Fastest spinning millisecond pulsars: indicators for quark matter in neutron stars?

Abstract

We study rotating hybrid stars, with a particular emphasis on the effect of a deconfinement phase transition on their properties at high spin. Our analysis is based on a hybrid equation of state with a phase transition from hypernuclear matter to color-superconducting quark matter, where both phases are described within a relativistic density functional approach. By varying the vector meson and diquark couplings in the quark matter phase, we obtain different hybrid star sequences with varying extension of the quark matter core, ensuring consistency with astrophysical constraints from mass, radius, and tidal deformability measurements. As a result, we demonstrate the impact of an increasing rotational frequency on the maximum gravitational mass, the central energy density of compact stars, the appearance of the quasiradial oscillations, and nonaxisymmetric instabilities. We demonstrate that for the most favorable parameter sets with a strong vector coupling, hybrid star configurations with a color superconducting quark matter core can describe the fastest spinning and heaviest galactic neutron star PSR J0952-0607, while it is out of reach for the purely hadronic hypernuclear star configuration. We also revise the previously proposed empirical relation between the Kepler frequency, gravitational mass, and radius of nonrotating neutron stars, obtained based on the assumption that all neutron stars, up to the heaviest, are hadronic. We show how the phase transition to quark matter alters this relation and, consequently, the constraints on the dense matter equation of state. Our findings reveal that incorporating the hybrid equation of state has significant implications for the constraints on the properties of strongly interacting matter and neutron stars, placing the upper limit on R1.4≤14.90 and R0.7<11.49 km (considering 716 Hz frequency limit from PSR J1748+2446ad) and R1.4≤11.90 km (for 1000 Hz).

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