What can we learn about quasars from alpha_OX measurements in galactic black hole binaries?

We draw a comparison between AGN and Galactic black hole binaries using a uniform description of spectral energy distribution of these two classes of accreting X-ray sources. We parametrize spectra of GBHs with an alpha_GBH parameter which we define as a slope of a nominal power law function between 3 and 20 keV. We show that this parameter can be treated as an equivalent of the X-ray loudness, alpha_OX, used to describe AGN spectra. We do not find linear correlation between the alpha_GBH and disc flux (similar to that between alpha_OX and optical/UV luminosity found in AGN). Instead, we show that alpha_GBH follows a well defined pattern during a GBH outburst. We find that alpha_GBH tend to cluster around 1, 1.5 and 2, which correspond to a hard, very high/intermediate and soft spectral state, respectively. We conclude that majority of the observed Type 1 radio quiet AGN are in a spectral state corresponding to a very high/intermediate state of GBHs. The same conclusion is valid for radio loud AGN. We also study variations of the spectral slopes (alpha_GBH and the X-ray photon index, Gamma) as a function of disc and Comptonization fluxes. We discuss these dependencies in the context of correlations of alpha_OX and Gamma with the optical/UV and X-ray 2 keV fluxes considered for AGN and quasars.

💡 Research Summary

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The paper presents a systematic comparison between active galactic nuclei (AGN) and Galactic black‑hole binaries (GBHs) by introducing a unified spectral descriptor that can be applied to both classes of accreting black‑hole systems. In AGN studies the X‑ray loudness parameter α_OX – the logarithmic slope between the monochromatic flux at 2500 Å (optical/UV) and at 2 keV (X‑ray) – is widely used to quantify the relative strength of the high‑energy corona compared with the thermal accretion‑disk emission. Because GBHs are observed primarily in the X‑ray band, a direct analogue of α_OX cannot be defined using the same wavelengths. The authors therefore define a new parameter, α_GBH, as the slope of a hypothetical power‑law connecting the observed fluxes at 3 keV and 20 keV:

α_GBH = –log