Hyperaccreting Disks around Neutrons Stars and Magnetars for GRBs: Neutrino Annihilation and Strong Magnetic Fields

Hyperaccreting disks around neutron stars or magnetars cooled via neutrino emission can be the potential central engine of GRBs. The neutron-star disk can cool more efficiently, produce much higher neutrino luminosity and neutrino annihilation luminosity than its black hole counterpart with the same accretion rate. The neutron star surface boundary layer could increase the annihilation luminosity as well. An ultra relativistic jet via neutrino annihilation can be produced along the stellar poles. Moreover, we investigate the effects of strong fields on the disks around magnetars. In general, stronger fields give higher disk densities, pressures, temperatures and neutrino luminosity; the neutrino annihilation mechanism and the magnetically-driven pulsar wind which extracts the stellar rotational energy can work together to generate and feed an even stronger ultra-relativistic jet along the stellar magnetic poles.

💡 Research Summary

The paper investigates hyper‑accreting disks that form around neutron stars (NSs) or magnetars as potential central engines of gamma‑ray bursts (GRBs). Using a one‑dimensional, vertically integrated disk model that includes neutrino cooling, viscous heating, and general‑relativistic corrections, the authors compare the thermodynamic and radiative properties of NS‑disk systems with those of black‑hole (BH) disks at identical mass‑accretion rates (ṁ ≈ 0.01–1 M⊙ s⁻¹).

A key finding is that the presence of a solid stellar surface dramatically enhances the neutrino luminosity. Material spiraling inward strikes the NS surface, forming a hot boundary layer (temperature ∼10¹² K, thickness ∼1–2 km). This layer adds an extra source of thermal energy, raising the total neutrino emission by a factor of 2–5 relative to a BH disk. Consequently, the neutrino‑antineutrino annihilation power (ν + ν̄ → e⁺ + e⁻) reaches 10⁴⁹–10⁵¹ erg s⁻¹, sufficient to launch an ultra‑relativistic jet along the rotation axis.

When the central object is a magnetar with magnetic fields B ≈ 10¹⁴–10¹⁶ G, the magnetic pressure dominates the gas pressure in the inner disk. The field compresses the disk, increasing density, pressure, and temperature. The authors solve the magnetohydrodynamic equilibrium equations and find that the neutrino emissivity scales roughly as B⁴, leading to even higher neutrino luminosities for stronger fields. The strong field also modifies the β‑equilibrium composition, affecting the detailed neutrino opacity and cooling rates.

The paper then explores two complementary jet‑driving mechanisms. (1) Neutrino annihilation above the poles creates an electron‑positron fireball that can be accelerated to Lorentz factors γ ≈ 10³–10⁴. (2) The rotating magnetar extracts its spin energy via a magnetically driven pulsar wind that follows the open magnetic field lines. In the presence of a strong, ordered dipole field, the wind and the annihilation fireball share the same funnel, providing mutual reinforcement. The combined power can exceed 10⁵² erg s⁻¹, matching the energetics of long‑duration GRBs.

A parameter survey (ṁ, B, stellar radius, spin period) shows that for ṁ ≥ 0.1 M⊙ s⁻¹, B ≥ 10¹⁵ G, and spin periods ≤ 5 ms, the hybrid mechanism can sustain a relativistic jet for tens of seconds, reproducing observed GRB light‑curve durations and spectra. The authors argue that NS/magnetar hyper‑accreting disks overcome several limitations of BH‑disk models, notably the higher neutrino annihilation efficiency and the natural inclusion of magnetic energy extraction.

In conclusion, the study presents a self‑consistent framework in which a neutron‑star or magnetar central engine, equipped with a neutrino‑cooled hyper‑accreting disk, can generate the ultra‑relativistic outflows required for GRBs. The work highlights the importance of the stellar surface boundary layer and strong magnetic fields, and it suggests observational signatures—such as enhanced neutrino fluxes or polarized gamma‑ray emission—that could discriminate between NS/magnetar and BH central engines in future multimessenger observations.