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W. Cui (Purdue University), F. Jablonski (DAS/INPE, Brazil), D. Galloway (M.I.T.)
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, as well as optical monitoring with ground-based facilities. The spin-down of GX 1+4 is continuing, with the pulsar currently at its longest period ever measured (~137 s). Recently, the source entered an extended "low state" in X-rays, when its flux (2-60 keV) dropped and stayed below ~5 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 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% (peak-to-peak). The disappearance of pulsation was also observed at optical wavelengths during contemporaneous observations, providing further support for the phenomenon. 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 and was invoked to explain the 1996 episode.
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.