Long-term evolution of the force-free twisted magnetosphere of a magnetar

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Title: Long-term evolution of the force-free twisted magnetosphere of a magnetar
Authors: Akgün, Taner | Cerdá-Durán, Pablo | Miralles, Juan A. | Pons, José A.
Research Group/s: Astrofísica Relativista
Center, Department or Service: Universidad de Alicante. Departamento de Física Aplicada
Keywords: Magnetic fields | MHD | Stars: magnetars | Stars: neutron
Knowledge Area: Astronomía y Astrofísica
Issue Date: 21-Dec-2017
Publisher: Oxford University Press
Citation: Monthly Notices of the Royal Astronomical Society. 2017, 472(4): 3914-3923. doi:10.1093/mnras/stx2235
Abstract: We study the long-term quasi-steady evolution of the force-free magnetosphere of a magnetar coupled to its internal magnetic field. We find that magnetospheric currents can be maintained on long time-scales of the order of thousands of years. Meanwhile, the energy, helicity and twist stored in the magnetosphere all gradually increase over the course of this evolution, until a critical point is reached, beyond which a force-free magnetosphere cannot be constructed. At this point, some large-scale magnetospheric rearrangement, possibly resulting in an outburst or a flare, must occur, releasing a large fraction of the stored energy, helicity and twist. After that, the quasi-steady evolution should continue in a similar manner from the new initial conditions. The time-scale for reaching this critical point depends on the overall magnetic field strength and on the relative fraction of the toroidal field. The energy stored in the force-free magnetosphere is found to be up to ∼30 per cent larger than the corresponding vacuum energy. This implies that for a 1014 G field at the pole, the energy budget available for fast magnetospheric events is of the order of a few 1044 erg. The spin-down rate is estimated to increase by up to ∼60 per cent, since the dipole content in the magnetosphere is enhanced by the currents present there. A rough estimate of the braking index n reveals that it is systematically n < 3 for the most part of the evolution, consistent with actual measurements for pulsars and early estimates for several magnetars.
Sponsor: This work is supported in part by the Spanish MINECO grants AYA2015-66899-C2-1-P, AYA2015-66899-C2-2-P, the grant of Generalitat Valenciana PROMETEOII-2014-069 and by the New Compstar COST action MP1304.
URI: http://hdl.handle.net/10045/72135
ISSN: 0035-8711 (Print) | 1365-2966 (Online)
DOI: 10.1093/mnras/stx2235
Language: eng
Type: info:eu-repo/semantics/article
Rights: © 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
Peer Review: si
Publisher version: http://dx.doi.org/10.1093/mnras/stx2235
Appears in Collections:INV - Astrofísica Relativista - Artículos de Revistas

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