Critical Phenomena in Neutron Stars II: Head-on Collisions

TL;DR

This paper investigates the critical phenomena during head-on collisions of neutron stars, demonstrating that near the threshold of black-hole formation, the system exhibits type-I critical behavior with a metastable star acting as a transitional state.

Contribution

It provides evidence that the critical solution is a perturbed equilibrium star on the unstable branch, extending understanding of critical phenomena to perfect fluids in neutron star collisions.

Findings

Confirmation of type-I critical phenomena in neutron star collisions

Identification of the metastable star as a perturbed equilibrium solution

Observation of fine structure in the critical exponent for perfect fluids

Abstract

We consider the head-on collision of equal-mass neutron stars boosted towards each other and we study the behavior of such systems near the threshold of black-hole formation. In particular, we confirm the previous findings by [1] that a type-I critical phenomenon can be observed by fine-tuning the initial mass of the two neutron stars. At the same time, we argue against the interpretation that the critical solution is not a perturbed spherical star and show instead that the metastable star corresponds to a (perturbed) equilibrium solution on the unstable branch of the equilibrium configurations. As a result, the head-on collision of two neutron stars near the critical threshold can be seen as a transition in the space of configurations from an initial stable solution over to a critical metastable one which can either migrate to a stable solution or collapse to a black hole. The critical exponent for this process shows a fine structure which was already observed in the case of the critical collapse of scalar fields but never before for perfect fluids.