Off-axis prompt X-ray transients from the cocoon of short gamma-ray
  bursts
  We present the results of numerical simulations of the prompt emission of
short-duration gamma-ray bursts. We consider emission from the relativistic
jet, the mildly relativistic cocoon, and the non-relativistic shocked ambient
material. We find that the cocoon material is confined between off-axis angles
15<theta<45 degrees and gives origin to X-ray transients with a duration of a
few to ~10 seconds, delayed by a few seconds from the time of the merger. We
also discuss the distance at which such transients can be detected, finding
that it depends sensitively on the assumptions that are made about the
radiation spectrum. Purely thermal cocoon transients are detectable only out to
a few Mpc, Comptonized transients can instead be detected by the FERMI GBM out
to several tens of Mpc.
