Nuclear Reactions Induced by Pre-breakout Shock Passage in BNS Merger Ejecta

Abstract: The first electromagnetic signal emitted form different types of stellar explosions (SNe, GRBs, BNS mergers), is released during the breakout of a shock created in the explosion from the envelope surrounding the source. A notable example is the faint gamma-ray flash (GRB 170817A) that followed the gravitational wave signal, GW170817, released during a BNS merger. This flash has been interpreted as shock breakout emission, most likely driven by a relativistic jet launched from the compact remnant. We recently discovered that the passage of such a fast shock in the dense r-process ejecta can induce nuclear transmutations that might change the composition behind the shock, potentially affecting the very early kilonova emission. After a brief introduction to shock breakout and the properties of radiation mediated shocks, I’ll introduce a multi-ion shock model constructed recently to account for the conditions anticipated in such environments, and explain why and when nuclear reactions are expected to be ignited in the post shock plasma.