A reconnection switch to trigger gamma-ray burst jet dissipation

Abstract

Prompt gamma-ray burst (GRB) emission requires some mechanism to dissipate an ultrarelativistic jet. Internal shocks or some form of electromagnetic dissipation are candidate mechanisms. Any mechanism needs to answer basic questions, such as what is the origin of variability, what radius does dissipation occur at, and how does efficient prompt emission occur. These mechanisms also need to be consistent with how ultrarelativistic jets form and stay baryon pure despite turbulence and electromagnetic reconnection near the compact object and despite stellar entrainment within the collapsar model. We use the latest magnetohydrodynamical models of ultrarelativistic jets to explore some of these questions in the context of electromagnetic dissipation due to the slow collisional and fast collisionless reconnection mechanisms, as often associated with Sweet-Parker and Petschek reconnection, respectively. For a highly magnetized ultrarelativistic jet and typical collapsar parameters, we find that significant electromagnetic dissipation may be avoided until it proceeds catastrophically near the jet photosphere at large radii (r~ 1013-1014cm), by which the jet obtains a high Lorentz factor (γ~ 100-1000), has a luminosity ofLj~ 1050-1051ergs-1, has observer variability time-scales of the order of 1s (ranging from 0.001 to 10s), achieves γθj~ 10-20 (for opening half-angle θj) and so is able to produce jet-breaks, and has comparable energy available for both prompt and afterglow emission. A range of model parameters are investigated and simplified scaling laws are derived. This reconnection switch mechanism allows for highly efficient conversion of electromagnetic energy into prompt emission and associates the observed prompt GRB pulse temporal structure with dissipation time-scales of some number of reconnecting current sheets embedded in the jet. We hope this work helps to motivate the development of self-consistent radiative compressible relativistic reconnection models. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.