  Despite its importance to our understanding of physics at supranuclear
densities, the equation of state (EoS) of matter deep within neutron stars
remains 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. Pulsar
timing - the process of accounting for every rotation of a pulsar over 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
pulsar binary 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 pulsar systems. 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 interval is
$\mathbf{2.14^{+0.20}_{-0.18}}$ solar masses). It is highly likely to be the
most massive neutron star yet observed, and serves as a strong constraint on
the neutron star interior EoS.
