Relativistic Shapiro delay measurements of an extremely massive millisecond pulsar.

Despite its importance to our understanding of physics at supranuclear densities, the equation of state (EoS) of matter deep within neutron starsremains poorly understood. Millisecond pulsars(MSPs) are among the most useful astrophysical objects in the Universe for testing fundamental physics, and place some of the most stringent constraints on this high-density EoS. Pulsartiming - the process of accounting for every rotation of a pulsarover long time periods - can precisely measure a wide variety of physical phenomena, including those that allow the measurement of the masses of the components of a pulsarbinary system (Lorimer & Kramer 2005). One of these, called relativistic Shapiro delay(Shapiro 1964), can yield precise masses for both an MSP and its companion; however, it is only easily observed in a small subset of high-precision, highly inclined (nearly edge-on) binary pulsarsystems. By combining data from the North American Nanohertz Observatory for Gravitational Waves (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 $\mathbf{2.14^{+0.10}_{-0.09}}$ solar masses(68.3% credibility interval; 95.4% credibility intervalis $\mathbf{2.14^{+0.20}_{-0.18}}$ solar masses). It is highly likely to be the most massive neutron staryet observed, and serves as a strong constraint on the neutron starinterior EoS.