An analysis of Chandra deep follow-up gamma-ray bursts: implications for off-axis jets

We present a sample of 27 gamma-ray bursts (GRBs) with detailed Swift light curves supplemented by late-time Chandra observations. To answer the missing jet-break problem in general, we develop a numerical-simulation-based model that can be directly fit to the data using Monte Carlo methods. Our numerical model takes into account all the factors that can shape a jet break: (i) lateral expansion, (ii) edge effects, and (iii) off-axis effects. Our results provide improved fits to the light curves and constraints on physical parameters. More importantly, our results suggest that off-axis effects are important and must be included in interpretations of GRB jet breaks.