The soft-gamma repeater SGR 1900 + 14 became active again on June 1998 after a long period of quiescence ; it remained at a low state of activity until August 1998 , when it emitted a series of extraordinarily intense outbursts .
We have observed the source with RXTE twice , during the onset of each active episode .
We confirm the pulsations at the 5.16 s period reported earlier ( [ Hurley et al .
1998b , Hurley et al .
1998e ] ) from SGR 1900 + 14 .
Here we report the detection of a secular spindown of the pulse period at an average rate of 1.1 \times 10 ^ { -10 } s/s .
In view of the strong similarities between SGRs , we attribute the spindown of SGR 1900 + 14 to magnetic dipole radiation , possibly accelerated by a quiescent flux , as in the case of SGR 1806 - 20 ( [ Kouveliotou et al .
1998a ] ) .
This allows an estimate of the pulsar dipolar magnetic field , which is 2 - 8 \times 10 ^ { 14 } G. Our results confirm that SGRs are magnetars .