A milli-Tidal Disruption Event Model for GRB$\;$250702B: Main Sequence Star Disrupted by an IMBH

TL;DR

This work investigates GRB 250702B as a milli-Tidal Disruption Event powered by an intermediate-mass black hole (~10^4 solar masses) tidally disrupting a main-sequence star off-nucleus. Multiband afterglow modeling reveals a Bondi-like, stratified external medium with n(r) ∝ r^{-3/2}, enabling a self-consistent IMBH mass estimate constrained by the source-frame variability timescale. The proposed MS–IMBH mTDE scenario naturally accounts for the day-scale soft X-ray pre-peak rise, the multi-hour main emission, and the long-lived afterglow, while WD–IMBH and micro-TDE alternatives face significant challenges. The paper outlines falsifiable observational tests (late-time calorimetry, VLBI imaging, and host-environment studies) and discusses event-rate implications, suggesting IMBH mTDEs could represent a viable channel for ultra-long gamma-ray transients and a probe of wandering IMBHs in distant galaxies.

Abstract

GRB$\;$250702B is the longest 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 $\sim1\;$day gradually rising emission. It is offset from its host galaxy center by $\sim5.7\;$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 ultra-long 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 $k=1.60\pm0.17$, 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 $M_\bullet\approx\left(6.55^{+3.51}_{-2.29}\right)\times10^3\,n_0^{-2/3}\,c_{s,6}^{2}(1+\mathcal{M}^2)\;M_\odot$ 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 $M_\bullet<\frac{c^3}{G}\frac{t_{\rm MV}}{1+z} \lesssim5\times10^4\,M_\odot$ 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 a 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 disk buildup, leading to an increase in the jet's power and Lorentz factor.

Figures (3)

• Figure 1: Best-fit model lightcurve obtained from an MCMC fit to the multiwavelength afterglow data of GRB 250702B. The model parameter posterior distributions are shown in Fig. \ref{['fig:fit-posteriors']}. The bump in the radio lightcurves at $10^5\,{\rm s}\lesssim t \lesssim 10^6\,{\rm s}$ is produced by a contribution from the reverse shock emission.
• Figure 2: Critical radii $R$ versus black hole mass $M_\bullet$: the tidal radius $R_t=R_*(M_\bullet/M_*)^{1/3}$ for a main-sequence (MS) solar-like star ($M_*=M_\odot$, $R_*=R_\odot$) and a white dwarf (WD; $M_*=M_\odot$, $R_*=0.01R_\odot$), the Schwarzschild radius $R_s = \frac{2GM_\bullet}{c^2}$, the Bondi radius $R_B=\frac{2GM_\bullet}{c_s^2}$ (for $c_{s,6}=1$), and the radius of influence $R_h$ within which the ISM mass (for $n_0=c_{s,6}=1$) equals $M_\bullet$. Two upper limits on $M_\bullet$ are indicated by a vertical line and arrow. The narrow shaded gray region shows the inferred 1-$\sigma$ confidence interval on $M_\bullet$ (from Eq. (\ref{['eq:M_IMBH']})) for $n_0^{-2/3}\,c_{s,6}^{2}=1$. The lighter and wider shading indicates a factor of 5 uncertainty in the value of $n_0^{-2/3}\,c_{s,6}^{2}$. Approximate mass ranges are indicated for stellar, intermediate mass (IMBH), and super-massive black holes (SMBH).
• Figure 3: Model parameter posterior distributions obtained from an MCMC fit to the GRB 250702B multiwavelength afterglow data using the afterglow model of Gill-Granot-23.