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?

Abstract

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-(P)over dot diagram and giving a characteristic age >2 x 10(7) yr, much older than the 10(5) 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 10(12) G range and initial rotation period P(0) similar to 100 ms to the 9.1 s period of SGR 0418+5729 in a few 10(4) to similar to 10(5) yr. The current upper limit to the period derivative gives a lower limit of similar to 10(5) 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 10(12) G dipole component.