On the black hole from merging binary neutron stars: how fast can it spin?

TL;DR

This study investigates the maximum spin achievable by black holes formed from merging binary neutron stars, revealing a spin limit around 0.89 and the distribution of excess angular momentum to the surrounding torus.

Contribution

It introduces a method to determine the maximum black hole spin from neutron star mergers and demonstrates a spin limit consistent with cosmic censorship.

Findings

Maximum black hole spin is approximately 0.89.

Additional angular momentum is transferred to the torus.

Constraint-damping formulation improves simulation accuracy.

Abstract

The merger of two neutron stars will in general lead to the formation of a torus surrounding a black hole whose rotational energy can be tapped to potentially power a short gamma-ray burst. We have studied the merger of equal-mass binaries with spins aligned with the orbital angular momentum to determine the maximum spin the black hole can reach. Our initial data consists of irrotational binaries to which we add various amounts of rotation to increase the total angular momentum. Although the initial data violates the constraint equations, the use of the constraint-damping CCZ4 formulation yields evolutions with violations smaller than those with irrotational initial data and standard formulations. Interestingly, we find that a limit of $J/M^2 \simeq 0.89$ exists for the dimensionless spin and that any additional angular momentum given to the binary ends up in the torus rather than in the black hole, thus providing another nontrivial example supporting the cosmic censorship hypothesis.