Two Major Constraints on the Inner Radii of Accretion Disks

• Published in: Astrophysical journal. Letters 2020-06, Vol.896 (2), p.L36
• Main Authors: Zdziarski, Andrzej A., De Marco, Barbara
• Format: Article
• Language: English
• Subjects: Accretion; Accretion disks; Black holes; Blackbody; Circular orbits; Color temperature; Constraint modelling; Corona; Fluorescence; Ionization; Luminosity; Non-thermal radiation sources; Orbital stability; Reflection; X ray binaries; X ray stars; X-ray binary stars
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/ab9899

The Stefan-Boltzmann law yields a fundamental constraint on the geometry of inner accretion disks in black hole X-ray binaries. It follows from considering the irradiating flux and the effective temperature of the inner parts of the disk, which implies that a strong quasi-thermal component with the average energy higher than that of a blackbody at the effective temperature has to be present whenever relativistic Fe K fluorescence and reflection features are observed. The apparent absence of such quasi-thermal component with the color temperature of ∼1 keV in high-luminosity hard states is not compatible with a strongly irradiated disk extending close to the innermost stable circular orbit. Instead, the disk should be either truncated at a relatively large radius or irradiated by a corona at a large height, which would reduce the effective temperature and bring it to an agreement with the data. We also study constraints on disk/corona models following from comparing the disk densities fitted in literature using variable-density reflection codes with those calculated by us from the ionization parameter, the luminosity, and the disk inner radius. We find that the fitted densities are much higher/lower in the hard/soft state of binaries, implying significant problems with the used assumptions and methods.