[Paper Review] Damping of GRR instability by direct URCA reactions

J. L. Zdunik|arXiv (Cornell University)|Nov 28, 1995

Gamma-ray bursts and supernovae3 citations

TL;DR

This paper investigates how direct URCA processes suppress the gravitational radiation driven (GRR) instability in neutron stars by enhancing bulk viscosity. Using analytical methods based on recent bulk viscosity calculations, it shows that direct URCA reactions significantly reduce the parameter space—particularly at higher temperatures—where the GRR instability can occur, effectively damping the instability over a broader range of neutron star temperatures and rotation frequencies.

ABSTRACT

The role of direct URCA reactions in damping of the gravitational radiation driven instability is discussed. The temperature at which bulk viscosity suppresses completely this instability is calculated. The results are obtained analytically using recent calculations performed in the case of bulk viscosity due to the modified URCA processes (Lindblom 1995; Yoshida & Eriguchi 1995). The bulk viscosity caused by direct URCA reactions is found to reduce significantly the region of temperatures and rotation frequencies where a neutron star is subject to GRR instability.

Motivation & Objective

• To understand the role of direct URCA processes in suppressing the gravitational radiation driven (GRR) instability in neutron stars.
• To determine the critical temperature at which bulk viscosity from direct URCA reactions fully suppresses the GRR instability.
• To extend previous results on modified URCA bulk viscosity to the case of direct URCA processes.
• To quantify the reduction in the region of temperatures and rotation frequencies where the GRR instability is active due to direct URCA effects.
• To provide analytical estimates of the damping effect of direct URCA on the GRR instability in neutron star cores.

Proposed method

• Analytical derivation of bulk viscosity contributions from direct URCA processes in neutron star matter.
• Adaptation of recent results on modified URCA bulk viscosity (Lindblom 1995; Yoshida & Eriguchi 1995) to the direct URCA case.
• Use of equations of state and particle reaction rates to model the viscosity enhancement in dense neutron star matter.
• Comparison of the growth timescale of the GRR instability with the damping timescale due to bulk viscosity.
• Calculation of the critical temperature above which direct URCA bulk viscosity fully suppresses the instability.
• Assumption of equilibrium conditions and weak interaction rates appropriate for neutron star core conditions.

Experimental results

Research questions

• RQ1 At what temperature does bulk viscosity from direct URCA reactions fully suppress the gravitational radiation driven instability in neutron stars?
• RQ2 How does the inclusion of direct URCA processes alter the region of instability in the temperature-rotation frequency plane?
• RQ3 What is the relative effectiveness of direct URCA versus modified URCA in damping the GRR instability?
• RQ4 How does the critical temperature for instability suppression depend on the equation of state and particle composition?
• RQ5 In what range of neutron star parameters is the GRR instability rendered irrelevant due to direct URCA damping?

Key findings

• Direct URCA processes significantly enhance bulk viscosity in neutron star cores, leading to strong damping of the gravitational radiation driven (GRR) instability.
• The critical temperature above which the GRR instability is fully suppressed due to direct URCA bulk viscosity is found to be substantially higher than in the absence of direct URCA processes.
• The region of neutron star parameters—characterized by temperature and rotation frequency—where the GRR instability can grow is markedly reduced when direct URCA reactions are active.
• The analytical results indicate that direct URCA reactions can effectively quench the instability over a broad range of realistic neutron star conditions.
• The findings suggest that neutron stars with cores capable of supporting direct URCA processes are less likely to exhibit persistent GRR-driven gravitational wave emission.
• The study confirms that direct URCA reactions play a crucial role in stabilizing neutron stars against GRR instability, especially at higher temperatures.