X-ray observations of the accreting X-ray pulsar 4U 1907+09 , obtained during February 1996 with the Proportional Counter Array on the Rossi X-ray Timing Experiment ( RXTE ) , have enabled the first measurement of the intrinsic pulse period P _ { pulse } since 1984 : P _ { pulse } = 440.341 ^ { +0.012 } _ { -0.017 } s. 4U 1907+09 is in a binary system with a blue supergiant .
The orbital parameters were solved and this enabled the correction for orbital delay effects of a measurement of P _ { pulse } obtained in 1990 with Ginga .
Thus , three spin down rates could be extracted from four pulse periods obtained in 1983 , 1984 , 1990 , and 1996 .
These are within 8 % equal to a value of \dot { P } _ { pulse } = +0.225 s yr ^ { -1 } .
This suggest that the pulsar is perhaps in a monotonous spin down mode since its discovery in 1983 .
Furthermore , the RXTE observations show transient \sim 18 s oscillations during a flare that lasted about 1 hour .
The oscillations may be interpreted as Keplerian motion of an accretion disk near the magnetospheric radius .
This , and the notion that the co-rotation radius is much larger than any conceivable value for the magnetospheric radius ( because of the long spin period ) , renders it unlikely that this pulsar spins near equilibrium like is suspected for other slowing accreting X-ray pulsars .
We suggest as an alternative that perhaps the frequent occurrence of a retrograde transient accretion disk may be consistently slowing the pulsar down .
Further observations of flares can provide more evidence of this .