Discovery of a magnetar associated with the soft gamma repeater SGR 1900+14

The soft gamma repeater SGR 1900+14 became active again on 1998 June after a long period of quiescence; it remained at a low state of activity until 1 998 August, when it emitted a. series of extraordinarily intense outbursts. We have observed the source with the Rossi X-Ray Timing Explorer twice, du ring the onset of each active episode. We confirm the pulsations at the 5.1 6 s period reported earlier from SGR 1900+14. Here we report the detection of a secular spin-down of the pulse period at an average rate of 1.1 x 10(- 10) s s(-1). In view of the strong similarities between SGRs, we attribute the spin-down of SGR 1900+14 to magnetic dipole radiation, possibly acceler ated by a quiescent flux, as in the case of SGR 1806-20. This allows an est imate of the pulsar dipolar magnetic field, which is (2-8) x 10(14) G. Our results confirm that SGRs are magnetars.