Misc,
The First Day in the Life of a Magnetar: Evolution of the Inclination Angle, Magnetic Dipole Moment and Braking Index of Millisecond Magnetars During Gamma-Ray Burst Afterglows
(2019)cite arxiv:1904.06769Comment: 16 pages, 8 figures, 4 tables.
Abstract
The afterglow light curves of some gamma-ray bursts (GRBs) show a shallow
decay (plateau) phase implying continuous injection of energy. The source of
this energy is very commonly assumed to be the spin-down power of a nascent
millisecond magnetar. The magnetic dipole radiation torque is considered to be
the mechanism causing the spin-down of the neutron star. This torque has a
component working for the alignment of the angle between rotation and magnetic
axis, i.e. inclination angle. Here, we demonstrate the evolution of the
inclination angle and magnetic dipole moment of nascent magnetars associated
with GRBs. We constrain the initial inclination angle, magnetic dipole moment
and rotation period of seven magnetars by modelling the seven long-GRB
afterglow light curves. We find that, in its first day, the inclination angle
of a magnetar decreases rapidly. The rapid alignment of the magnetic and
rotation axis may address the lack of persistent radio emission from mature
magnetars. We also find that in three cases the magnetic dipole moments of
magnetars decrease exponentially to a value a few times smaller than the
initial value. The braking index of nascent magnetars, as a result of the
alignment and magnetic dipole moment decline, is variable during the afterglow
phase and always greater than three.
