/ 99 12 23 7 v 1 1 0 D ec 1 99 9 Magnetic Field Limits on SGRs

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 ∼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 4×10 13 G and that the efficiency for conversion of wind luminosity to x-ray luminosity is <2%. (Figure 1) is characterized by three intervals of time for which the spin-down rate was essentially constant within the interval. This characterization of the SGR 1900+14 spin-down is based upon both direct measurements of ˙ P as part of the period determination, and upon differences in measured spin periods between two different observations. The first interval begins with the RXTE observation in September of 1996 and ends with the ASCA observation at the beginning of May, 1998. The mean spin-down ˙ P ∼ 6×10 −11 s/s [1–3]. The second interval begins with the onset of bursting on May 26, 1998 and continues until mid-September 1998. The mean spin-down during this time ˙ P ∼13×10 −11 s/s [4,1,5]. The third interval begins in mid-September 1998 and continues at least until March 30, 1999. The mean spin-down rate at that time is again ∼ 6×10 −11 s/s [8]. Woods et al. [8] (1999b) have suggested that the data may be consistent with a discontinuous spin-down event during the second interval as a result of the giant burst of August 27, 1998, as opposed to a doubling of ˙ P during the entire second interval [1]. This suggestion, however, appears to be at odds with the measurement of ˙