Explaining the X-ray Precursor, Ultra-long Prompt Emission, and Week-long Decay of GRB250702B with a Jetted Micro-TDE

Abstract

The longest detected gamma-ray burst, GRB250702B, exhibited seven hours of prompt $γ$-ray emission, preceded by a soft X-ray precursor ($\sim1$ day earlier) and followed by a weeks-long fading X-ray tail. Lacking an established progenitor for all three phases, we propose that this ultra-long GRB (ULGRB) is powered by a jetted micro-tidal disruption event (micro-TDE), in which a spinning stellar-mass black hole (BH) disrupts a Sun-like star and launches a relativistic jet via the Blandford-Znajek mechanism. Micro-TDE debris disks have hours-to-days viscous timescales, naturally explaining ULGRB durations. Using 3D hydrodynamic AREPO simulations of a $1\,M_\odot$ star disrupted by a $10\,M_\odot$ BH, we show that within $\sim1$ day the debris forms a quasi-steady envelope with a low-density polar funnel ($ρ\propto r^{-2}$, half-opening angle $\approx15^\circ$). Applying an analytic jet-stability framework to these profiles, we find that the $r^{-2}$ funnel keeps the jet below the kink-instability threshold, enabling stable propagation and breakout for jet powers, $L_{\rm jet}\gtrsim10^{47}$ erg s$^{-1}$. We attribute the X-ray precursor to pre-disk stream-fed accretion; the prompt GRB to a tightly beamed jet ($θ_{\rm b}\lesssim1^\circ$, $L_{γ,\rm iso}\sim10^{51}$ erg s$^{-1}$) escaping the funnel, launched by a rapidly spinning BH ($a_\bullet\sim0.9$); and the weeks-long X-ray decline to disk-wind mass loss ($L_{\rm jet}\propto t^{-2}$) combined with jet widening ($θ_{\rm b}\propto t$, initially steepening the decay to $L_{\rm X,iso}\propto L_{\rm jet}/θ_{\rm b}^{2}\propto t^{-4}$). Our model reproduces the multi-phase evolution of GRB250702B and establishes jetted micro-TDEs as a physically motivated ULGRB engine.