  The three gravitational wave events detected by LIGO are opening a new era
for high- energy astrophysics. Nevertheless, location of such events remain
unknown. A promising solution to the localization problem is to find an
electromagnetic (EM) counterpart of GW- generating events, such as binary
neutron star mergers (BNS). Indeed, their GW emission will be above sensitivity
threshold in the near future. BNS are also considered as short Gamma Ray Bursts
(sGRB) progenitors. However, sGRB are highly beamed. In this study, we will
therefore focus on another EM counterpart candidate: the cocoon afterglow. The
propagation of the GRB jet inside the matter ejected by the BNS produces a
cocoon. Then, similarly to the GRB afterglow, a cocoon afterglow is produced,
but with a mildly relativistic velocity. Firstly, we propose a model that gives
the full hydrodynamic evolution of the cocoon including the mildly relativistic
regime. Then we calculate the cocoon afterglow emission in X-ray, optical and
radio wavelengths. Finally, we compare the cocoon afterglow emission to the GRB
afterglow emission and conclude that the cocoon afterglow is a promising EM
counterpart.
