Observational Properties Of Puffy Accretion Disc

We have performed a general relativistic radiative magnetohydrodynamics (GRRMHD) simulations of a stable accretion disc around a stellar-mass black hole (BH) with sub-Eddington mass accretion rates. The numerical solutions revealed an elevated vertical structure - above a dense geometrically thin core of dimensionless thickness hρ/r ∼ 0.1, resembling a classic thin accretion disc, a puffed-up, optically and geometrically thick layer of lower density and hτ/r ∼ 1.0 is formed. We refer to this solution as the Puffy disc. We discuss the observational properties of a puffy discs, in particular the geometrical obscuration at higher observing inclinations of the inner area by the puffed-up region, and collimation of radiation along the accretion disc spin axis. These effects may explain the apparent super-Eddington luminosity of some ultraluminous X-ray binaries. We also present synthetic spectra of puffy discs, and show that they are qualitatively similar to Comptonized thin disc spectra. We demonstrate that the existing XSPEC spectral models provide good fits to synthetic spectra of puffy discs but cannot correctly recover the input luminosity, nor the black hole spin. We suggest that puffy discs may correspond to X-ray binary systems in the intermediate spectral state with luminosities above 0.3 of the Eddington luminosity.
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