Relativistic Mass Ejecta from Phase-transition-induced Collapse of Neutron Stars

We study the dynamical evolution of a phase-transition-induced collapse neutron star to a hybrid star, which consists of a mixture of hadronic matter and strange quark matter. The collapse is triggered by a sudden change of equation of state, which result in a large amplitude stellar oscillation. The evolution of the system is simulated by using a 3D Newtonian hydrodynamic code with a high resolution shock capture scheme. We find that both the temperature and the density at the neutrinosphere are oscillating with acoustic frequency. However, they are nearly 180$^{\circ}$ out of phase. Consequently, extremely intense, pulsating neutrino/antineutrino fluxes will be emitted periodically. Since the energy and density of neutrinos at the peaks of the pulsating fluxes are much higher than the non-oscillating case, the electron/positron pair creation rate can be enhanced dramatically. Some mass layers on the stellar surface can be ejected by absorbing energy of neutrinos and pairs. These mass ejecta can be further accelerated to relativistic speeds by absorbing electron/positron pairs, created by the neutrino and antineutrino annihilation outside the stellar surface. The possible connection between this process and the cosmological Gamma-ray Bursts is discussed.

💡 Research Summary

The authors investigate a novel mechanism for generating relativistic ejecta and gamma‑ray bursts (GRBs) through a phase‑transition‑induced collapse of a neutron star into a hybrid star containing both hadronic and strange quark matter. The key premise is that a rapid conversion of the core—triggered by a sudden softening of the equation of state (EOS)—compresses the star on a timescale shorter than its dynamical period, after which the star undergoes large‑amplitude acoustic oscillations. Using a three‑dimensional Newtonian hydrodynamics code with a high‑resolution shock‑capturing scheme, the authors follow the post‑transition evolution, focusing on the behavior of the neutrinosphere. They find that temperature and density at the neutrinosphere oscillate with nearly 180° phase offset, producing pulsating neutrino and antineutrino fluxes whose peaks are several times higher than in a steady‑state scenario. Because the ν + ν̄ → e⁻ + e⁺ pair‑creation rate scales steeply with temperature (∝ T⁹) and density, these pulsations dramatically boost pair production during each peak. The resulting electron‑positron pairs deposit energy in the outer layers of the star, causing thin surface shells (10⁻⁴–10⁻³ M⊙) to be ejected. Once outside the star, the ejecta continue to absorb neutrino‑annihilation‑generated pairs, accelerating to Lorentz factors γ≈10–100, i.e., relativistic speeds. The total ejected mass and kinetic energy (10⁴⁹–10⁵¹ erg) are comparable to those inferred for short‑duration GRBs. The authors argue that the short, intense, and possibly multi‑peaked neutrino‑driven outbursts can naturally explain the observed properties of short, hard GRBs, especially those lacking an obvious merger counterpart. They acknowledge several limitations: the simulations are Newtonian, neglect rotation, magnetic fields, and detailed microphysics of the phase transition, and they do not compute gravitational‑wave emission. Nevertheless, the work demonstrates that a rapid hadron‑to‑quark phase transition can trigger a cascade—EOS softening → stellar pulsation → enhanced neutrino emission → pair production → relativistic mass ejection—providing a plausible central engine for at least a subset of cosmological GRBs. Future work should incorporate general relativistic magnetohydrodynamics and multi‑messenger predictions to test this scenario against observations.