HEAD Division Meeting 1999, April 1999
Session 38. Neutron Star Binaries
Oral, Thursday, April 15, 1999, 2:00pm-3:31pm, Colonial Room

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[38.07] A Serendipitous RXTE Observation of the Strong Field X-ray Pulsar GX 1+4

D.K. Galloway (UTas/SRCfTA), A.B. Giles (UTas/LHEA GSFC), J.G. Greenhill (UTas), M.C. Storey (SRCfTA), J. Swank (LHEA GSFC)

The X-ray pulsar GX 1+4 was observed with the RXTE satellite for a total of 5.1 \times 104 s between 1996 July 19 - 21. During this period the flux decreased smoothly from an initial mean level of \approx 6 \times 1036\,{\rm erg\,s-1} to a minimum of \approx 4 \times 1035\,{\rm erg\,s-1} (assuming a source distance of 10 kpc) before partially recovering towards the initial level at the end of the observation.

BATSE measurements and timing analysis indicate that a torque reversal may have taken place close to the time of observation. Both the mean pulse profile and the photon spectrum varied significantly in a manner which has not been previously observed in this source. The single best-fitting spectral model was based on a component originating from thermal photons Comptonised by a hot plasma. The observed variation in the source may be an important clue as to the (presently unknown) mechanism of torque reversals in this and other sources.

Pulse phase spectroscopy during the first and brightest interval suggests that the flux variations with phase are due primarily to variations in the Compton scattered model component, in particular the normalisation and the scattering optical depth. These spectral variations with pulse phase may provide an indication of the distribution of matter close to the neutron star.

A strong flare of duration \approx 50 s was observed during the period of flux minimum, with the flux increasing to 4-5 times the mean level. The timing of a modest increase in flux prior to the flare is consistent with dual episodes of accretion resulting from successive orbits of a locally dense patch of matter in the accretion disk. This unexpected result provides an elegant confirmation of theoretical estimates of the inner disk radius given the estimated surface magnetic field strength of \approx 3 \times 1013 G.

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