The Mass and Radius of the Neutron Star in 4u 1820–30

We report on the measurement of the mass and radius of the neutron star in the low-mass X-ray binary 4U 1820−30. The analysis of the spectroscopic data on multiple thermonuclear bursts yields well-constrained values for the apparent emitting area and the Eddington flux, both of which depend in a distinct way on the mass and radius of the neutron star. The distance to the source is that of the globular cluster NGC 6624, where the source resides. Combining these measurements, we uniquely determine the probability density over the stellar mass and radius. We find the mass to be M = 1.58 ± 0.06 M ☉ and the radius to be R = 9.1 ± 0.4 km.