Braking indices of young radio pulsars: theoretical perspective

Abstract

Recently, Parthsarathy et al. analysed long-term timing observations of 85 young radio pulsars. They found that 15 objects have absolute values of braking indices ranging ∼10 − 3000, far from the classical value n = 3. They also noted a mild correlation between measured value of n and characteristic age of a radio pulsar. In this article we systematically analyse possible physical origin of large braking indices. We find that a small fraction of these measurements could be caused by gravitational acceleration from an unseen ultra-wide companion of a pulsar or by precession. Remaining braking indices cannot be explained neither by pulsar obliquity angle evolution, nor by complex high-order multipole structure of the poloidal magnetic field. The most plausible explanation is a decay of the poloidal dipole magnetic field which operates on a time scale ∼104 − 105 years in some young objects, but has significantly longer time scale in other radio pulsars. This decay can explain both amplitude of measured n and some correlation between n and characteristic age. The decay can be caused by either enhanced crystal impurities in the crust of some isolated radio pulsars, or more likely, by enhanced resistivity related to electron scattering off phonons due to slow cooling of low-mass neutron stars. If this effect is indeed the main cause of the rapid magnetic field decay manifesting as large braking indices, we predict that pulsars with large braking indices are hotter in comparison to those with n ≈ 3.

Magnetic field decay in young radio pulsars

“…In this paper, we discuss how the results from Igoshev & Popov (2020) are compatible with our early findings and give some additional (technical) considerations on the magnetic field evolution.…”

“…In this section, we, at first, briefly summarize results presented in Igoshev & Popov (2020), and then discuss how they are related with our previous modeling.…”

“…In Igoshev & Popov (2020), we analyzed several scenarios to explain these atypical values of the braking index. The main one is related to magnetic field decay.…”

“…If τ ohm evolves with the NS temperature, as in our model, we can solve the original Equation (4) numerically using for example, Runge–Kutta integrator (as it was done in Igoshev & Popov 2020), or use Equation (17) computing it at a fine grid and substituting value at the previous grid point instead of B 0 , that is, …”

“…In Igoshev & Popov (2015), we concluded that the first variant is more probable. In our new study (Igoshev & Popov 2020), we focused on the fact that most of reliably measured braking indices for young radio pulsars in a new sample (Parthasarathy et al 2020) are positive and deviate significantly from a classical value of three, typical for both magneto‐dipolar braking in vacuum and filled magnetosphere. Positive braking indices indicate that magnetic field might be decaying and this decay also seems to terminate at timescales comparable to 100 kyr.…”