부산대학교 도서관

In the gravitational wave event GW170817, there was a ∼10 h gap before electromagnetic (EM) observations, without detection of the cocoon. The cocoon is heated by a short gamma-ray burst (s GRB) jet propagating through the ejecta of a neutron star (NS) merger, and a part of the cocoon escapes the ejecta with an opening angle of 20°–30°. Here, we model the cocoon and calculate its EM emission. Our 2D hydrodynamic simulations suggest that the density and energy distributions, after entering homologous expansion, are well-fitted with power-law functions, in each of the relativistic and non-relativistic parts of the escaped cocoon. Modelling these features, we calculate the cooling emission analytically. We find that the cocoon outshines the r-process kilonova/macronova at early times (10–103 s), peaking at UV bands. The relativistic velocity of the cocoon's photosphere is measurable with instruments such as Swift ,  ULTRASAT , and LSST. We also imply that energetic cocoons, including failed jets, might be detected as X-ray flashes. Our model clarifies the physics and parameter dependence, covering a wide variety of central engines and ejecta of NS mergers and s GRBs in the multimessenger era. [ABSTRACT FROM AUTHOR]