Alternative explanations for extreme supersolar iron abundances inferred from the energy spectrum of Cygnus X-1

TL;DR

This study analyzes Cygnus X-1's X-ray spectrum, revealing that previous high iron abundance estimates are likely unphysical and can be explained by considering higher disk densities in spectral models.

Contribution

The paper introduces a modified reflection model with free electron density, providing more realistic iron abundance estimates for Cygnus X-1.

Findings

Previous models overestimated iron abundance due to fixed density assumptions.

Higher disk density models yield solar iron abundances, resolving previous discrepancies.

The inner disk radius and inclination are affected by the new density parameter.

Abstract

Here we study a 1-200 keV energy spectrum of the black hole binary Cygnus X-1 taken with NuSTAR and Suzaku. This is the first report of a NuSTAR observation of Cyg X-1 in the intermediate state, and the observation was taken during the part of the binary orbit where absorption due to the companion's stellar wind is minimal. The spectrum includes a multi-temperature thermal disk component, a cutoff power-law component, and relativistic and non-relativistic reflection components. Our initial fits with publicly available constant density reflection models (relxill and reflionx) lead to extremely high iron abundances (>9.96 and 10.6(+1.6)(-0.9) times solar, respectively). Although supersolar iron abundances have been reported previously for Cyg X-1, our measurements are much higher and such variability is almost certainly unphysical. Using a new version of reflionx that we modified to make the electron density a free parameter, we obtain better fits to the spectrum even with solar iron abundances. We report on how the higher density (n_e = (3.98(+0.12)(-0.25))E20 cm-3) impacts other parameters such as the inner radius and inclination of the disk.