Origin of large braking-index deviations

Determine what physical mechanism drives the very large positive or negative braking-index values observed in middle-aged and old pulsars, beyond crustal magnetic-field evolution and accounting for observational uncertainties.

Background

The paper discusses the evolution of the surface dipolar field and its contribution to the braking index. Crustal simulations, including models with Hall-induced oscillations and core-field coupling, can produce braking-index variations, but generally not the largest positive or negative values measured in middle-aged and old pulsars.

The authors therefore identify the physical origin of these extreme braking indices as unresolved, while noting that observational caveats and short-term torque fluctuations may contribute. This problem motivates the paper’s investigation of spontaneous dipolar-axis drift and its possible competition with magnetospheric or superfluid effects.

References

Therefore, what drives the very large positive or negative $n$ values often observed in middle-age and old pulsars therefore remains an open question, though the above-mentioned observational caveats need to be kept in mind.

— Spontaneous wandering of the magnetic axis in pulsars: 3D magneto-thermal simulations and the imprint on braking indices  (2609.17436 - Dehman et al., 15 Sep 2026) in Section 3.1, subsection “Surface dipolar field evolution”

On the one hand, since our models are not fine-tuned, we cannot exclude more extreme variations of $n$ caused by the obliquity drift and the field decay.

— Spontaneous wandering of the magnetic axis in pulsars: 3D magneto-thermal simulations and the imprint on braking indices  (2609.17436 - Dehman et al., 15 Sep 2026) in Section 6, Discussion