Comptonization in the accretion column of the X-ray pulsar GX~1+4

X-ray observations of the binary pulsar GX 1+4 made using the Rossi X-ray Timing Explorer (RXTE) satellite between February 1996 and May 1997 were analysed to quantify source spectral variation with luminosity. Mean Proportional Counter Array (PCA) spectra over the range 2-40 keV are best fitted with a Comptonization model, with source spectrum temperature T_0 approx 1-1.3 keV, plasma temperature T_e approx 6-10 keV, and optical depth tau approx 2-6. The range of fitted T_0 was consistent with the source spectrum originating at the neutron star polar cap, with Compton scattering taking place primarily in the hot plasma of the accretion column. Both the fitted optical depth and plasma temperature vary significantly with the source flux. The variation of the optical depth (and hence the density of the scattering region) with luminosity strongly suggests increasing cross-sectional area of the accretion column at higher accretion rate Mdot. The wide range of source luminosity spanned by archival observations of GX 1+4 offers evidence for two distinct spectral states above and below L_X approx 2*10^{37} erg/s (2-60 keV, assuming a source distance of 10 kpc). GX 1+4 additionally exhibits dramatic hourly variations in neutral column density n_H indicative of density variations in the stellar wind from the giant companion.