AN ENVIRONMENTAL ORIGIN FOR DIVERSITY IN GAMMA-RAY BURST AFTERGLOWS

The most powerful and energetic explosions in the known universe – gamma-ray bursts (GRBs) – are thought to originate from the collapse of massive stars or the merger of compact objects. Often marking the birth of a black hole, GRBs produce broadband emission via synchrotron radiation as the resulting ultra-relativistic jet interacts with the surrounding medium. This emission, known as the afterglow, carries rich information about the nature of the burst and its environment. To better understand these cataclysmic events and the environments that surround them, we developed a Python-based modeling and inference framework, AMPy, capable of modeling afterglows in arbitrary and smoothly varying power-law density profiles, extending beyond the standard assumption of either a constant-density interstellar medium (k = 0) or an idealized stellar wind profile (k = 2). This dissertation presents detailed results from modeling 15 events with AMPy which reveal a wide diversity of environmental structures, including steep “superwinds”, shallow “subwinds,” stratified media with smooth transitions between regimes, and cavity-like regions consistent with effective density gradients of k < 0. The recovered stratified and cavity-like profiles are consistent with evolving progenitor winds, shocked regions, termination shocks, or the presence of shells ejected prior to the burst. These findings also reveal that radiative cooling plays a significant role in many events, that synchrotron self-absorption can influence modeling even when unobserved, and that calibration systematics and unaccounted-for statistical uncertainties (“slop”) must be incorporated to avoid biased fits, underestimated uncertainties, and consequently overstated conclusions. AMPy handles all of this self-consistently and efficiently. These results demonstrate that GRB afterglow environments are far more diverse than has been traditionally assumed for the past three decades and that oversimplified models likely misrepresent the physical nature of these explosions.