Spin down and oscillations in 4U 1907+09: a retrograde disk?

Abstract

X-ray observations of the accretion-powered X-ray pulsar 4U 1907+09, obtained during February 1996 with the Proportional Counter Array on the Rossi X-ray Timing Experiment (RXTE), have shown interesting features of which three are: 1) a flare occurred lasting at least a few hours and close to the time of apastron; 2) the spin down rate, when measured since the pulsar discovery in 1983, is within 8% equal to a value of P pulse = +0.225 s yr −1, this suggest that the pulsar is in a monotonous spin down mode; 3) transient ∼ 18 s oscillations appeared during the flare. 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.