Crust–magnetosphere coupling during magnetar evolution and implications for the surface temperature

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Title: Crust–magnetosphere coupling during magnetar evolution and implications for the surface temperature
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: magnetic field | Stars: neutron
Knowledge Area: Astronomía y Astrofísica
Issue Date: 21-Dec-2018
Publisher: Oxford University Press
Citation: Monthly Notices of the Royal Astronomical Society. 2018, 481(4): 5331-5338. doi:10.1093/mnras/sty2669
Abstract: We study the coupling of the force-free magnetosphere to the long-term internal evolution of a magnetar. We allow the relation between the poloidal and toroidal stream functions – that characterizes the magnetosphere – to evolve freely without constraining its particular form. We find that, on time-scales of the order of kyr, the energy stored in the magnetosphere gradually increases, as the toroidal region grows and the field lines expand outwards. This continues until a critical point is reached beyond which force-free solutions for the magnetosphere can no longer be constructed, likely leading to some large-scale magnetospheric reorganization. The energy budget available for such events can be as high as several 1045 erg for fields of 1014 G. Subsequently, starting from the new initial conditions, the evolution proceeds in a similar manner. The time-scale to reach the critical point scales inversely with the magnetic field amplitude. Allowing currents to pass through the last few metres below the surface, where the magnetic diffusivity is orders of magnitude larger than in the crust, should give rise to a considerable amount of energy deposition through Joule heating. We estimate that the effective surface temperature could increase locally from ∼0.1 keV to ∼0.3-0.6 keV, in good agreement with observations. Similarly, the power input from the interior into the magnetosphere could be as high as 1035-1036 erg s−1, which is consistent with peak luminosities observed during magnetar outbursts. Therefore, a detailed treatment of currents flowing through the envelope may be needed to explain the thermal properties of magnetars.
Sponsor: This work is supported in part by the Spanish MINECO/FEDER grants AYA2015-66899-C2-1-P, AYA2015-66899-C2-2-P, the grant of Generalitat Valenciana PROMETEOII-2014-069, and by the PHAROS COST action CA16214. PC acknowledges the support from the Ramón y Cajal program of the Spanish MINECO (RYC-2015-19074).
ISSN: 0035-8711 (Print) | 1365-2966 (Online)
DOI: 10.1093/mnras/sty2669
Language: eng
Type: info:eu-repo/semantics/article
Rights: © 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
Peer Review: si
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Appears in Collections:INV - Astrofísica Relativista - Artículos de Revistas

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