The optical light curve of the energetic \gamma -ray burst GRB 991216 is consistent with jet-like behavior in which a power-law decay steepens from t ^ { -1.22 \pm 0.04 } at early times to t ^ { -1.53 \pm 0.05 } in a gradual transition at around 2 d. The derivation of the late-time decay slope takes into account the constant contribution of a host or intervening galaxy which was measured 110 days after the event at R = 24.56 \pm 0.14 , although the light curve deviates from a single power law whether or not a constant term is included .
The early-time spectral energy distribution of the afterglow can be described as F _ { \nu } \propto \nu ^ { -0.74 \pm 0.05 } or flatter between optical and X-ray , which , together with the slow initial decay , is characteristic of standard adiabatic evolution in a uniformly dense medium .
Assuming that a reported absorption-line redshift of 1.02 is correct , the apparent isotropic energy of 6.7 \times 10 ^ { 53 } ergs is reduced by a factor of \approx 200 in the jet model , and the initial half-opening angle is \approx 6 ^ { \circ } .
GRB 991216 is the third good example of a jet-like afterglow ( following GRB 990123 and GRB 990510 ) , supporting a trend in which the apparently most energetic \gamma -ray events have the narrowest collimation and a uniform ISM environment .
This , plus the absence of evidence for supernovae associated with jet-like afterglows , suggests that these events may originate from a progenitor in which angular momentum plays an important role but a massive stellar envelope or wind does not , e.g . , the coalescence of a compact binary .