Mapping the QCD Phase Transition with Accreting Compact Stars

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 Ω−M plane, the phase border between neutron stars and hybrid stars with a quark matter core. Along this line a drop in the pulsar’s moment of inertia entails a waiting point phenomenon in the accreting millisecond pulsar (AMXP) evolution: most of these objects should therefore be found along the phase border in the Ω−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 AMXPs with sufficiently accurate determination of their masses, spin frequencies and magnetic fields.