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Abstract

In the past few years, new observations of neutron stars and neutron-star
mergers have provided a wealth of data that allow one to constrain the equation
of state of nuclear matter at densities above nuclear saturation density.
However, most observations were based on neutron stars with masses of about 1.4
solar masses, probing densities up to $\sim$ 3-4 times the nuclear saturation
density. Even higher densities are probed inside massive neutron stars such as
PSR J0740+6620. Very recently, new radio observations provided an update to the
mass estimate for PSR J0740+6620 and X-ray observations by the NICER and XMM
telescopes constrained its radius. Based on these new measurements, we revisit
our previous nuclear-physics multi-messenger astrophysics constraints and
derive updated constraints on the equation of state describing the neutron-star
interior. By combining astrophysical observations of two radio pulsars, two
NICER measurements, the two gravitational-wave detections GW170817 and
GW190425, detailed modeling of the kilonova AT2017gfo, as well as the gamma-ray
burst GRB170817A, we are able to estimate the radius of a typical 1.4-solar
mass neutron star to be $11.94^{+0.76}_{-0.87} \rm{km}$ at 90\% confidence. Our
analysis allows us to revisit the upper bound on the maximum mass of neutron
stars and disfavours the presence of a strong first-order phase transition from
nuclear matter to exotic forms of matter, such as quark matter, inside neutron
stars.