A milli-tidal disruption event model for GRB 250702B: main-sequence star disrupted by an IMBH

GRB 250702B is the longest gamma-ray burst (GRB) r ecor ded so far, with multiple gamma-ray emission episodes spread over a duration exceeding 25 ks and a weaker soft X-ray pre-peak ∼1 day gradually rising emission. It is offset from its host galaxy centre by ∼5.7 kpc, and displays a long-lived afterglow emission in radio to X-ray. Its true nature is unclear, with the two leading candidate classes of objects being a peculiar type of ultralong GRB and a tidal disruption event (TDE) by an int ermediat e mass black hole (IMBH). Here, w e consider the latter, mTDE origin. We model the afterglow data, finding a stratified external density profile ∝ r^-k with k = 1.60 ±0.17, consistent with Bondi accretion of the interstellar medium (of initial number density n_ISM = n₀cm^-3 and sound speed c_s=c_s,6 10⁶ cm s^-1) for which n(r) ≈n_ISM (r/R_B)^-3/2 within the Bondi radius R_B. Moreover, we use the implied density normalization to infer the IMBH mass within this model, finding M• ≈(6.55^+3.51_-2.29 ×10³ n^-2/3₀ c²_s,6 (1+M²) Mo where M Ξv_BH/c_s is the IMBH's Mach number relative to the ISM. Together with an upper limit on M• <c₃t_MV/G1+ z≲ 5 ×10⁴ M⊙ from the source-frame minimum variability time t_{MV, src}= t_MV/ 1+z ≈0.5 s this implies v_BH ≲ 28 n^1/3₀ km s^-1. We show that an mTDE of a main-sequence star (but not of a white dwarf) can explain the duration and energetics of GRB 250702B. The gradual rise to the peak may be caused by gradual circularization and accretion disc buildup, leading to an increase in the jet's power and Lorentz factor.