The evolution of a beamed gamma-ray-burst afterglow: the non-relativistic case |
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| Authors: | D M Wei T Lu |
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| Institution: | Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China;National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100080, China;Department of Astronomy, Nanjing University, Nanjing, 210093, China;Laboratory for Cosmic‐Ray and High‐Energy Astrophysics, Institute of High‐Energy Physics, Chinese Academy of Sciences, Beijing, China |
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| Abstract: | There has been increasing evidence that at least some gamma-ray bursts (GRBs) are emission beamed. The beamed GRB-afterglow evolution has been discussed by several authors in the ultrarelativistic case. It has been shown that the dynamics of the blast wave will be significantly modified by the sideways expansion, and there may be a sharp break in the afterglow light curves under certain circumstances. However, this is only true when the fireball is still relativistic. Here we present an analytical approach to the evolution of the beamed GRB blast wave expanding in the surrounding medium (density  in the non-relativistic case, our purpose is to explore whether the sideways expansion will strongly affect the blast-wave evolution as in the relativistic case. We find that the blast-wave evolution is strongly dependent on the speed of the sideways expansion. If it expands with the sound speed, then the jet angle θ increases with time as  which means that the sideways expansion has little effect on the afterglow light curves, the flux  for  and  for  It is clear that the light curve of  is not always steeper than that of  as in the relativistic case. We also show that if the expansion speed is a constant, then the jet angle  and the radius  in this case the sideways expansion has the most significant effect on the blast-wave evolution, the flux  independent of s , and we expect that there should be a smooth and gradual break in the light curve. |
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| Keywords: | gamma-rays: bursts |
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