Pulsar timing irregularities and the imprint of magnetic field evolution

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Title: Pulsar timing irregularities and the imprint of magnetic field evolution
Authors: Pons, José A. | Viganò, Daniele | Geppert, Ulrich
Research Group/s: Astrofísica Relativista
Center, Department or Service: Universidad de Alicante. Departamento de Física Aplicada
Keywords: Pulsars: general | Stars: neutron | Stars: magnetic field | Stars: evolution
Knowledge Area: Astronomía y Astrofísica
Issue Date: 18-Oct-2012
Publisher: EDP Sciences
Citation: Astronomy & Astrophysics. 2012, 547: A9. doi:10.1051/0004-6361/201220091
Abstract: Context. The rotational evolution of isolated neutron stars is dominated by the magnetic field anchored to the solid crust of the star. Assuming that the core field evolves on much longer timescales, the crustal field evolves mainly though Ohmic dissipation and the Hall drift, and it may be subject to relatively rapid changes with remarkable effects on the observed timing properties. Aims. We investigate whether changes of the magnetic field structure and strength during the star evolution may have observable consequences in the braking index n. This is the most sensitive quantity to reflect small variations of the timing properties that are caused by magnetic field rearrangements. Methods. We performed axisymmetric, long-term simulations of the magneto-thermal evolution of neutron stars with state-of-the-art microphysical inputs to calculate the evolution of the braking index. Relatively rapid magnetic field modifications can be expected only in the crust of neutron stars, where we focus our study. Results. We find that the effect of the magnetic field evolution on the braking index can be divided into three qualitatively different stages depending on the age and the internal temperature: a first stage that may be different for standard pulsars (with n ~ 3) or low field neutron stars that accreted fallback matter during the supernova explosion (systematically n < 3); in a second stage, the evolution is governed by almost pure Ohmic field decay, and a braking index n > 3 is expected; in the third stage, at late times, when the interior temperature has dropped to very low values, Hall oscillatory modes in the neutron star crust result in braking indices of a high absolute value and both positive and negative signs. Conclusions. Current magneto-thermal evolution models predict a large contribution to the timing noise and, in particular, to the braking index, from temporal variations of the magnetic field. Models with strong (≳ 1014 G) multipolar or toroidal components, even with a weak (~1012 G) dipolar field are consistent with the observed trend of the timing properties.
Sponsor: This research was supported by the grants AYA 2010-21097-C03-02 and ACOMP/2012/135. D.V. is supported by a fellowship from the Prometeo program for research groups of excellence of the Generalitat Valenciana (Prometeo/2009/103).
URI: http://hdl.handle.net/10045/33789
ISSN: 0004-6361 (Print) | 1432-0746 (Online)
DOI: 10.1051/0004-6361/201220091
Language: eng
Type: info:eu-repo/semantics/article
Rights: © ESO, 2012
Peer Review: si
Publisher version: http://dx.doi.org/10.1051/0004-6361/201220091
Appears in Collections:INV - Astrofísica Relativista - Artículos de Revistas

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