Large Sound Speed in Dense Matter and the Deformability of Neutron Stars

TL;DR

This paper suggests that the speed of sound in neutron star cores is likely very high, possibly exceeding the conformal limit, based on gravitational wave and radio observations indicating small deformability and large mass.

Contribution

It provides a lower bound on the maximum sound speed in neutron stars using observational data and simple constant sound-speed models.

Findings

Sound speed likely exceeds the conformal limit in neutron star cores.

Observations imply rapid pressure increase with density.

Neutron star matter may be more exotic than previously thought.

Abstract

The historic first detection of the binary neutron star merger GW170817 by the LIGO-Virgo collaboration has set a limit on the gravitational deformability of neutron stars. In contrast, radio observations of PSR J0740+6620 find a very massive neutron star. Tension between the small deformability and the large maximum mass may suggest that the pressure rises rapidly with density and thus the speed of sound in dense matter is likely a large fraction of the speed of light. We use these observations and simple constant sound-speed model equations of state to set a lower bound on the maximum speed of sound in neutron stars. If the tidal deformability of a 1.4$M_\odot$ neutron star is less than 600, as is suggested by subsequent analyses of GW170817, we find that the sound speed in the cores of neutron stars is likely larger than the conformal limit of $c/\sqrt{3}$. Implications of this for our understanding of both hadronic and quark-gluon descriptions of dense matter are discussed.