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

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 nonrelativistic reflection components. Our initial fits with publicly available constant density reflection models (relxill and reflionx) lead to extremely high iron abundances (>9.96 and ${10.6}_{-0.9}^{+1.6}$ 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 (${n}_{e}=({3.98}_{-0.25}^{+0.12})\times {10}^{20}$ cm−3) impacts other parameters such as the inner radius and inclination of the disk.

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ORCID for Poshak Gandhi: orcid.org/0000-0003-3105-2615

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