Journal Article

Magnetar activity mediated by plastic deformations of neutron star crust

Maxim Lyutikov

in Monthly Notices of the Royal Astronomical Society

Volume 447, issue 2, pages 1407-1417
ISSN: 0035-8711
Published online December 2014 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1093/mnras/stu2413
Magnetar activity mediated by plastic deformations of neutron star crust

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We advance a ‘solar flare’ model of magnetar activity, whereas a slow evolution of the magnetic field in the upper crust, driven by electron magnetohydrodynamic flows, twists the external magnetic flux tubes, producing persistent emission, bursts, and flares. At the same time, the neutron star crust plastically relieves the imposed magnetic field stress, limiting the strain ϵt to values well below the critical strain ϵcrit of a brittle fracture, ϵt ∼ 10−2ϵcrit. Magnetar-like behaviour, occurring near the magnetic equator, takes place in all neutron stars, but to a different extent. The persistent luminosity is proportional to cubic power of the magnetic field (at a given age), and hence is hardly observable in most rotationally powered neutron stars. Giant flares can occur only if the magnetic field exceeds some threshold value, while smaller bursts and flares may take place in relatively small magnetic fields. Bursts and flares are magnetospheric reconnection events that launch Alfvén shocks which convert into high-frequency whistlers upon hitting the neutron star surface. The resulting whistler pulse induces a strain that increases with depth both due to the increasing electron density (and the resulting slowing of the waves), and due to the increasing coherence of a whistler pulse with depth. The whistler pulse is dissipated on a time-scale of approximately a day at shallow depths corresponding to ρ ∼ 1010 g cm−3; this energy is detected as enhanced post-flare surface emission.

Keywords: stars: magnetars; stars: magnetic fields; stars: neutron

Journal Article.  9954 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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