Journal Article

Magnetic field decay in neutron stars: from soft gamma repeaters to ‘weak-field magnetars’

S. Dall’Osso, J. Granot and T. Piran

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 422, issue 4, pages 2878-2903
Published in print June 2012 | ISSN: 0035-8711
Published online May 2012 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2012.20612.x
Magnetic field decay in neutron stars: from soft gamma repeaters to ‘weak-field magnetars’

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The recent discovery of the ‘weak-field, old magnetar’ soft gamma repeater (SGR) J0418+5729, whose dipole magnetic field, Bdip, is less than 7.5 × 1012 G, has raised perplexing questions: how can the neutron star produce SGR-like bursts with such a low magnetic field? What powers the observed X-ray emission when neither the rotational energy nor the magnetic dipole energy is sufficient? These observations, which suggest either a much larger energy reservoir or a much younger true age (or both), have renewed the interest in the evolutionary sequence of magnetars. We examine here a phenomenological model for the magnetic field decay: and compare its predictions with the observed period, P, the period derivative, , and the X-ray luminosity, LX, of magnetar candidates. We find a strong evidence for a dipole field decay on a time-scale of ∼103 yr for the strongest (Bdip∼ 1015 G) field objects, with a decay index within the range 1 ≤α < 2 and more likely within 1.5 ≲α≲ 1.8. The decaying field implies a younger age than what is implied by . Surprisingly, even with the younger age, the energy released in the dipole field decay is insufficient to power the X-ray emission, suggesting the existence of a stronger internal field, Bint. Examining several models for the internal magnetic field decay, we find that it must have a very large (≳ 1016 G) initial value. Our findings suggest two clear distinct evolutionary tracks – the SGR/anomalous X-ray pulsar branch and the transient branch, with a possible third branch involving high-field radio pulsars that age into low-luminosity X-ray dim isolated neutron stars.

Keywords: magnetic fields; stars: magnetars; stars: neutron; X-rays: stars

Journal Article.  18465 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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