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

Abstract

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 $F_{5-12\mathrm{keV}}/F_{1.33-3\mathrm{keV}}$ 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 $\sim (3-7)\times 10^{37}$~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 $10^{17}$ to 1.2$\times 10^{18}$~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 ${\dot{M}}_{cr}\sim (6-8)\times 10^{17}$~g~s${}^{-1}$, the hight 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.

11 pages, 12 figures, accepted to MNRAS

References in corpus (6)

• The critical accretion luminosity for magnetized neutron stars
• Discovery of a flux-related change of the cyclotron line energy in Her X-1
• Patterns of variability in Be/X-ray pulsars during giant outbursts
• 4U 0115+63 from RXTE and INTEGRAL Data: Pulse Profile and Cyclotron Line Energy
• Transient X-ray pulsar V0332+53: pulse phase-resolved spectroscopy and the reflection model
• Study of the cyclotron feature in MXB 0656-072

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• Discovery and Modeling of a Flattening of the Positive Cyclotron Line/Luminosity Relation in GX 304-1 with RXTE
• Dynamical and Radiative Properties of X-ray Pulsar Accretion Columns: Phase-Averaged Spectra
• Accretion regimes in the X-ray pulsar 4U 1901+03
• Spectral and Timing Analysis of the accretion-powered pulsar 4U 1626-67 observed with Suzaku and NuSTAR
• Looking at A 0535+26 at low luminosities with NuSTAR
• Understanding the nature of the intriguing source X Persei: A deep look with a Suzaku observation
• Evidence for different accretion regimes in GRO J1008-57
• A Study of the 20 Day Superorbital Modulation in the High-Mass X-ray Binary IGR J16493-4348
• The giant outburst of 4U 0115+634 in 2011 with Suzaku and RXTE
• Variability in high-mass X-ray binaries