2026/08/04 by A. J. Goodwin, James C. A. Miller-Jones, Itai Sfaradi +18
Physics and Astronomy · #astro-ph.HE
29 pages, 15 figures, submitted to MNRAS. All data in Table D1 available in machine readable format upon request. Comments welcome
arxiv created 2026/08/04 · arxiv updated 2026/08/05
We present an extensive radio monitoring campaign of the unique extragalactic transient GRB 250702B, with observations spanning 0.65-233 GHz from 6-356 d (observer frame) post-discovery. The radio emission shows a smoothly evolving peaked synchrotron spectrum consistent with an adiabatic shock expanding into a stratified ambient medium (ne∝ R-k; k= 1.5-2). We detect significant variability in the low frequency (≤3 GHz) light curves which we interpret as interstellar scintillation, placing an approximate bound on the blast wave image size of 1.2×1016\lesssim R⊥ \lesssim 5×1017 cm. The temporal evolution of the flux density and critical synchrotron frequencies suggest the shock that powers the radio emission is potentially a wide-angle θj\gtrsim15 deg, low Lorentz factor (Γ\lesssim10) jet, or a narrow θj\lesssim2 deg highly relativistic jet. A narrow jet is expected for a stellar-mass black hole engine, such as a helium star merger, and the beaming-corrected kinetic energy in this scenario is consistent with the known distribution for long GRBs (EK∼1051 erg). The wide-angle jet scenario would instead require a progenitor involving prolonged accretion. We derive and show an intermediate or stellar-mass black hole tidal disruption event are viable possibilities. The beaming-corrected kinetic energy in this scenario is on the low end of the known distribution for relativistic SMBH TDEs (EK∼1050 erg). We disfavour an SMBH TDE due to lack of compatibility with the observed timescales. The detection of a jet shut off within the next year would favour a WD-IMBH TDE due to the shorter theoretical duration of super-Eddington accretion than the main-sequence TDE channels.