The cores of neutron stars contain matter at densities inaccessible in terrestrial laboratories. Hence observing neutron stars is an important way to improve our understanding of such matter. Thanks to its large collecting area and ~ 50 ns timing resolution, NASA's Neutron Star Interior Composition Explorer (NICER) has obtained high-precision phase- and energy-resolved pulse profiles of a handful of non-accreting millisecond X-ray pulsars. These profiles are affected by the pulsar's radius and compactness, which allows them to be used to constrain the mass and radius of the star and consequently the equation of state of the cold dense matter in its interior. In this talk I will discuss recent NICER measurements of neutron star masses and radii, particularly our updated analysis of the high-mass pulsar PSR J0740+6620. Using the NICER data accumulated through 21 April 2022, we find that the equatorial circumferential radius of this pulsar is 12.92+2.09−1.13 km (68% credibility). The fractional uncertainty of this estimate is about 20% smaller than the uncertainty of our previous estimate, in line with statistical expectations given the additional data we were able to use.
