Superradiant and dynamical spin-down of neutron stars with gravitational wave implications

Abstract

Neutron stars such as pulsars and magnetars lose angular momentum primarily through electromagnetic dipole radiation, gravitational waves, $r$-mode oscillation. However, spin variations, particularly enhanced spin-down rates, indicate additional spin-down mechanisms. We propose superradiant spin-down as a potential faster spin-down process. By modelling the interplay between conventional and superradiant spin-down channels, we evaluate their impact on neutron star rotational evolution. We also discuss gravitational-wave emission produced by quadrupole deformation, $r$-mode oscillations, and axion-induced bosonic clouds around an isolated neutron star, highlighting their potential as distinct multi-messenger probes in upcoming detectors.

18 pages, 8 figures; discussion on fallback accretion removed, new discussion on braking index and other clarifications added, references updated, typos fixed; matches published version