Off-axis emission of short gamma-ray bursts and the detectability of
  electromagnetic counterparts of gravitational wave detected binary mergers
  We present calculations of the wide angle emission of short-duration
gamma-ray bursts from compact binary merger progenitors. Such events are
expected to be localized by their gravitational wave emission, fairly
irrespective of the orientation of the angular momentum vector of the system,
along which the gamma-ray burst outflow is expected to propagate. We show that
both the prompt and afterglow emission are dim and challenging to detect for
observers lying outside of the cone within which the relativistic outflow is
propagating. If the jet initially propagates through a baryon contaminated
region surrounding the merger site, however, a hot cocoon forms around it. The
cocoon subsequently expands quasi-isotropically producing its own prompt
emission and external shock powered afterglow. We show that the cocoon prompt
emission is detectable by Swift BAT and Fermi GBM, We also show that the cocoon
afterglow peaks a few hours to a few days after the burst and is detectable for
up to a few weeks at all wavelengths. The timing and brightness of the
transient are however uncertain due to their dependence on unknown quantities
such as the density of the ambient medium surrounding the merger site, the
cocoon energy, and the cocoon Lorentz factor. For a significant fraction of the
gravitationally-detected neutron-star-binary mergers, the cocoon afterglow
could possibly be the only identifiable electromagnetic counterpart, at least
at radio and X-ray frequencies.
