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Durham University

Department of Mathematical Sciences

Academic Staff

Publication details for Konstantinos Gourgouliatos

Gourgouliatos, Konstantinos N. & Hollerbach, Rainer (2018). Magnetic Axis Drift and Magnetic Spot Formation in Neutron Stars with Toroidal Fields. The Astrophysical Journal 852(1): 21.

Author(s) from Durham

Abstract

We explore magnetic field configurations that lead to the formation of magnetic spots on the surface of neutron
stars and the displacement of the magnetic dipole axis. We find that a toroidally dominated magnetic field is
essential for the generation of a single spot with a strong magnetic field. Once a spot forms, it survives for several
million years, even after the total magnetic field has decayed significantly. We find that the dipole axis is not
stationary with respect to the neutron star’s surface and does not in general coincide with the location of the
magnetic spot. This is due to non-axisymmetric instabilities of the toroidal field that displace the poloidal dipole
axis at rates that may reach 0°. 4 per century. A misaligned poloidal dipole axis with the toroidal field leads to more
significant displacement of the dipole axis than the fully aligned case. Finally we discuss the evolution of neutron
stars with such magnetic fields on the P P– ˙ diagram and the observational implications. We find that neutron stars
spend a very short time before they cross the Death Line of the P P– ˙ diagram, compared to their characteristic ages.
Moreover, the maximum intensity of their surface magnetic field is substantially higher than the dipole component
of the field. We argue that SGR 0418+5729 could be an example of this type of behavior, having a weak dipole
field, yet hosting a magnetic spot responsible for its magnetar behavior. The evolution on the pulse profile and
braking index of the Crab pulsar, which are attributed to an increase of its obliquity, are compatible with the
anticipated drift of the magnetic axis.