Gravitational shocks as a key ingredient of Gamma-Ray Bursts

We identify a novel physical mechanism that may be responsible for energy release in $\gamma$-ray bursts. Radial perturbations in the neutron core, induced by its collision with collapsing outer layers during the early stages of supernova explosions, can trigger a gravitational shock, which can readily eject a small but significant fraction of the collapsing material at ultra-relativistic speeds. The development of such shocks is a strong-field effect arising in near-critical collapse in General Relativity and has been observed in numerical simulations in various contexts, including in particular radially perturbed neutron star collapse, albeit for a tiny range of initial conditions. Therefore, this effect can be easily missed in numerical simulations if the relevant parameter space is not exhaustively investigated. In the proposed picture, the observed rarity of $\gamma$-ray bursts would be explained if the relevant conditions for this mechanism appear in only about one in every $10^4-10^5$ core collapse supernovae. We also mention the possibility that near-critical collapse could play a role in powering the central engines of Active Galactic Nuclei.

💡 Research Summary

Gamma‑ray bursts (GRBs) are among the most energetic transients in the universe, releasing 10⁵¹–10⁵³ erg of gamma‑ray photons within seconds. Traditional fireball models invoke a compact central engine that first creates an ultra‑relativistic outflow (Lorentz factor γ ≫ 1) and then converts its kinetic energy into radiation via internal or external shocks. However, the physical mechanism that can accelerate a sizable fraction of baryonic mass to such high Lorentz factors remains elusive; neutrino‑antineutrino annihilation appears far too inefficient, and magnetically‑driven models require extreme field strengths and rotation rates.

The paper proposes a fundamentally different engine: a “gravitational shock” generated by near‑critical type II gravitational collapse. In the language of critical phenomena in general relativity, one considers a one‑parameter family of initial data p. For p < p* the matter disperses, for p > p* a black hole forms, and exactly at p = p* the solution approaches a self‑similar, zero‑mass black‑hole critical solution with a universal scaling exponent γ. Near, but not exactly at, the threshold (p ≈ p*), the system exhibits a dual behavior: a black hole of mass M ∝ (p‑p*)^γ forms while a finite fraction of the original mass is expelled to infinity with arbitrarily large velocities. The expelled material’s Lorentz factor grows as the deviation from criticality shrinks, following a power‑law governed by the same exponent γ.

In a core‑collapse supernova, the outer stellar layers fall inward at speeds of order 0.1–0.3 c and collide with the dense neutron‑star core. This impact can provide precisely the radial perturbation needed to push the core’s collapse into the near‑critical regime. Numerical studies cited (e.g., Novak, Choptuik, and collaborators) show that for realistic equations of state (k = p/ρ ≈ 0.5–1) a modest inward velocity of ~0.1 c is sufficient to trigger type II critical behavior. When the system is slightly super‑critical, up to ~30 % of the neutron‑star mass can be ejected with Lorentz factors γ ≈ 10–100; when sub‑critical, most of the mass still escapes but with a broader, lower‑γ distribution. The shock surface where the gravitational shock forms is thin, and the acceleration is essentially gravitational, not hydrodynamic, thereby bypassing the efficiency problems of neutrino‑driven mechanisms.

The authors argue that this mechanism naturally explains two key observational facts. First, the required ultra‑relativistic bulk motion is produced without invoking exotic magnetic fields or fine‑tuned neutrino physics, matching the internal‑shock requirement for non‑thermal spectra. Second, the near‑critical initial conditions occupy only a tiny fraction of the full parameter space—estimated at 10⁻⁴–10⁻⁵ of all core‑collapse events—providing a straightforward statistical explanation for the rarity of observable GRBs (≈1 per million years per galaxy). The paper also speculates that similar near‑critical collapses could operate in active galactic nuclei, where continuous mass inflow and larger scales might produce recurrent gravitational shocks, potentially contributing to jet launching and high‑energy emission. However, the authors acknowledge that extending the model to AGN will require three‑dimensional simulations including angular momentum, magnetic fields, and radiative processes.

In summary, the work introduces a novel, gravity‑driven engine for GRBs based on near‑critical type II collapse. By showing that modest radial perturbations in a collapsing neutron core can generate a gravitational shock that ejects a substantial mass fraction at ultra‑relativistic speeds, the authors provide a plausible solution to the long‑standing energy‑conversion problem in GRB theory. The proposal is testable: future high‑resolution numerical relativity studies can map the critical surface more completely, and gravitational‑wave detectors might eventually capture the characteristic burst associated with such a shock. If confirmed, gravitational shocks could become a cornerstone of our understanding of the most violent explosions in the cosmos.