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

TL;DR

This work investigates neutron-star spin-down including standard channels (electromagnetic dipole, GW emission, $r$-modes) and fallback accretion, plus a nonstandard superradiant mechanism driven by ultralight axions. It develops a framework for how these channels combine to shape spin evolution and identifies distinctive multimessenger GW signatures from quadrupolar deformations, $r$-modes, and axion-cloud annihilation, with axion-induced GWs characterized by $f_{\mathrm{GW}}^0 = 2 m_A/h$. The authors show that superradiant instability can yield transient spin-down episodes and potentially explain non-recoverable anti-glitches, while the resulting axion clouds produce continuous GWs with peak strains around $h_{0,\mathrm{peak}} \sim 10^{-28}$, which are challenging for current detectors but accessible to future observatories. Collectively, the results highlight a multimessenger pathway to constrain axion properties (mass $m_A$ and coupling $g_{a\gamma\gamma}$) and to probe neutron-star interior physics through combined timing and gravitational-wave observations.

Abstract

Neutron stars such as pulsars and magnetars lose angular momentum primarily through electromagnetic dipole radiation, gravitational waves, $r$-mode oscillation, and also affected by fallback accretion processes. However, anomalous spin variations, particularly sudden enhanced spin-down rates, indicate additional spin-down mechanisms. We propose superradiant spin-down as a potential explanation for these events. 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 multimessenger probes in upcoming detectors.

Figures (9)

• Figure 1: Pulsar spin parameters $P$-$\dot{P}$ distribution from the ATNF catalogue Manchester:2004bp.
• Figure 2: Spin period evolution of an isolated milisecond neutron star due to different spindown mechanisms.
• Figure 3: Comparative analysis of spin period evolution of an isolated neutron star having different magnetic fields for combined effect of dipole and accretion torque and dipole torque only.
• Figure 4: (Left) Spin period evolution of an isolated neutron star due to fallback accretion torques and GW torques. (Right) Spin period evolution of an isolated neutron star due to accretion and dipole torques for different values of $\eta$.
• Figure 5: Spindown during superradiant instability of an isolated neutron star.