Pulse Profiles, Accretion Column Dips, and a Flare in GX 1+4 During a Faint State

The Rossi X-ray Timing Explorer (RXTE) spacecraft observed the X-ray pulsarGX 1+4 for a period of 34 hours on July 19/20 1996. The source faded from anintensity of ~20 mCrab to a minimum of <~0.7 mCrab and then partially recoveredtowards the end of the observation. This extended minimum lasted ~40,000seconds. Phase folded light curves at a barycentric rotation period of124.36568 +/- 0.00020 seconds show that near the center of the extended minimumthe source stopped pulsing in the traditional sense but retained a weak dipfeature at the rotation period. Away from the extended minimum the dips areprogressively narrower at higher energies and may be interpreted asobscurations or eclipses of the hot spot by the accretion column. The pulseprofile changed from leading-edge bright before the extended minimum totrailing-edge bright after it. Data from the Burst and Transient SourceExperiment (BATSE) show that a torque reversal occurred <10 days after ourobservation. Our data indicate that the observed rotation departs from aconstant period with a Pdot/P value of ~-1.5% per year at a 4.5 sigmasignificance. We infer that we may have serendipitously obtained data, withhigh sensitivity and temporal resolution about the time of an accretion diskspin reversal. We also observed a rapid flare which had some precursoractivity, close to the center of the extended minimum.Comment: 19 pages, 6 figures, accepted for publication in Astrophysical Journal (tentatively scheduled for vol. 529 #1, 20 Jan 2000