GAMMA-RAY BURST AFTERGLOW LIGHT CURVES FROM A LORENTZ-BOOSTED SIMULATION FRAME AND THE SHAPE OF THE JET BREAK

The early stages of decelerating gamma-ray burst (GRB) afterglow jets have been notoriously difficult to resolve numerically using two-dimensional hydrodynamical simulations even at very high resolution, due to the extreme thinness of the blast wave and high outflow Lorentz factors. However, these resolution issues can be avoided by performing the simulations in a boosted frame, which makes it possible to calculate afterglow light curves from numerically computed flows in sufficient detail to accurately quantify the shape of the jet break and the post-break steepening of the light curve. Here, we study afterglow jet breaks for jets with opening angles of 0.05, 0.1, and 0.2 radians decelerating in a surrounding medium of constant density, observed at various angles ranging from on-axis to the edge of the jet. A single set of scale-invariant functions describing the time evolution of afterglow synchrotron spectral break frequencies and peak flux, depending only on jet opening angle and observer angle, are all that is needed to reconstruct light curves for arbitrary explosion energy, circumburst density and synchrotron particle distribution power law slope p. These functions are presented in the paper. Their time evolutions change directly following the jet break, although an earlier reported temporary post-break steepening of the cooling break is found to have been resolution-induced. We compare synthetic light curves to fit functions using sharp power law breaks as well as smooth power law transitions. We confirm our earlier finding that the measured jet break time is very sensitive to the angle of the observer and can be postponed significantly. We find that the difference in temporal indices across the jet break is larger than theoretically anticipated and is about −(0.5 + 0.5p) below the cooling break and about −(0.25 + 0.5p) above the cooling break, both leading to post-break slopes of roughly about 0.25 − 1.3p, although different observer angles, jet opening angles and heuristic descriptions of the break introduce a wide range of temporal indices. Nevertheless, the post-break slope from our constant density interstellar medium simulations is sufficiently steep to be hard to reconcile with post-break slopes measured for the Swift sample, suggesting that Swift GRBs mostly do not explode in a homogeneous medium or that the jet breaks are hidden from view by additional physics such as prolonged energy injection or viewing angle effects. A comparison between different smooth power law fit functions shows that although smooth power law transitions of the type introduced by Harrison et al. often provide better fits, smooth power law transitions of the type introduced by Beuermann et al. or even sharp power law fits are easier to interpret in terms of the underlying model. Light curves and spectral break and peak flux evolution functions will be made publicly available online at http://cosmo.nyu.edu/afterglowlibrary.