Large sound speed in dense matter and the deformability of neutron stars
Brendan T. Reed, C. J. Horowitz
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 $\mathrm{J}0740+6620$ 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 $1.4\phantom{\rule{0.16em}{0ex}}{M}_{\ensuremath{\bigodot}}$ neutron star is less than 600, as is suggested by subsequent analyses of GW170817, then 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.