Perspective on Afterglows: Numerically Computed Views, Light Curves, and the Analysis of Homogeneous and Structured Jets with Lateral Expansion

Herein I present numerical calculations of lightcurves of homogeneous andstructured afterglows with various lateral expansion rates as seen from anyvantage point. Such calculations allow for direct simulation of observablequantities for complex afterglows with arbitrary energy distributions andlateral expansion paradigms. A simple, causal model is suggested for lateralexpansion of the jet as it evolves; namely, that the lateral expansion kineticenergy derives from the forward kinetic energy. As such the homogeneous jetmodel shows that lateral expansion is important at all times in the afterglowevolution and that analytical scaling laws do a poor job at describing theafterglow decay before and after the break. In particular, I find that lateralexpansion does not cause a break in the lightcurve as had been predicted. Aprimary purpose of this paper is to study structured afterglows, which do agood job of reproducing global relationships and correlations in the data andthus suggest the possibility of a universal afterglow model. Simulations ofstructured jets show a general trend in which jet breaks become more pronouncedwith increasing viewing angle with respect to the jet axis. In fact, undercertain conditions a bump can occur in the lightcurve at the jet break time. Iderive scaling relations for this bump and suggest that it may be a source ofsome bumps in observed lightcurves such as that of GRB 000301C. A couple oflateral expansion models are tested over a range of efficiencies and viewingangles and it is found that lateral expansion can, in some cases, substantiallysharpen the jet break. I show flux surface contour maps and simulated images ofthe afterglows which give insight into how they evolve and determine theirlightcurves.