A geometric model for the off axis emission by short gamma ray bursts.

In this work I will present a model relating the flux emitted by a gamma-ray burst and the angle between the line of sight and the jet orientation, considering at first a homogeneous jet, then a more realistic structured jet with power law distribution of velocity and density. The model is based on the geometry of the phenomenon and not strictly dependent on the emission nature. Under the assumption of isotropic specific intensity, it is considered the emission by a sphere expanding at relativistic speed, then the region of interest is reduced to that of the cone defined by the jet aperture. Inserting the parameters of the first peak of GRB170817A obtained by other works in the geometrical model resulted a not accurate fit of the light curve, but considering an angle of view of 0.35 the on axis equivalent luminosity obtained is 1051 erg s􀀀1, in agreement with the one derived in other works and also with the canonical luminosity of short gamma ray bursts.