Revisiting the maximum mass of differentially rotating neutron stars in general relativity: Übermassive stars with realistic equations of state

Abstract: We study the solution space of general relativistic, axisymmetric, equilibria of differentially rotating neutron stars with realistic, nuclear equations of state. We find that different types of stars, which were identified by earlier works for polytropic equations of state, arise for realistic equations of state, too. Scanning the solution space for the sample of realistic equations of state we treat, we find lower limits on the maximum rest masses supported by cold, differentially rotating stars for each type of stars. We often discover equilibrium configurations that can support more than 2 times the mass of a static star. We call these equilibria "\"ubermassive", and in our survey we find \"ubermassive stars that can support up to 2.5 times the maximum rest mass that can be supported by a cold, non-rotating star with the same equation of state. This is nearly two times larger than what previous studies employing realistic equations of state had found, and which did not uncover \"ubermassive neutron stars. Moreover, we find that the increase in the maximum rest mass with respect to the non-spinning stellar counterpart is larger for moderately stiff equations of state. These results may have implications for the lifetime and the gravitational wave and electromagnetic counterparts of hypermassive neutron stars formed following binary neutron star mergers.