GRB Afterglow (2D) — Jet Break Cross-Section

A gamma-ray burst launches a narrow, ultra-relativistic jet that ploughs into the surrounding gas and decelerates. This 2D cross-section tracks that deceleration in real time along the jet's meridional plane — the axis-containing slice that fully captures an axisymmetric jet's geometry. The bulk Lorentz factor Γ falls as the blast wave sweeps up mass, and with it the relativistic beaming wedge (half-angle 1/Γ) steadily widens around the jet axis. While that wedge stays narrower than the jet's true physical opening angle θ_j, a distant on-axis observer sees what looks like ordinary spherical-fireball emission. The moment 1/Γ grows past θ_j, the observer becomes sensitive to the jet's finite edges and the afterglow light curve steepens abruptly — the achromatic "jet break" that lets astronomers measure a GRB's true (beaming-corrected) energy instead of the wildly overestimated isotropic-equivalent value. Tune the opening angle, initial Lorentz factor and electron energy index, then scrub or play forward in time to watch the beaming wedge catch up to the jet boundary exactly as the light curve kinks.