Is SGR 1900+14 a magnetar?

We present Rossi X-Ray Timing Explorer observations of the soft gamma-ray r epeater SGR 1900+14 taken 1996 September 4-18, nearly 2 yr before the 1998 active period of the source. The pulsar period (P) of 5.1558199 +/- 0.00000 29 s and period derivative ((P) over dot) of (6.0 +/-: 1.0) x 10(-11) s s(- 1) measured during the 2 week observation are consistent with the mean(P) o ver dot of(6.126 +/- 0.006) x 10(-11) s s(-1) over the time up to the comme ncement of the active period. This (P) over dot is less than half that of ( 12.77 +/- 0.01) x 10(-11) s s(-1) observed during and after the active peri od. If magnetic dipole radiation were the primary cause of the pulsar spin- down, the implied neutron star magnetic field would exceed the critical fie ld of approximate to 4.4 x 10(13) G by more than an order of magnitude, and such field estimates for this and other soft gamma repeaters (SGRs) have b een offered as evidence that the SGRs are magnetars, in which the neutron s tar magnetic energy exceeds the rotational energy. The observed doubling of (P) over dot, however, would suggest that the pulsar magnetic field energy increased by more than 100% as the source entered an active phase, which s eems very hard to reconcile with models in which the SGR bursts are powered by the release of magnetic energy. Because of this, we suggest that the sp in-down of SGR 1900+14 is not driven by magnetic dipole radiation, but by s ome other process, most likely a relativistic wind. The (P) over dot, there fore, does not provide a measure of the pulsar magnetic field strength, nor evidence for a magnetar.