Temporal behaviour of emissions from γ − ray bursts and optical / near - IR afterglows of GRB 991208 and GRB 991216

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 γ−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 × 1053 erg for GRB 991208 and ∼ 6.7 × 1053 erg for GRB 991216 by these bright γ−ray flashes. The enormous amount of released energy will be reduced, if the radiation is beamed which seems to be 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.