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

TL;DR

This paper proposes a model where a white dwarf on a highly eccentric orbit around an intermediate-mass black hole undergoes successive partial tidal disruptions, explaining the peculiar long-duration GRB 250702B with irregular flares and predicting observable late-time radio afterglow enhancements.

Contribution

It introduces a novel scenario of repeated partial disruptions with orbital precession to explain complex GRB light curves, linking orbital dynamics to observable signatures.

Findings

Reproduces GRB 250702B's duration and flare structure with a WD-BH interaction model.

Predicts late-time radio afterglow enhancement due to off-axis jet emissions.

Suggests orbital precession causes irregular flare intervals consistent with observations.

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 $\Delta M \approx 2\times10^{-2}\,M_\odot$. 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 $\theta_{\rm min}\gtrsim0.12 {\rm rad}$, 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.