SGR 0418+5729 - How does a Young Neutron Star Spin Down to a 9 s Period with a Dipole Field less than 10^13 G?

The period derivative bound for SGR 0418+5729 (Rea et al. 2010) 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 - dP/dt diagram and giving a characteristic age > 2 \times 10^{7} years, much older than the 10^5 year age range of SGRs and AXPs. So the spindown 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 10^{12} G range and initial rotation period P_0 ~ 100 ms to the 9.1 s period of SGR 0418+5729 in a few 10^4 to ~10^5 years. The current upper limit to the period derivative gives a lower limit of \sim 10^5 years 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 10^{12} G dipole component.
Comment: 17 pages, 2 figures, accepted by ApJL