Constraining the Neutron Star Interior Equation of State with Radio and X-ray Pulsar Observations

Electromagnetic observations of pulsars play a key role in determining the equation of state (EoS) of supranuclear-density matter in neutron star interiors. High-precision radio timing of massive millisecond pulsars—especially those with well-measured post-Keplerian effects such as the relativistic Shapiro delay—provides straightforward constraints on the EoS, as valid models must explain the existence of neutron stars with masses greater than ~2 solar masses. X-ray observations obtained with instruments such as the Neutron Star Interior Composition Explorer (NICER) telescope provide similarly strong, but conceptually different, constraints on the compactness (M/R) of neutron stars through light curve modeling. This talk will touch on recent radius and mass measurements obtained with X-ray and radio observations, including improved measurements of some of the most massive neutron stars known. The synergistic method of using radio millisecond pulsar mass constraints to inform X-ray light curve modeling, as well as efforts to measure the masses of more poorly determined pulsar systems and quantify the potential covariances in these measurements, will also be discussed.