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Keywords:

  • accretion, accretion discs;
  • galaxies: active;
  • X-rays: galaxies

ABSTRACT

Active galactic nuclei (AGN) and galactic black hole binaries (GBHs) represent two classes of accreting black holes. They both contain an accretion disc emitting a thermal radiation, and a non-thermal X-ray emitting ‘corona’. GBHs exhibit state transitions, and the spectral states are characterized by different luminosity levels and shapes of the spectral energy distribution (SED). If AGN transitioned in a similar way, the characteristic time-scales of such transitions would exceed ∼105 yr. Thus the probability to observe an individual AGN transiting between different spectral states is very low. In this paper we follow the spectral evolution of the GBH GRO J165540 and then apply its SED evolution pattern to a simulated population of AGN under the reasonable assumption that a large sample of AGN should contain a mixture of sources in different spectral states. We model the X-ray spectra of GRO J165540 with the eqpair model and then scale the best-fitting models with the black hole mass to simulate the AGN spectra. We compare the simulated and observed AGN SEDs to determine the spectral states of observed Type 1 AGN, LINER and NLS1 populations. We conclude that bright Type 1 AGN and NLS1 galaxies are in a spectral state similar to the soft spectral state of GBHs, while the spectral state of LINERs may correspond to the hard spectral state of GBHs. We find that taking into account the spread of the black hole masses over several orders of magnitude, as they are in the observed AGN samples, leads to a correlation between the X-ray loudness, αox, and the monochromatic luminosity at 2500 Å. We predict that the αox correlates positively with the Eddington luminosity ratio down to the critical value of λcrit= L/LE≈ 0.01, and that this correlation changes its sign for the accretion rates below λcrit.