From Neutron Star Binaries to Gamma-ray bursts

TL;DR

This paper explores how neutron star binary mergers can produce short gamma-ray bursts through neutrino annihilation and magnetic field amplification, explaining observed energies and durations.

Contribution

It presents a detailed analysis of two mechanisms—neutrino annihilation and magnetic field amplification—that can generate short GRBs from neutron star mergers, emphasizing the plausibility of the central object resisting collapse.

Findings

Neutrino annihilation drives relativistic outflows with high Lorentz factors.

Outflows are collimated into narrow angles, achieving energies near 10^{51} ergs.

Magnetic field amplification can spin down the central object within 0.2 seconds.

Abstract

I summarize recent results about how a neutron star binary coalescence can produce short gamma-ray bursts (GRBs). Two possibilities are discussed: the annihilation of neutrino anti-neutrino pairs above the merged remnant and the exponential amplification of magnetic fields in the central object up to values close to equipartition. We find that the neutrino annihilation drives bipolar, relativistic outflows with Lorentz-factors large enough to circumvent the GRB 'compactness problem'. The total energy within these outflows is moderate by GRB-standards ($\sim 10^{48}-10^{49}$ ergs), but the interaction with the baryonic material blown-off by the neutrinos collimates the outflows into opening angles of typically 0.1 sterad, yielding isotropic energies close to $10^{51}$ ergs. We further want to stress the plausibility of the central object resisting the immediate collapse to a black hole. In this case the central object will --similar to a proto-neutron star-- be subject to neutrino driven convection that --together with the rapid, differential rotation-- will lead to a drastic amplification of pre-existing magnetic fields. Within fractions of a second, field strengths comparable to equipartition field strength ($> 10^{17}$ G) will be reached. These will produce large torques that will spin-down the object within about 0.2 s, and would thus naturally explain the duration of short GRBs.