The Braking Index of Millisecond Magnetars

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 ± 0.1 and n 2.6 ± 0.1 for millisecond magnetars born in GRB 130603B and GRB 140903A, respectively. The neutron star born in GRB 130603B has the only known a 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 < 3braking 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.