The Discovery of a New Instability in a Hyperaccretion Flow and its Implication for Gamma-ray Bursts

TL;DR

This paper identifies a new viscous instability in hyperaccretion flows around black holes, which can cause clumpy structures and may explain the rapid variability observed in gamma-ray burst emissions.

Contribution

It reveals a previously unrecognized viscous instability in hyperaccretion flows, with implications for the structure and variability of gamma-ray bursts.

Findings

Discovered a negative slope in the thermal equilibrium curve indicating viscous instability.

Confirmed the flow remains thermally stable despite the instability.

Estimated clump formation timescale to be less than 0.1 seconds.

Abstract

A hyperaccretion flow around a stellar mass black hole is thought to be the most plausible engine that powers gamma-ray bursts (GRBs). The flow efficiently cools via neutrino emission at >~ 0.003-0.01 M_sun s^{-1} (corresponding to a luminosity of ~10^50 erg s^{-1}), while neither neutrino nor photon emission is efficient below this rate, so the flow should be advection dominated. We carefully solve how a transition occurs from the advection-dominated to the neutrino-dominated branches, and find that the slope of the thermal equilibrium curve is negative in the surface density - accretion rate (Sigma-Mdot) plane, a condition for viscous instability, at radii smaller than ~12R_g (with R_g being the gravitational radius). We also confirm that the flow is thermally stable. The consequence of this instability is the formation of a clumpy structure in the flow. This is because the larger (respectively smaller) surface density is, the smaller (respectively larger) the mass accretion rate from the region in question becomes, leading to growth of the density contrast. The timescale for clump formation is estimated to be shorter than 0.1s. The observational implication is discussed in the context of GRBs. We suggest that this might explain the origin of the large variability observed in the prompt emission of GRBs.