222 afterglow. We construct the light curve of the afterglow emission in B,R,R and I passbands, and from a broken power-law fit determine the early and late time power-law flux decay indices as 0.74 ± 0.05 and 1.35 ± 0.04 respectively. Steepening in the flux decay seems to have started around 0.7 day after the burst. Negligible Galactic extinction amounting E(B - V) = 0.023 mag is derived in the direction of the GRB. We derive the value of the spectral index in the X-ray to optical region to be 0.61 ± 0.02 and 0.75 ± 0.02 at Δt = 0.35 and 9.13 day. We attempted radio observations of the afterglow from RATAN-600 telescope during=20 23-26 February 2001 and from the Giant Metrewave Radio Telescope on 8 March 2001, yielding upper limits of ~ 5 mJy at 3.9 GHz and ~ 1 mJy at 610 MHz respectively. The millimeter wave observations obtained with the IRAM Plateau de Bure Interferometer during 24 and 25 February 2001 and 16 March 2001 at 93 GHz and 230 GHz also indicate no flux detection from the afterglow. The light curve and the spectrum indicate that the synchrotron cooling frequency lies in the sub-millimeter region, which also explains the observed sub-millimeter excess. Attributing the observed break in the light curve to the sideways expansion of coolimated ejecta, we estimate a jet opening angle of ~ 2.0n^1/8 deg.