2011ApJ...732L...4A

The period derivative bound for the soft gamma-ray repeater SGR 0418+5729 establishes the magnetic dipole moment to be distinctly lower than the magnetar range, placing the source beyond the regime of isolated pulsar activity in the P-{dot}P diagram and giving a characteristic age >2x10⁷ yr, much older than the 10⁵ yr age range of SGRs and anomalous X-ray pulsars. So the spin-down must be produced by a mechanism other than dipole radiation in vacuum. A fallback disk will spin down a neutron star with surface dipole magnetic field in the 10¹² G range and initial rotation period P₀∼ 100 ms to the 9.1 s period of SGR 0418+5729 in a few 10⁴ to ∼10⁵ yr. The current upper limit to the period derivative gives a lower limit of ∼10⁵ yr to the age that is not sensitive to the neutron star's initial conditions. The total magnetic field on the surface of SGR 0418+5729 could be significantly larger than its 10¹² G dipole component.