Alternative Explanations for Extreme Supersolar Iron Abundances Inferred from the Energy Spectrum of Cygnus X-1

• Published in: The Astrophysical journal Vol. 855; no. 1; pp. 3 - 14
• Main Authors: Tomsick, John A., Parker, Michael L., García, Javier A., Yamaoka, Kazutaka, Barret, Didier, Chiu, Jeng-Lun, Clavel, Maïca, Fabian, Andrew, Fürst, Felix, Gandhi, Poshak, Grinberg, Victoria, Miller, Jon M., Pottschmidt, Katja, Walton, Dominic J.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01-03-2018; IOP Publishing; American Astronomical Society
• Subjects: Abundance; accretion, accretion disks; Astrophysics; Binary stars; black hole physics; Companion stars; Density; Electron density; Energy spectra; Inclination; Iron; Parameter modification; Physics; Power law; Reflection; stars: individual (Cygnus X-1); Stellar winds; X-rays: general; X-rays: stars; accretion; X-rays: general; X-rays: stars; black hole physics; stars: individual; accretion disks
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aaaab1

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 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 ( cm−3) impacts other parameters such as the inner radius and inclination of the disk.