Constraining the Braking Indices of Magnetars

TL;DR

This paper estimates the mean braking indices of eight magnetars using their associated supernova remnants' ages, revealing insights into their spin-down mechanisms and magnetic field evolution, despite uncertainties in SNR age measurements.

Contribution

It introduces a method to constrain magnetar braking indices based on SNR ages, offering new insights into their magnetic and rotational evolution.

Findings

Magnetars have mean braking indices ranging from 1 to 42.

Different braking index ranges suggest different dominant spin-down mechanisms.

The method can be refined with more accurate SNR age measurements.

Abstract

Due to the lack of long term pulsed emission in quiescence and the strong timing noise, it is impossible to directly measure the braking index $n$ of a magnetar. Based on the estimated ages of their potentially associated supernova remnants (SNRs), we estimate the values of the mean braking indices of eight magnetars with SNRs, and find that they cluster in a range of $1\sim$42. Five magnetars have smaller mean braking indices of $1<n<3$, and we interpret them within a combination of magneto-dipole radiation and wind aided braking, while the larger mean braking indices of $n>3$ for other three magnetars are attributed to the decay of external braking torque, which might be caused by magnetic field decay. We estimate the possible wind luminosities for the magnetars with $1<n<3$, and the dipolar magnetic field decay rates for the magnetars with $n>3$ within the updated magneto-thermal evolution models. Although the constrained range of the magnetars' braking indices is tentative, due to the uncertainties in the SNR ages, which come from distance uncertainties and the unknown conditions of the expanding shells, our method provides an effective way to constrain the magnetars' braking indices if the measurements of the SNRs' ages are reliable, which can be improved by future observations.