Constraining neutron star matter from the slope of the mass-radius curves

We analyze the implications of information about local derivatives from the mass-radius diagram in neutron star matter. It is expected that the next generation of gravitational wave and electromagnetic detectors will allow the determination of the neutron star radius and mass with a small uncertainty. Observations of neutron stars clustered around a given neutron star mass allow the estimation of local derivatives in the $M(R)$ diagram, which can be used to constrain neutron star properties. From a model-independent description of the neutron star equation of state, it is shown that a $M(R)$ curve with a negative slope at $1.4{M}_{\ensuremath{\bigodot}}$ predicts a $2{M}_{\ensuremath{\bigodot}}$ neutron star radius below 12 km. Furthermore, a maximum mass below $2.3{M}_{\ensuremath{\bigodot}}$ is obtained if the $M(R)$ slope is negative in the whole range of masses above $1{M}_{\ensuremath{\bigodot}}$, and a maximum mass above $2.4{M}_{\ensuremath{\bigodot}}$ requires the $M(R)$ slope to be positive in some range of masses. Constraints on the mass-radius curve of neutron stars will place strong constraints on microscopic models.