Causality for first-order phase transition and its implication on the maximum mass of neutron stars

TL;DR

This paper investigates how causality constraints affect the maximum mass of neutron stars with first-order phase transitions, revealing significant differences from smooth equations of state and implications for neutron star observations.

Contribution

It introduces a causality condition specific to first-order phase transitions in neutron star matter, highlighting deviations in mass-radius relations from smooth equations of state.

Findings

Causality line deviates for first-order phase transition equations of state.

A narrow mass-radius band is identified for stars with smooth equations of state.

Implications for ruling out certain phase transition densities based on pulsar observations.

Abstract

Causality is an essential factor in determining the maximum mass of a neutron star. Previous works study causality for smooth equations of state. The density at the core of neutron stars can be a few times nuclear saturation density, where the occurrence of first-order phase transition has not been ruled out. The causality condition for first-order phase transition is characteristically different from that of smooth EOS, which becomes evident in the mass-radius relation. In this letter, we find that equations of state having first-order phase transition, the causality line deviates considerably from the smooth equation of state. Depending on the onset density of phase transition, there is a narrow band in the mass-radius plot, which is available only to the stars having a smooth equation of state. This can have significant consequences in the sense that if some pulsars are to lie in this band, one can rule out the equation of state having first-order phase transition occurring at that particular onset density.