Colliding Relativistic Shells: New Insights

Shock waves are abundant in astrophysical sources, and are responsible for much of the electromagnetic emission that we observe from many sources. Therefore, the shock dynamics can significantly affect the observable signatures, and are important to account for when interpreting the observations. I will first discuss a planar collision between two cold shells, which may be relevant either for internal shocks within an outflow (e.g. in GRBs or AGN) or for an ejected shell colliding with a preexisting external shell (e.g. in magnetar giant flares, superluminous supernovae or possibly also in FRBs). Both the bulk velocity of the two shells and their relative velocity can range from Newtonian to ultra-relativistic, within the same formalism. The possible observable implications for prompt GRB emission from internal shocks will be outlined as an important case study. Finally, I will discuss an oblique collision between two cold shells, which can be reduced to two coupled oblique collisions of a single cold shell with a wall. The latter problem can be solved analytically and has a lot in common with the classical problem of shock reflection, which we have recently generalized from the well studied Newtonian regime to the relativistic regime.