Braking indices as probes of r-mode spin-down in young pulsars

Abstract

We present a timing-based framework for interpreting braking-index measurements in young pulsars using a four-channel spin-down model that includes particle-wind, magnetic-dipole, mass-quadrupole, and current-quadrupole torques. The observed braking index is a torque-weighted average of the channel exponents, enabling equation-of-state-independent constraints on the torque fraction of a possible r-mode-like current-quadrupole component from timing data alone. For positive, slowly varying secular torques, the allowed range is (1\le n_{\rm obs}\le 7), with (f_{7,\min}=\max[0,(n_{\rm obs}-5)/2]) and (f_{7,\max}^{\rm phys}=\min{1,\max[0,(n_{\rm obs}-1)/6]}). These bounds rely on non-negative, slowly evolving torque coefficients; if these assumptions fail, a large braking index need not uniquely indicate a current-quadrupole torque. Applied to young pulsars with measured braking indices, the analysis shows that most sources do not require gravitational-wave spin-down and are consistent with wind-plus-dipole braking. Sources with (3<n_{\rm obs}<5) require an additional higher-order contribution, but timing alone cannot identify it uniquely as an r-mode torque. PSR~J0537$-$6910 is the most suggestive case: its large inter-glitch braking index approaches the (n\simeq7) limit and is consistent, within the restricted secular model, with a strong current-quadrupole-like contribution. However, vortex-creep and superfluid-recovery effects may produce similar inter-glitch behaviour without gravitational-wave emission. We also derive stellar-model-dependent r-mode amplitude bounds, braking-index-corrected ages, and continuous-wave ranking metrics for current and future detectors, including the reduced sensitivity expected for glitch-limited semi-coherent searches.