Successive Partial Disruptions with Orbital Precession in a White Dwarf-Black Hole System for Repeating GRB 250702B

TL;DR

GRB 250702B is interpreted as arising from repeated partial tidal disruptions of a white dwarf by a rapidly spinning intermediate-mass black hole on a highly eccentric orbit. The model links the four observed ~100 s prompt flares, their irregular hour-scale spacing, and the day-long activity to three hierarchical timescales set by viscous disk evolution, orbital period, and secular orbital evolution, respectively, with a total of ~40 potential jet-launch episodes before complete disruption. Relativistic frame dragging induces orbital precession, causing most jets to be off-axis and only a few to be observed on-axis, while accumulated off-axis jets produce a pronounced late-time radio brightening; the jets are powered by a Blandford–Znajek mechanism requiring near-maximal BH spin and MAD-like accretion. The authors support the scenario with afterglow modeling that shows distinct max/min radio predictions and propose multimessenger tests, including LISA-band gravitational waves, to constrain the BH mass and spin and the orbital evolution. This work provides a concrete WD–IMBH tidal-disruption pathway to explain an extreme GRB and offers clear, testable observational signatures across electromagnetic and gravitational-wave channels.

Abstract

The peculiar gamma-ray burst GRB 250702B is the longest event ever observed, lasting about one day and exhibiting four prompt-emission flares of $\sim100$ s with irregular recurrence intervals of at least one hour. To explain this hierarchy of timescales, we consider a scenario in which a stellar object undergoes repeated partial tidal disruptions by a black hole (BH). We find that if a white dwarf (WD) is on a highly eccentric orbit ($e\approx0.97$) around an intermediate-mass black hole (BH) with $M_{\rm BH}\lesssim10^{6}\,M_\odot$ and $a = 50\,R_\odot\left(M_{\rm BH}/10^{6}\,M_\odot\right)^{1/3}$, the observed properties of GRB 250702B can be naturally reproduced. In this framework, the duration of each flare is determined by the viscous accretion timescale of material stripped near pericenter, with a typical mass $ΔM \approx 2\times10^{-2}\,M_\odot$. The minimum recurrence time corresponds to the orbital period, while the total activity period is set by the secular orbital evolution timescale leading to the complete disruption of the WD. Furthermore, if $M_{\rm BH}\gtrsim10^{5}\,M_\odot$ and the orbit has a minimum polar angle relative to the BH equatorial plane of $θ_{\rm min}\gtrsim0.12 {\rm rad}$, relativistic frame dragging induces $\gtrsim0.1$ rad precession of the orbital angular momentum between successive pericenter passages, comparable to a typical GRB jet half-opening angle, resulting in intermittent alignment with the observer and irregular flare spacing. The WD experiences $\approx40$ jet-launch episodes before complete disruption, but only four are expected to be observed on-axis. The remaining off-axis jets become visible at late times, enhancing the radio afterglow by about an order of magnitude, providing a testable prediction of this scenario.

Figures (3)

• Figure 1: Our physical picture for GRB 250702B. Panel (a) shows the geodesic orbit of the WD, where the red line represents our numerical result with $a = 20\,R_\odot$, $e = 0.97$, $M_{\rm BH} = 1\times10^5\,M_\odot$, $a_{\rm spin}=0.9$, and $\theta_{\rm min} = 0.12~\mathrm{rad}$ ($7^\circ$). The BH spin axis is indicated by the vertical black line. The orbit exhibits relativistic precession. Mass loss from the WD is not included in this calculation. Panel (b) illustrates an unobservable configuration. An accretion disk forms with its angular-momentum vector tilted by an angle $\theta_{\rm d}$ with respect to the BH spin axis. As a result, the jet undergoes Lense--Thirring precession. If $|\theta_{\rm v,BH} - \theta_{\rm d}| > \theta_{\rm j}$, the jet does not intersect the line of sight, and the prompt emission remains unobservable. Panel (c) shows an observable configuration. If $|\theta_{\rm v,BH} - \theta_{\rm d}| \lesssim \theta_{\rm j}$, the jet remains continuously observable throughout the Lense--Thirring precession.
• Figure 2: Afterglow light curves in the X-ray (5 keV; orange), near-infrared ($2\times10^{14}$ Hz; blue), and radio (1.3 GHz; red) bands, compared with the observed data of GRB 250702B (X-ray: orange circles; near-infrared: blue diamonds; radio: red squares). The thick solid and thin dashed lines denote the predicted emission for the maximum (40 jets) and minimum (4 jets) cases, respectively. Black arrows indicate the times of the observed gamma-ray flares.
• Figure 3: Predicted radio (1.3 GHz) afterglow light curves for the maximum (40 jets; red solid line) and minimum (4 jets; red dashed line) cases. The gray solid lines show the emission from individual jets in the maximum case, while the black dashed lines represent the emission from individual jets in the minimum case. At $T \sim 10^{8}$ s, the maximum case is approximately an order of magnitude brighter than the minimum case.