2021ApJ...910...62Z

The nature of GW190814's secondary component m₂ of mass 2.50-2.67 M_☉ in the mass gap between the currently known maximum mass of neutron stars and the minimum mass of black holes is currently under hot debate. Among the many possibilities proposed in the literature, m₂ was suggested to be a superfast pulsar, while its r-mode stability against runaway gravitational radiation through the Chandrasekhar-Friedman-Schutz mechanism is still unknown. Previously, Fortin et al. constructed a sample of 33 unified equations of state using the same nuclear interactions from the crust to the core consistently; from that sample we use those equations that fulfill all currently known astrophysical and nuclear physics constraints to compare the minimum frequency required for m₂ to rotationally sustain a mass greater than 2.50 M_☉ with the critical frequency above which the r-mode instability occurs. We use two extreme damping models assuming that the crust is either perfectly rigid or elastic. Using the stability of 19 observed low-mass X-ray binaries as an indication that the rigid crust damping of the r-mode dominates within the models studied, we find that m₂ is r-mode-stable while rotating with a frequency higher than 870.2 Hz (0.744 times its Kepler frequency of 1169.6 Hz) as long as its temperature is lower than about 3.9 x 10⁷ K, further supporting the proposal that GW190814's secondary component is a supermassive and superfast pulsar.