  The production of both gravitational waves and short gamma-ray bursts (sGRBs)
is widely associated with the merger of compact objects. Several studies have
modelled the evolution of the electromagnetic emission using the synchrotron
emission produced by the deceleration of both a relativistic top-hat jet seen
off-axis, and a wide-angle quasi-spherical outflow (both using numerical
studies). In this study we present an analytical model of the synchrotron and
synchrotron self-Compton (SSC) emission for an off-axis top-hat jet and a
quasi-spherical outflow. We calculate the light curves obtained from an
analytic model in which the synchrotron and SSC emission (in the fast- or
slow-cooling regime) of an off-axis top-hat jet and a quasi-spherical outflow
are decelerated in either a homogeneous or a wind-like circumburst medium. We
show that the synchrotron emission of the quasi-spherical outflow is stronger
than that of the off-axis jet during the first $\sim$ 10 - 20 days, and weaker
during the next $\gtrsim$ 80 days. Moreover, we show that if the off-axis jet
is decelerated in a wind-like medium, then the SSC emission is very likely to
be detected. Applying a MCMC code to our model (for synchrotron emission only),
we find the best-fit values for the radio, optical and X-ray emission of GRB
170817A which are in accordance with observations. For GRB 170817A, we find
using our model that the synchrotron emission generated by the quasi-spherical
outflow and off-axis top-hat jet increase as $F_\nu\propto t^{\alpha}$ with
$\alpha\lesssim 0.8$ and $\alpha>3$, respectively. Finally, we obtain the
correspondent SSC light curves which are in accordance with the
very-high-energy gamma-ray upper limits derived with the GeV - TeV
observatories.
