One year of broadband radio monitoring of the enigmatic transient GRB 250702B reveals the evolution of the relativistic jet

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 (n e proportional to R-k; k= 1.5-2). We detect significant variability in the low frequency (at most3 GHz) light curves which we interpret as interstellar scintillation, placing an approximate bound on the blast wave image size of 1.2 times10 16 R 5 times10 17 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 theta j 15 deg, low Lorentz factor (10) jet, or a narrow theta j 2 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 (E K about10 51 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 (E K about10 50 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.