Spectral variation in the X-ray pulsar GX 1+4 during a low-flux episode

The X-ray pulsar GX 1 + 4 was observed with the RXTE satellite for a total of 51 ks between 1996 July 19 and 21. During this period the flux decreased smoothly from an initial mean level of ≈6 × 1036 erg s-1 to a minimum of ≈4 × 1035 erg s-1 (2-60keV, assuming a source distance of 10 kpc) before partially recovering towards the initial level at the end of the observation. BATSE pulse timing measurements indicate that a torque reversal took place approximately 10d after this observation. Both the mean pulse profile and the photon spectrum varied significantly. The observed variation in the source may provide important clues as to the mechanism of torque reversals. The single best-fitting spectral model was based on a component originating from thermal photons with kT0 ≈ 1 keV Comptonized by a plasma of temperature KT ≈ 7 keV. Both the flux modulation with phase during the brightest interval and the evolution of the mean spectra over the course of the observation are consistent with variations in this model component; with, in addition, a doubling of the column density nH contributing to the mean spectral change. A strong flare of duration ≤50 s was observed during the interval of minimum flux, with the peak flux ≈20 times the mean level. Although beaming effects are likely to mask the true variation in M thought to give rise to the flare, 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 disc.