2004ApJ...602..320C

We report results from our weekly monitoring campaign on the X-ray pulsar GX 1+4 with the Rossi X-Ray Timing Explorer satellite. The spin-down trend of GX 1+4 was continuing, with the pulsar being at its longest period ever measured (about 138.7 s). At the late stage of the campaign, the source entered an extended faint state, during which its X-ray (2-60 keV) flux decreased significantly, to an average level of ∼3x10^–10 ergs/cm2/s. It was highly variable in the faint state; the flux dropped to as low as ∼3x10^–11 ergs/cm2/s. In several observations during this period, the X-ray pulsation became undetectable. We can therefore conservatively conclude that the pulsed fraction, which is normally ≳70% (peak-to-peak), must have decreased drastically in those cases. This is very similar to what was observed in GX 1+4 in 1996, when it became similarly faint in X-ray. In fact, the flux at which the cessation of X-ray pulsation first occurred is nearly the same as it was in 1996. We suggest that we have once again observed the propeller effect in GX 1+4, a phenomenon that is predicted by theoretical models of accreting X-ray pulsars.