Radio observations of the ultra-long GRB 220627A reveal a hot cocoon supporting the blue supergiant progenitor scenario

• DOI: 10.48550/arxiv.2502.13435
• Publication Date: 2025-02-19
• Authors: James K. Leung; O. S. Salafia; Cristiana Spingola; G. Ghirlanda; S. Giarratana; M. Giroletti; Cormac Reynolds; Ziteng Wang; Tao An; Adam T. Deller; Maria R. Drout; D. L. Kaplan; E. Lenc; Tara Murphy; M. Á. Pérez-Torres; Lauren Rhodes
• Keywords: Progenitor

We present the discovery of radio afterglow most distant ultra-long gamma-ray burst (GRB) detected to date, GRB~220627A at redshift $z=3.084$. Its prompt light curve shows a double-pulse profile, with pulses separated by period quiescence lasting ${\sim} 15\,$min, leading early speculation it could be strongly gravitationally lensed GRB. However, our analysis $\textit{Fermi}$/GBM spectra taken during time intervals both show clear differences in their spectral energy distributions, disfavouring lensing scenario. observed from $7$ $456\,$d post-burst: an initial, steep decay ($F_{\nu} \propto t^{-2}$) is followed shallower decline t^{-1/2}$) after 20\,$d. Our modelling that these properties can explained presence slow, wide ejecta component addition fast, narrow component, consistent picture highly-collimated jet and its thermal cocoon decelerating into ambient medium. The point toward progenitor large stellar radius, supporting blue supergiant scenario proposed for GRBs. also conducted independent test hypothesis via Very Long Baseline Interferometry (VLBI) observations 12\,$d post-burst searching, first time, multiple images candidate GRB afterglow. experiment highlighted growing need developments real-time correlation capabilities time-critical VLBI experiments, particularly as we advance towards SKA ngVLA era astronomy.