Instabilities in the Gamma Ray Burst central engine. What makes the jet variable?

TL;DR

This paper investigates the internal instabilities in the accretion flow of gamma ray burst central engines, revealing how high accretion rates and black hole spin influence jet variability through neutrino-cooled disk instabilities.

Contribution

It provides a detailed numerical analysis of the microphysics and stability of neutrino-cooled tori around black holes, highlighting the role of accretion rate, spin, and magnetic coupling in jet variability.

Findings

Inner disk regions become opaque at high accretion rates.

Viscous and thermal instabilities develop due to pressure changes.

Black hole spin enhances and lowers the threshold for instabilities.

Abstract

Both types of long and short gamma ray bursts involve a stage of a hyper-Eddington accretion of hot and dense plasma torus onto a newly born black hole. The prompt gamma ray emission originates in jets at some distance from this 'central engine' and in most events is rapidly variable, having a form of spikes and subpulses. This indicates at the variable nature of the engine itself, for which a plausible mechanism is an internal instability in the accreting flow. We solve numerically the structure and evolution of the neutrino-cooled torus. We take into account the detailed treatment of the microphysics in the nuclear equation of state that includes the neutrino trapping effect. The models are calculated for both Schwarzschild and Kerr black holes. We find that for sufficiently large accretion rates (> 10 Msun/s for non-rotating black hole, and >1 Msun/s for rotating black hole, depending on its spin), the inner regions of the disk become opaque, while the helium nuclei are being photodissociated. The sudden change of pressure in this region leads to the development of a viscous and thermal instability, and the neutrino pressure acts similarly to the radiation pressure in sub-Eddington disks. In the case of rapidly rotating black holes, the instability is enhanced and appears for much lower accretion rates. We also find the important and possibly further destabilizing role of the energy transfer from the rotating black hole to the torus via the magnetic coupling.