Magnetic Field Limits on SGRs

Abstract

We measure the period and spin-down rate for SGR 1900+14 during the quiescient period two years before the recent interval of renewed burst activity. We find that the spin-down rate doubled during the burst activity which is inconsistent with both mangetic dipole driven spin down and a magnetic field energy source for the bursts. We also show that SGRs 1900+14 and 1806-20 have braking indices of $\sim$1 which indicate that the spin-down is due to wind torques and not magnetic dipole radiation. We further show that a combination of dipole radiation, and wind luminosity, coupled with estimated ages and present spin parameters, imply that the magnetic fields of SGRs 1900+14 and 1806-20 are less than the critical field of 4$\times10^{13}$ G and that the efficiency for conversion of wind luminosity to x-ray luminosity is <2%.