On the dependence of the X-ray continuum variations with luminosity in accreting X-ray pulsars

Using RXTE/ASM archival data, we investigate the behaviour of the spectral hardness ratio as a function of X-ray luminosity in a sample of six transient X-ray pulsars (EXO 2030+375, GX 304-1, 4U 0115+63, V 0332+63, A 0535+26 and MXB 0656-072). In all sources we find that the spectral hardness ratio defined as F_{5-12 keV}/F_{1.33-3 keV} increases with the ASM flux (1.33-12 keV) at low luminosities and then saturates or even slightly decreases above some critical X-ray luminosity falling into the range ∼(3-7) × 10³⁷ erg s^-1. Two-dimensional structure of accretion columns in the radiation-diffusion limit is calculated for two possible geometries (filled and hollow cylinder) for mass accretion rates dot{M} ranging from 10¹⁷ to 1.2 × 10¹⁸ g s^-1. The observed spectral behaviour in the transient X-ray pulsars with increasing dot{M} can be reproduced by a Compton-saturated sidewall emission from optically thick magnetized accretion columns with taking into account the emission reflected from the neutron star atmosphere. At dot{M} above some critical value dot{M}_cr∼ (6-8)× 10^{17} g s^-1, the height of the column becomes such that the contribution of the reflected component to the total emission starts decreasing, which leads to the saturation and even slight decrease of the spectral hardness. Hollow-cylinder columns have a smaller height than the filled-cylinder ones, and the contribution of the reflected component in the total emission does not virtually change with dot{M} (and hence the hardness of the continuum monotonically increases) up to higher mass accretion rates than dot{M}_cr for the filled columns.