Continuum correlations in accreting X-ray pulsars

Abstract

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 ratesṀ ranging from 10 17 to 1.2 × 10 18 g s −1 . The observed spectral hardening in the transient X-ray pulsars with increasingṀ 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. AtṀ above some critical valueṀcr ∼ (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 withṀ (and hence the hardness of the continuum monotonically increases) up to higher mass accretion rates thanṀcr for the filled columns. ObservationsAfter the discovery of X-ray pulsars in 1971 [1] it was realized that in bright pulsars the radiation plays a crucial role in braking of the accreting matter onto the surface of a neutron star with strong magnetic field. It is now well recognized that once the radiation density above the polar cap starts playing the role in the accreting matter dynamics, an optically thick accretion column above the polar cap is formed [2] (see [3] for the latest investigation). The characteristic height of the column increases with accretion rate, most of the emission escapes through sidewalls, and it can be expected that above some X-ray luminosity the dependence of the observed properties of the continuum emission on the X-ray luminosity can become different from that in the lowluminosity regime.Indeed, a similar bimodality is observed in the dependence of the X-ray continuum hardness on luminosity. This dependence can be studied using, for example, data from all-sky monitors such as RXTE/ASM and MAXI, i.e. without dedicated spectroscopic observations which are necessary to measure CRSFs. Following this approach, we measured the hardness ratio as a function of luminosity in the accreting pulsars GX 304−1, 4U 0115+63, V 0332+53, EXO 2030+375, A 0535+26 and MXB 0656−072 using the data from different energy bands of RXTE/ASM. The result is shown in Fig. 1. To convert the ASM count rates into X-ray luminosities, we used published distances to the sources (GX 304−1: ∼2 kpc, [4];