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Gravitational waves from coalescence of a Binary Neutron Star (BNS) and itsaccompagning short Gamma-Ray Burst GW/GRB~170817A confirmed the presumed originof these puzzeling transients and opened up the way for relating properties ofshort GRBs to those of their progenitor stars and their surroundings. Here wereview an extensive analysis of the prompt gamma-ray and late afterglows ofthis event. We show that a fraction of polar ejecta from the merger had beenaccelerated to ultra-relativistic speeds. This structured jet had an initialLorentz factor of about $260$ in our direction - $\mathcal{O}(10^\circ)$ fromthe jet's axis - and was a few orders of magnitude less dense than in typicalshort GRBs. At the time of arrival to circum-burst material theultra-relativistic jet had a close to Gaussian profile and a Lorentz factor$\gtrsim 130$ in its core. It had retained in some extent its internalcollimation and coherence, but had extended laterally to create mildlyrelativistic lobes - a {\it cocoon}. External shocks on the far from centerinhomogeneous circum-burst material and low density of colliding shellsgenerated slow rising afterglows. The circum-burst material was somehowcorrelated with the merger and it is possible that it contained recentlyejected material from glitching, which had resumed due to the deformation ofneutron stars crust by tidal forces in the latest stages of inspiral but wellbefore their merger. By comparing these findings with the results ofrelativistic MHD simulations and observed gravitational waves we conclude thatprogenitor neutron stars were old, had close masses and highly reduced magneticfields. In addition, they probably had oppositely directed spins due to theencounter and gravitational interaction with other stars.

Binary Neutron Star (BNS) merger: What we learned from relativistic ejecta of GW/GRB~170817A