LAUSR

GW 170817/GRB 170817A has offered unprecedented insight into binary neutron star post-merger systems. Prompt and afterglow emission light curves imply the presence of a tightly collimated relativistic jet with a smooth transverse structure. However, it remains unclear whether and how the central engine can produce such structured jets. Here, we utilize 3D GRMHD simulations starting with a black hole surrounded by a magnetized torus with properties typically expected of a post merger system. We follow the jet as it is self-consistently launched from the scale of the compact object out to more than 3 orders of magnitude in distance. We find that this results in a structured jet, which is collimated by the disk wind into a half-opening angle of roughly $10^\circ$ and whose emission can both explain the features of the prompt emission and afterglow light curves of GRB 170817A. Our simulation is the first to compute the afterglow, in the context of a binary merger, from a relativistic magnetized jet self-consistently generated by an accreting black hole. With the jet's transverse structure determined by the accretion physics rather than prescribed, this offers a stringent test of the physics of the central engine.