Abstract:
The CCD magnitudes in Cousins R and I photometric passbands are determined for GRB 991216 and GRB 991208 afterglows respectively ~ 1 and ~ 3 day after trigger of the corresponding gamma-ray bursts. Light curves of the afterglow emissions are obtained by combining the published data with the present measurements in R and I passbands for GRB 991208 and in R, Gunn i and J passbands for GRB 991216. They indicate that the flux decay constants of a GRB are almost the same in each passband with values ~ 2.2. for GRB 991208 and ~ 1.2 for GRB 991216 indicating very fast optical flux decay in the case of former which may be due to beaming effect. However, cause of steepening by 0.23 +/- 0.06 dex in the R light curve of GRB 991216 afterglow between 2 to 2.5 day after the burst is presently not understood. Redshift determinations indicate that both GRBs are at cosmological distance with a value of 4.2 Gpc for GRB 991208 and 6.2 Gpc for GRB 991216. The observed fluence >20 kev indicates, if isotropic, release of energy ~ 1.3 x 10^5 3 erg far GRB 991208 and ~ 6.7 x 10^53 erg for GRB 991216 by these bright gamma-ray flashes. The enormous amount of released energy will be reduced, if the radiation is beamed which seems to be the case for GRB 991208 afterglow. The quasi-simultaneous broad-band photometric spectral energy distributions of the afterglows are determined ~ 8.5 day and ~ 35 hour after the bursts of GRB 991208 and GRB 991216 respectively. The flux decreases exponentially with frequency. The value of spectral index in the optical-near IR region is ?0.75+/-0.03 for GRB 991208 and ?1.0+/-0.12 for GRB 991216.
