# The braking index of millisecond magnetars

## TL;DR

This paper measures the braking index of two millisecond magnetars from short gamma-ray bursts, finding values below the canonical 3, which challenges existing models of neutron star spin-down.

## Contribution

First measurement of the braking index for millisecond magnetars in short gamma-ray bursts, providing new observational constraints on neutron star spin-down models.

## Key findings

- Measured braking indices of 2.9 and 2.6 for two magnetars.
- One magnetar's index is consistent with the theoretical value of 3.
- The other magnetar's index is significantly below 3, challenging existing models.

## Abstract

We make the first measurement of the braking index $n$ of two putative millisecond magnetars born in short gamma-ray bursts. We measure $n=2.9\pm0.1$ and $n=2.6\pm0.1$ for millisecond magnetars born in GRB 130603B and GRB 140903A respectively. The neutron star born in GRB 130603B has the only known braking index consistent with the fiducial $n=3$ value. This value is ruled out with 99.95\% confidence for GRB 140903A. We discuss possible causes of $n<3$ braking indices in millisecond magnetars, showing that several models can account for the measurement of the braking index in GRB 140903A, while it is more difficult to account for a braking index consistent with $n=3$.