2025ApJ...993...68M

Recent observations from NICER in X-rays and LIGO/Virgo in gravitational waves have provided critical constraints on the mass, radius, and tidal deformability of neutron stars, imposing stringent limits on the equation of state (EOS) and the behavior of ultradense matter. However, several key parameters influencing the EOS, such as the maximum mass of neutron stars, spin-down rates, and the potential role of exotic matter in their cores, remain the subject of ongoing debate. Here we present a new approach to constraining the EOS by analyzing the X-ray afterglows of some short gamma-ray bursts, focusing on “the internal plateau” phase and its abrupt decay, which reflect the spin-down and possible collapse of a supramassive neutron star into a black hole. By linking critical neutron star masses with black hole formation criteria and the observational data from Swift's Swift's Burst Alert Telescope (BAT) and X-Ray Telescope (XRT)​​​ with compact object models, we explore three representative EOSs that range from “soft” to “stiff.” Our result supports a maximum mass for neutron stars of approximately 2.39 solar masses at the threshold of black hole formation. This conclusion holds under assumptions of magnetar-powered X-ray plateaus, constant radiative efficiency, isotropic emission, and full Kerr black hole energy extraction; deviations could influence the inferred results. Our results demonstrate the critical role of neutron star/black hole physics in probing dense nuclear matter and provide a novel framework for exploring extreme astrophysical environments.