(Robert Mochkovitch, Frédéric Daigne)
The detection of GRB170817A by Fermi triggered a hitherto unseen electromagnetic followup of the first directly witnessed binary neutron star merger. One year after the event, polarimetric, photometric and high resolution imagery observations of the remnant have brought forward the question of whether a relativistic jet had been produced in the merger event. This crucial question has profound impacts on our conception of these new phenomena and of short GRBs in general. We present simple analytical and numerical models of afterglows from jet outflows in binary neutron star mergers. These allow us to infer the physical conditions in the late phases of GRB170817A. Using these models, we also predict the phenomenology of afterglows for future events triggered by gravitational wave detections.