Mapping the QCD Phase Transition with Accreting Compact Stars

TL;DR

This paper proposes that accreting millisecond pulsars can reveal the QCD phase transition in dense nuclear matter by analyzing their spin and mass evolution, highlighting a characteristic phase border in their parameter space.

Contribution

It introduces a novel phase diagram approach linking pulsar spin and mass to the QCD phase transition, suggesting observable signatures in accreting pulsar populations.

Findings

A phase border line in the spin-mass plane indicates the quark matter transition.

Most accreting millisecond pulsars are expected to be found along this phase border.

Population statistics are crucial for confirming the high-density phase transition evidence.

Abstract

We discuss an idea for how accreting millisecond pulsars could contribute to the understanding of the QCD phase transition in the high-density nuclear matter equation of state (EoS). It is based on two ingredients, the first one being a ``phase diagram'' of rapidly rotating compact star configurations in the plane of spin frequency and mass, determined with state-of-the-art hybrid equations of state, allowing for a transition to color superconducting quark matter. The second is the study of spin-up and accretion evolution in this phase diagram. We show that the quark matter phase transition leads to a characteristic line in the Omega-M plane, the phase border between neutron stars and hybrid stars with a quark matter core. Along this line a change in the pulsar's moment of inertia entails a waiting point phenomenon in the accreting millisecond X-ray pulsar (AMXP) evolution: most of these objects should therefore be found along the phase border in the Omega-M plane, which may be viewed as the AMXP analog of the main sequence in the Hertzsprung-Russell diagram for normal stars. In order to prove the existence of a high-density phase transition in the cores of compact stars we need population statistics for AMXP's with sufficiently accurate determination of their masses and spin frequencies.