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

ABSTRACT GRB 250702B is the longest gamma-ray burst (GRB) recorded so far, with multiple gamma-ray emission episodes spread over a duration exceeding $25\,$ ks and a weaker soft X-ray pre-peak $\sim 1\,$ day gradually rising emission. It is offset from its host galaxy centre by $\sim 5.7\,$ 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 intermediate mass black hole (IMBH). Here, we consider the latter, mTDE origin. We model the afterglow data, finding a stratified external density profile $\propto r^{-k}$ with $k=1.60\pm 0.17$, consistent with Bondi accretion of the interstellar medium (of initial number density $n_{\rm ISM}=n_0\, {\rm cm^{-3}}$ and sound speed $c_s=c_{s,6}10^6\, {\rm cm\, s^{-1}}$) for which $n(r)\approx n_{\rm ISM}(r/R_{\rm B})^{-3/2}$ within the Bondi radius $R_{\rm B}$. Moreover, we use the implied density normalization to infer the IMBH mass within this model, finding $M_\bullet \approx \left(6.55^{+3.51}_{-2.29}\right)\times 10^3\, n_0^{-2/3}\, c_{s,6}^{2}(1+\mathcal {M}^2)\, \mathrm{ M}_\odot$ where $\mathcal {M}\equiv v_{\rm BH}/c_s$ is the IMBH’s Mach number relative to the ISM. Together with an upper limit on $M_\bullet \lt \frac{c^3}{G}\frac{t_{\rm MV}}{1+z} \lesssim 5\times 10^4\, \mathrm{ M}_\odot$ from the source-frame minimum variability time $t_{\rm MV,src}\!=\!\frac{t_{\rm MV}}{1+z}\!\approx \!0.5\,$ s this implies $v_{\rm BH}\lesssim 28\, n_0^{1/3}\, {\rm km\, s^{-1}}$. 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.