A larger-than-2-solar-mass pulsar in NANOGrav: leveraging Shapiro delay to constrain the neutron star equation of state

Despite its importance to our understanding of physics at supranuclear densities, the neutron star equation of state (EoS) remains poorly understood. Millisecond pulsar (MSP) timing — a technique which involves very accurately predicting the arrival time of every pulse from an MSP — continues to place some of the most stringent constraints on the neutron star interior EoS. In highly inclined MSP binary systems, the gravitational potential of the pulsar’s companion induces a discrepancy between expected and measured pulse times of arrival. Measurement of this general relativistic phenomenon, called Shapiro Delay, can yield the mass of both the MSP and its companion. By combining data from the NANOGrav 12.5-year data set with recent orbital-phase-specific observations using the Green Bank Telescope, we have measured the mass of the MSP J0740+6620 to be 2.10 ± 0.11 solar masses. It may therefore be the most massive neutron star ever observed, and would serve as a strong constraint on the neutron star EoS.