Neutron Star Mass-Radius Constraints for EXO 0748 - 676 from 2008-2025 Quiescent X-ray Spectra

We present new constraints on the mass and radius of the neutron star in the neutron star low-mass X-ray binary EXO 0748 - 676 obtained from a joint analysis of 20 quiescent X-ray observations obtained between 2008 and 2025, including 14 Chandra and 6 XMM-Newton exposures. These data sample two quiescent episodes separated by the 2024 - 2025 outburst. We model the 0.5 - 10 keV spectra with a hydrogen-atmosphere model, assuming a source distance of 7.1 kpc. In a global Markov Chain Monte Carlo analysis in which the hydrogen column density, neutron star mass, and radius are tied across all observations, we obtain a neutron-star mass of 1.77+0.17-0.22,M and a radius of 12.62+0.56-0.74 km (1 sigma credible intervals). We further perform independent fits to the first and second quiescent epochs and find that the combined data set significantly reduces the low-mass tail in the posterior distribution, leading to tighter lower bounds on the neutron-star mass. Incorporating the distance uncertainty of 7.1 plus or minus1.2 kpc, we conservatively constrain the neutron-star mass and radius to M 1.41-2.11 M and R 10.15-15.13 km, favoring relatively stiff dense-matter equations of state. We also trace the thermal evolution across two quiescent epochs and find evidence for renewed crust cooling following the 2024 - 2025 outburst, providing a unique opportunity to compare the thermal relaxation behavior after two distinct accretion episodes.