Off-axis prompt X-ray transients from the cocoon of short gamma-ray bursts

Understand

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.