Description of atypical bursts seen slightly off-axis

The detection of gravitational waves together with their electromagnetic counterpart, in the gamma-ray burst GRB 170817A, marked a new era of multi-messenger astronomy. Several theoretical models have been proposed to explain the atypical behavior of this event. Recently, it was shown that the multi-wavelength afterglow of GRB 170817A was consistent with a synchrotron forward-shock model when the outflow was viewed off-axis, decelerated in a uniform medium and parametrized through a power-law velocity distribution. Motivated by the upper limits on the very-high-energy emission, and the stratified medium in the close vicinity of a binary neutron star merger proposed to explain the gamma-ray flux in the short GRB 150101B, we extend the mechanism proposed to explain GRB 170817A to a more general scenario deriving the synchrotron self-Compton (SSC) and synchrotron forward-shock model when the off-axis outflow is decelerated in a uniform and stratified circumburst density. As particular cases, we show that the delayed and long-lasting afterglow emission observed in GRB 080503, GRB140903A, GRB 150101B, and GRB 160821B could be interpreted by a similar scenario to the one used to describe GRB 170817A. In addition, we show that the proposed scenario agrees with the MAGIC, Fermi-LAT and H.E.S.S upper limits on gamma-ray emission from GRB 160821B and GRB 170817A.