Critical Phenomena in Neutron Stars II: Head-on Collisions

Understand

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.