Continuum correlations in accreting X-ray pulsars

Abstract

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 Ṁ ranging from 1017 to 1.2 x 1018 g s-1. The observed spectral hardening in the transient X-ray pulsars with increasing Ṁ 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 Ṁ above some critical value Ṁcr ∼ (6 — 8) x 1017 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 Ṁ (and hence the hardness of the continuum monotonically increases) up to higher mass accretion rates than Ṁcr for the filled columns.