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

Brendan Reed, C. J. Horowitz

Published 2019-10-12Version 1

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 suggest that the pressure rises rapidly with density and thus the speed of sound in dense matter is a large fraction of the speed of light. We use these observations and simple model equations of state to set a lower bound on the maximum speed of sound in neutron stars. Implications of this bound for our understanding of both hadronic and quark gluon descriptions of dense matter are discussed.