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 ≲ θ ≲ 45° and gives origin to X-ray transients with a duration of a few to ∼10 s, 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, while omptonized transients can instead be detected by the Fermi Gamma-ray Burst Monitor (GBM) out to several tens of Mpc.