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W. Cui (Purdue University)
We report results on the X-ray pulsar GX 1+4, based on data from our weekly monitoring of the source with the Rossi X-ray Timing Explorer satellite. The spin-down of GX 1+4 is continuing, with the pulsar is currently at its longest period ever measured (~138 s). Recently, the source entered an extended "low state" in X-rays, when its flux (2-60 keV) dropped and stayed below ~5 \times 10-10 ergscm-2s-1. In several observations during the interval, the pulsation became undetectable, as the source flux reached the minimum flux of ~3\times 10-11 ergscm-2s-1. This is very similar to what was observed of GX 1+4 in 1996 when it became similarly faint in X-rays. In fact, the flux at which the cessation of X-ray pulsation first occurred is nearly the same as it was in 1996. The non-detection of pulsation indicates a dramatic change in the pulse fraction, which is normally greater than 80%. We argue that we have most likely observed the "propeller effect" in GX 1+4, a phenomenon that is predicted by theoretical models of accreting X-ray pulsars.
Bulletin of the American Astronomical Society,
© 2003. The American Astronomical Soceity.