The VHE emission of structured GRB jets: GW 170817 and short merger-time BNS progenitors.

Several GRB afterglows have now been observed at Very High energy (VHE, > 1 TeV). In addition, GW 170817 has allowed to probe the lateral structure of the associated relativistic jet. In this talk, I will present a complete numerical model of the afterglow of a laterally-structured relativistic ejecta from radio to VHE. This includes a self-consistent calculation of the synchrotron radiation, with its maximum frequency, and of Synchrotron Self-Compton (SSC) scatterings taking into account the Klein-Nishina regime. The attenuation due to pair production is also included. This model is computationally-efficient, allowing for multi-wavelength data fitting. While the SSC flux at the peak of GW 170817 was much dimmer than the H.E.S.S. upper limit, I show that smaller viewing angles or larger external densities would make similar off-axis events detectable in the future at VHE. Interestingly, large external densities are expected in the case of fast mergers, but the existence of a formation channel for such Binary Neutron Stars (BNS) is still uncertain. I will discuss how an evolutionary track including the accretion-induced collapse of a white dwarf into a neutron star could produce fast-merging BNSs, using results obtained with the population synthesis code COSMIC.