Ultra-long Gamma-ray Bursts from Micro-Tidal Disruption Events: The Case of GRB 250702B

TL;DR

ULGRBs pose a challenge to standard progenitor models. This paper proposes micro-TDEs ($bc$TDEs) as a unifying channel, detailing three formation pathways—dynamical partial/repeating disruptions, natal-kick-driven full disruptions, and a hybrid channel—that can reproduce the day-earlier X-ray pre-peak and the ultra-long main flare via fallback accretion. The framework naturally links tidal disruption physics to ULGRB energetics and timescales, predicts distinctive afterglow signatures, and provides observational discriminants to test the channels. If GRB 250702B is indeed a $bc$TDE, it would constitute the first strong evidence for such events and would open a new window on compact-object dynamics, natal kicks, and stellar encounters in dense environments; future high-energy missions with pre-peak sensitivity will be crucial to identify more events and constrain rates.

Abstract

Ultra-long gamma-ray bursts (ULGRBs), a rare class of high-energy transients with durations $>10^3$s, remain poorly understood. GRB 250702B is notable for its multi-hour prompt emission, an X-ray pre-peak emission starting $\sim$1 day earlier, off-nuclear host position, and hard, rapidly variable gamma-rays. This combination is difficult to explain with standard ULGRB progenitors such as blue-supergiant collapsars, magnetar engines, or white-dwarf tidal disruptions by intermediate-mass black holes. We interpret the event as a micro-tidal disruption event ($μ$TDE), where a stellar-mass black hole or neutron star partially or fully disrupts a main-sequence star. Three $μ$TDE pathways can reproduce the observed pre-peak emission to main flare delay: (i) a dynamical (partial/repeating) disruption, in which a grazing passage yields a faint precursor and the core returns after $\sim$day for a deeper encounter; (ii) a natal-kick disruption, where the delay reflects the ballistic motion of a newborn compact object relative to its companion, leading to full disruption; and (iii) a hybrid natal-kick + partial case, in which the kick seeds the close encounter but the first passage is only partial, with the core returning on the day-scale period. Cross-section scalings imply comparable rates for partial and full outcomes in both dynamical and natal-kick scenarios. The highly variable, hard $γ$-ray emission supports association with a stellar-mass compact object. Fallback and viscous accretion naturally explain the ultra-long duration, energetics, and ks-scale X-ray variability. We outline observational discriminants between the three channels and argue that $μ$TDEs offer a compelling framework for ULGRBs such as GRB 250702B.

Figures (1)

• Figure 1: Characteristics of $\mu$TDEs in the for different $M_{\ast}$ and $M_{\bullet}/M_{\ast}$. In all cases we assume main sequence stars (with $R_{\ast}\propto M_{\ast}^{0.8}$ at $M_{\ast}<M_{\odot}$ and $R_{\ast}\propto M_{\ast}^{0.57}$ otherwise). We also assume $\alpha=0.1,h=1,\beta=1,\eta=1,M_{\rm ej}=10^{0.5}M_{\odot},P_{\rm f}=1$ day (see text for details). Left: Viscous accretion timescale ($t_{\rm acc}$) as compared with the Keplerian timescale of the most bound material ($t_{\rm min}$). Typical values span $10^4<t_{\rm acc}<10^5$ s. The condition $t_{\rm acc}>t_{\rm min}$ is satisfied in the entire region plotted. Right: Probability that the SN kick results in the final orbit having a peri-center $r_{\rm p}<r_{\rm t}$. We assume 'momentum kicks' with a scaling relative to well-studied NS formation kicks, $v_{\rm kick}\approx v_{\rm kick,NS}M_{\rm NS}/M_{\bullet}$ where the NS kick distribution is taken as log-normal with median of $450\hbox{km s}^{-1}$ and $\sigma=0.5$Hobbs2005.