The 3.8 keV feature implies a compactness for 4U 1820-30 that overlaps only marginally with NICER EoS contours if maximum mass is capped at 2.3 solar masses, creating tension with touchdown-flux estimates.
The Mass and Radius of the Neutron Star in EXO 1745-248
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abstract
Bursting X-ray binaries in globular clusters are ideal sources for measuring neutron star masses and radii, and hence, for determining the equation of state of cold, ultradense matter. We use time-resolved spectroscopic data from EXO 1745-248 during thermonuclear bursts that show strong evidence for photospheric radius expansion to measure the Eddington flux and the apparent surface area of the neutron star. We combine this with the recent measurement of the distance to the globular cluster Terzan 5, where this source resides, to measure the neutron star mass and radius. We find tightly constrained pairs of values for the mass and radius, which are centered around M=1.4 M_sun and R=11 km or around M=1.7 M_sun and R=9 km. These values favor nucleonic equations of state with symmetry energy that is relatively low and has a weak dependence on density.
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The mass of the neutron star in 4U 1820-30 revisited
The 3.8 keV feature implies a compactness for 4U 1820-30 that overlaps only marginally with NICER EoS contours if maximum mass is capped at 2.3 solar masses, creating tension with touchdown-flux estimates.