Astronomy and Astrophysics – Astrophysics – High Energy Astrophysical Phenomena
Scientific paper
2011-10-11
Astronomy and Astrophysics
Astrophysics
High Energy Astrophysical Phenomena
47 pages, 11 figures, accepted for publication on MNRAS, in press
Scientific paper
The recent discovery of the "weak field, old magnetar", the soft gamma repeater SGR 0418+5729, whose dipole magnetic field is less than 7.5 \times 10^{12} 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 are sufficient? These observations, that 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: B_dip} \propto (B_dip)^{1+a} and compare its predictions with the observed period, P,the period derivative, \dot{P}, and the X-ray luminosity, L_X, of magnetar candidates. We find a strong evidence for a dipole field decay on a timescale of \sim 10^3 yr for the strongest (\sim 10^{15} G) field objects, with a decay index within the range 1 \leq a < 2 and more likely within 1.5\lesssim a \lesssim 1.8. The decaying field implies a younger age than what is implied by the spinown age. 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^{16} G) initial value. Our findings suggest two clear distinct evolutionary tracks -- the SGR/AXP 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.
Dall'Osso Simone
Granot Jonathan
Piran Tsvi
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