Possible X-ray cocoon emission from GRB 050709.

This article focuses on the reanalysis of the prompt emission of the short gamma-ray burst (SGRB) GRB 050709, detected by the HETE-2 satellite, revealing a two-component structure consisting of an initial hard spike followed by a soft-tail emission. The soft tail, observed predominantly below ~10 keV, is interpreted as thermal emission from a cocoon formed by the relativistic jet interacting with surrounding material, with estimated cocoon parameters (radius ~10^9 cm and pressure ~10^19 erg cm^−3) consistent with numerical simulations of narrow-jet scenarios. Comparisons with other SGRBs exhibiting similar features—GRB 170817A and GRB 150101B—show that GRB 050709 has a relatively higher cocoon temperature and pressure, suggesting a viewing angle closer to the jet axis. The study also discusses the extended soft X-ray emission following the prompt phase, which may originate from different mechanisms than the prompt emission, and highlights observational challenges in detecting such soft components with current instruments, emphasizing the potential of future missions like HiZ-GUNDAM to improve detection and understanding of SGRB emission components.