A White Dwarf Tidal Disruption by an Intermediate-Mass Black Hole as the Progenitor of Ultra-long GRB 250702B

Abstract

The recent detection of GRB 250702B, the longest gamma-ray burst observed to date with prompt emission lasting $\sim 2.5\times 10^4$ seconds, challenges the conventional collapsar model. Its remarkable features--including an extraordinary X-ray flare at $\sim 1.3$ days post-detection, a late-time transition from steep to shallow decay in the X-ray afterglow, and hard spectra extending from keV to MeV energies--point to a novel progenitor. Here we show that these multiwavelength signatures can be consistently explained by a relativistic jet powered by successive partial tidal disruptions of a white dwarf (WD) by an intermediate-mass black hole (IMBH). By modeling the time-dependent accretion rate from repeated partial disruptions and the resulting jet evolution, we show that the external forward and reverse shocks account for the long-term X-ray, near-infrared, and radio afterglow, whereas the luminous X-ray flare originates from internal energy dissipation caused by collisions between fast and slow relativistic ejecta associated with the final complete disruption. Our findings establish IMBH-WD tidal disruption events as a viable engine for ultra-long GRBs.

Figures (4)

• Figure 1: Schematic illustration of the partial disruption of a WD by an IMBH, along with the structure of a relativistic jet propagating through the external medium. The dotted curve represents the orbit of the WD, while the debris stream and the resulting accretion disk are also shown. The pericenter distance is denoted as $R_p$. Within the jet, the structures from right to left correspond to the forward shock, contact discontinuity, reverse shock, and internal shell collisions.
• Figure 2: Accretion rate (defined by Eq. \ref{['eq:acc_rate']}) multiplied by $\eta_{\rm acc}^{-1}$ as a function of time during the successive partial disruptions of the WD and following the final complete disruption (e.g., $\dot M_{\rm BH}\propto t^{-5/3}$ for $t\gtrsim3\times10^{4}~\rm s$). The reference time is defined as the time of the first disruption.
• Figure 3: X-ray light curves ($0.3-10$ keV band, left panel) and spectra (right panel) of GRB 250702B. The data points show observations by EP Li:2025mae. In the left panel, the magenta solid, blue dashed, and green dash-dotted curves depict the contributions from the jet reverse shock, forward shock, and internal energy dissipation regions, respectively. In the right panel, the green, red, and blue points illustrate the X-ray spectra during the X-ray flare stage, early afterglow (EP-FXT follow-up observation on 3 July 2025, $\sim2$ days after the trigger), and late-time stacked spectrum (data taken from 12 July to 15 July), respectively. The dashed and solid curves show the corresponding spectra predicted by the TDE jet model before and after accounting for absorption.
• Figure 4: Near-infrared (H and K bands) and radio (3.1 GHz) light curves: external shock emission (solid curves) versus observations by Very Large Telescope (VLT) and MeerKAT Levan:2025mrtLi:2025mae.