Explaining high braking indices of magnetars<scp>SGR</scp>0501+4516 and<scp>1E</scp>2259+586 using the double magnetic‐dipole model
Fang‐Zhou Yan, Zhi‐Fu Gao, Wen‐Shen Yang, Ai‐Jun Dong
Abstract
Abstract In this paper, we attribute high braking indices n > 3 of two magnetars SGR 0501+4516 and 1E 2259+586 to the decrease in their inclination angles using the double magnetic‐dipole model proposed by Hamil et al. (2016). In this model, there are two magnetic moments inside a neutron star—one is generated by the rotation effect of a charged sphere, M 1 , and the other is generated by the magnetization of ferromagnetically ordered material, M 2 . Our calculations indicate that the magnetic moment M 2 would evolve toward alignment with the spin axis of the two magnetars and cause their magnetic inclination angles to decrease. We also define a ratio η = M 2 / M 1 , which reflects the magnetization degree, and find that the values of η of the two magnetars are about two orders of magnitude higher than that of rotationally powered pulsar PSR J1640‐4631 with n = 3.15(3) , assuming that they have the same rate of decrease in their inclination angles.