Due to some refinements in the dynamics , we can follow the overall evolution of a realistic jet numerically till its bulk velocity being as small as \beta c \sim 10 ^ { -3 } c .
We find no obvious break in the optical light curve during the relativistic phase itself .
However , an obvious break does exist at the transition from the relativistic phase to the non-relativistic phase , which typically occurs at time t \sim 10 ^ { 6 } — 10 ^ { 6.5 } s ( i.e. , 10 — 30 d ) .
The break is affected by many parameters , such as the electron energy fraction \xi _ { e } , the magnetic energy fraction \xi _ { B } ^ { 2 } , the initial half opening angle \theta _ { 0 } , and the medium number density n .
Increase of any of them to a large enough value will make the break disappear .
Although the break itself is parameter-dependent , afterglows from jetted GRB remnants are uniformly characterized by a quick decay during the non-relativistic phase , with power law timing index \alpha \geq 2.1 .
This is quite different from that of isotropic fireballs , and may be of fundamental importance for determining the degree of beaming in \gamma -ray bursts observationally .