Continuum correlations in accreting X-ray pulsars

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 M ranging from 10(17) to 1.2 x 10(18) g s(-1). The observed spectral hardening in the transient X-ray pulsars with increasing (M)over dot 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 (M)over dot above some critical value (M)over dot(cr) similar to (6 - 8) x 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 (M)over dot (and hence the hardness of the continuum monotonically increases) up to higher mass accretion rates than (M)over dot(cr) for the filled columns.