Gamma-ray burst afterglow light curves from a Lorentz-boosted simulation frame and the shape of the jet break

TL;DR

This paper uses boosted frame simulations to accurately model gamma-ray burst afterglow light curves, revealing that observed jet break features are more complex than simple models suggest, with implications for understanding GRB environments.

Contribution

It introduces a novel boosted frame simulation method to resolve high Lorentz factors and provides scale-invariant functions for light curve reconstruction, advancing GRB afterglow modeling.

Findings

Jet break times can be delayed by viewing angle effects.

Post-break slopes are steeper than theoretical predictions.

Most Swift GRBs may not occur in homogeneous media or have hidden jet breaks.

Abstract

GRB afterglow jets have been notoriously difficult to resolve numerically using 2D hydrodynamical simulations due to high outflow Lorentz factors. By performing simulations in a boosted frame, it is possible to calculate 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. We study jet breaks for jets with opening angles of theta_0 = 0.05, 0.1 and 0.2 radians decelerating in a constant density medium, observed at angles (theta_obs) ranging from on-axis to the jet edge. We present a single set of scale-invariant functions describing the time evolution of synchrotron spectral break frequencies and peak flux, depending only on theta_0 and theta_obs, sufficient to reconstruct light curves for arbitrary explosion energy, circumburst density and synchrotron slope p. We compare our light curves to fit functions using sharp breaks and smooth transitions and confirm that the measured jet break time can be delayed due to theta_obs. We find that the difference in temporal indices across the jet break is larger than theoretically anticipated leading to post-break slopes of roughly 0.25 - 1.3 p, sufficiently steep to not easily reconcile with post-break slopes measured for the Swift sample, suggesting that most Swift GRBs do not explode in homogeneous media or that jet breaks are hidden by additional physics such as prolonged energy injection or viewing angle effects. Light curves and spectral break and peak flux evolution functions will be made publicly available on-line at http://cosmo.nyu.edu/afterglowlibrary .