Ultra-long gamma-ray bursts (ULGRBs), with prompt durations of order 10310^3 s or more, remain without a settled central-engine identification. Ror et al. (2024) showed that beaming-corrected energetics exclude a newborn-magnetar engine only for the most extreme events (GRB 221009A, GRB 210619B), that three other ULGRB candidates favor magnetars, and that the engine is left poorly constrained for most of the population because beaming angles, plateau detections, and kinetic-energy estimates are incomplete; they also found no observational criterion besides duration T90T_{90} that robustly separates ULGRBs from ordinary long GRBs. Later work has sharpened rather than resolved the question: one ULGRB (GRB 211024B) shows internal-plateau evidence for a magnetar, and the 2025 event GRB 250702B/EP250702a has attracted three mutually incompatible engine models (helium-merger black hole, micro-tidal disruption, fallback-accreting collapsar). The open problem is to determine what powers ULGRBs — hyper-accreting black holes, millisecond magnetars, or tidal-disruption-like engines — and to establish robust, T90T_{90}-independent observational discriminants that classify individual bursts and the population as a whole.

Background

Gamma-ray bursts (GRBs) are flashes of MeV gamma-ray radiation conventionally classified by T90T_{90}, the time interval containing 90% of the prompt photon fluence: bursts shorter than about 2 s are produced by compact-object mergers, while longer bursts (LGRBs) arise from the collapse of massive stars (the collapsar scenario). A rare group with prompt emission lasting of order 10310^3 s or longer — ultra-long GRBs (ULGRBs) — sits at the extreme tail of the long population, and whether it is a physically distinct class has been debated since its recognition. Candidate explanations include collapsars powered by blue supergiant progenitors (whose large radius delays the jet's breakout and extends the engine activity), newborn millisecond magnetars, and tidal disruption events, in which a star or white dwarf is torn apart by a black hole (a micro-tidal disruption when the disruptor is a stellar-mass compact object rather than a supermassive black hole); Ioka, Hotokezaka & Piran (2016) framed this as a three-way discrimination problem. Two engine families dominate the discussion. A hyper-accreting black hole (a stellar-mass black hole fed at far above the Eddington rate by a fallback disc) can extract energy from accretion (neutrino-antineutrino annihilation) and from black-hole spin (the Blandford–Znajek mechanism, in which magnetic fields thread the rotating hole), with essentially no ceiling on the available energy. A newborn magnetar — a rapidly rotating, strongly magnetized neutron star — dissipates its rotational energy Erot=12IΩ2E_{\rm rot}=\frac{1}{2}I\Omega^2 (with II the moment of inertia and Ω\Omega the spin rate), which is bounded by roughly 2×10522\times10^{52} erg for a 1.4 M⊙M_\odot, 10 km, millisecond rotor; this bound underlies the standard energetic test. Because GRB emission is beamed into a jet, the directly measured isotropic-equivalent energy Eγ,isoE_{\gamma,\rm iso} overstates the true budget; it is corrected by the beaming factor fb=1−cos⁡θjf_b=1-\cos\theta_j, where θj\theta_j is the jet half-opening angle usually inferred from an achromatic break in the afterglow light curve. If the beaming-corrected energy exceeds the magnetar ceiling, a magnetar engine is excluded and a black hole is favored; energies below the ceiling, however, do not prove a magnetar, so the test is one-sided. X-ray light curves of the afterglow, monitored by the X-Ray Telescope (XRT) on the Neil Gehrels Swift Observatory satellite, often show an early plateau followed by a decay; an internal plateau terminated by a very steep drop is interpreted as magnetar spin-down ending in collapse to a black hole, while plateaus within the external-shock afterglow carry information on the kinetic energy EK,isoE_{\rm K,iso}, which depends on assumed microphysical shock parameters (the fractions ϵe\epsilon_e and ϵB\epsilon_B of energy in electrons and magnetic fields) and on the circumburst density profile. GRB 221009A, the brightest GRB ever observed (prompt duration exceeding 1000 s), made these questions concrete. Ror, Gupta, Aryan, Pandey, Oates, Castro-Tirado & Kumar (2024) built a nearly complete sample of Swift-detected GRBs with redshifts, graded ULGRB candidates into Bronze, Silver, and Gold subsamples by their deviation in the log-normal T90T_{90} distribution (GRB 221009A falls in Gold), and applied the energetic and plateau tests: beaming-corrected energies exclude a magnetar only for GRB 210619B and GRB 221009A (implied black-hole masses of roughly 3.4 and 9.1 M⊙M_\odot), while GRB 060218, GRB 091024A, and GRB 100316D favor a magnetar; for the remaining bursts the engine stays poorly constrained because jet-break-based beaming angles, plateau detections, and kinetic-energy estimates are missing or assumption-laden. They also cautioned that, apart from T90T_{90} itself, their analysis found no other robust observational criterion separating ULGRBs from LGRBs, and that residual selection biases in the Swift and Fermi gamma-ray observatories (detector saturation, Earth occultation, survey-mode coverage) affect population inferences. Stellar-evolution simulations with MESA (Modules for Experiments in Stellar Astrophysics, a standard open-source one-dimensional stellar-evolution code) in the same work suggest rotating (Ω≥0.2Ωc\Omega\ge0.2\Omega_c) low-metallicity massive stars as viable progenitors, but conditional on the chosen one-dimensional physics.

Problem Statement

Determine the central engines that power ultra-long gamma-ray bursts — bursts with prompt durations of order 10310^3 s or more — and establish an observational basis for assigning individual bursts to engine classes. A complete answer must (i) provide discriminant criteria, independent of the duration T90T_{90} alone and robust against beaming-angle, plateau-detection, kinetic-energy, and selection uncertainties, that distinguish a hyper-accreting black hole (collapsar-type) engine from a newborn millisecond magnetar and from tidal-disruption-like alternatives such as white-dwarf disruption by an intermediate-mass black hole or micro-tidal disruption of an ordinary star by a stellar-mass compact object; and (ii) apply those criteria to classify the observed ULGRB population, and thereby state whether ULGRBs constitute a physically distinct class of explosions or the long-duration tail of the ordinary long-GRB population with mixed engines. Prior per-burst engine assignments — the magnetar exclusion for GRB 221009A and GRB 210619B and the magnetar-favoring results for GRB 060218, GRB 091024A, and GRB 100316D in Ror et al. (2024), the magnetar interpretation of GRB 211024B in Fu et al. (2024), and the competing models proposed for GRB 250702B/EP250702a — are model-dependent prior results that a proposed classification should confront and explain, not fixed labels that a correct answer is required to reproduce.

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