Spectral Formation in Accreting X-Ray Pulsars: Bimodal Variation of the Cyclotron Energy with Luminosity

TL;DR

This paper investigates the bimodal spectral variability in accreting X-ray pulsars, linking it to radiation pressure and Coulomb interactions, and introduces a new critical luminosity formula to explain observed behaviors.

Contribution

It proposes a new expression for critical luminosity that distinguishes supercritical and subcritical accretion regimes, explaining the bimodal CRSF energy variation.

Findings

Confirmed group 1 sources are supercritical and group 2 are subcritical.

Derived a new formula for critical luminosity based on magnetic field estimates.

Explained CRSF energy variation as due to changes in emission height with opposite signs in different regimes.

Abstract

Accretion-powered X-ray pulsars exhibit significant variability of the Cyclotron Resonance Scattering Feature (CRSF) centroid energy on pulse-to-pulse timescales, and also on much longer timescales. Two types of spectral variability are observed. For sources in group 1, the CRSF energy is negatively correlated with the variable source luminosity, and for sources in group 2, the opposite behavior is observed. The physical basis for this bimodal behavior is currently not understood. We explore the hypothesis that the accretion dynamics in the group 1 sources is dominated by radiation pressure near the stellar surface, and that Coulomb interactions decelerate the gas to rest in the group 2 sources. We derive a new expression for the critical luminosity such that radiation pressure decelerates the matter to rest in the supercritical sources. The formula for the critical luminosity is evaluated for 5 sources, using the maximum value of the CRSF centroid energy to estimate the surface magnetic field strength. The results confirm that the group 1 sources are supercritical and the group 2 sources are subcritical, although the situation is less clear for those highly variable sources that cross over the critical line. We also explain the variation of the CRSF energy with luminosity as a consequence of the variation of the characteristic emission height. The sign of the height variation is opposite in the supercritical and subcritical cases, hence creating the observed bimodal behavior.