TY - JOUR
T1 - Magnetic field decay in neutron stars
T2 - From soft gamma repeaters to 'weak-field magnetars'
AU - Dall'Osso, S.
AU - Granot, J.
AU - Piran, T.
PY - 2012/6
Y1 - 2012/6
N2 - The recent discovery of the 'weak-field, old magnetar' soft gamma repeater (SGR) J0418+5729, whose dipole magnetic field, B dip, is less than 7.5 × 10 12G, 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, L X, of magnetar candidates. We find a strong evidence for a dipole field decay on a time-scale of ∼10 3yr for the strongest (B dip∼ 10 15G) 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, B int. Examining several models for the internal magnetic field decay, we find that it must have a very large (≳ 10 16G) 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.
AB - The recent discovery of the 'weak-field, old magnetar' soft gamma repeater (SGR) J0418+5729, whose dipole magnetic field, B dip, is less than 7.5 × 10 12G, 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, L X, of magnetar candidates. We find a strong evidence for a dipole field decay on a time-scale of ∼10 3yr for the strongest (B dip∼ 10 15G) 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, B int. Examining several models for the internal magnetic field decay, we find that it must have a very large (≳ 10 16G) 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.
KW - Magnetic fields
KW - Stars: magnetars
KW - Stars: neutron
KW - X-rays: stars
UR - http://www.scopus.com/inward/record.url?scp=84861231162&partnerID=8YFLogxK
U2 - 10.1111/j.1365-2966.2012.20612.x
DO - 10.1111/j.1365-2966.2012.20612.x
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AN - SCOPUS:84861231162
SN - 0035-8711
VL - 422
SP - 2878
EP - 2903
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
IS - 4
ER -