  The supermassive black holes in active galactic nuclei (AGN) and stellar-mass
black holes in X-ray binaries (XRBs) are believed to work in a similar way.
While XRBs evolve rapidly and several sources have undergone a few complete
cycles from quiescence to an outburst and back, most AGN remain in the same
state over periods of decades, due to their longer characteristic timescale
proportional to their size. However, the study of the AGN spectral states is
still possible with a large sample of sources. Multi-wavelength observations
are needed for this purpose since the AGN thermal disc emission dominates in
the ultraviolet energy range, while the up-scattered hot-corona emission is
detected in X-rays. We compared simultaneous UV and X-ray measurements of AGN
obtained by the XMM-Newton satellite. The non-thermal flux was constrained from
the 2-12 keV X-ray luminosity, while the thermal disc component was estimated
from the UV flux at 2900A. The hardness (ratio between the X-ray and UV plus
X-ray luminosity) and the total luminosity were used to construct the AGN state
diagrams. For sources with reliable mass measurements, the Eddington ratio was
used instead of the total luminosity. The state diagrams show that the
radio-loud sources have on average higher hardness, due to the lack of the
thermal disc emission in the UV band, and have flatter intrinsic X-ray spectra.
In contrast, the sources with high luminosity and low hardness are radio-quiet
AGN. The hardness-Eddington ratio diagram reveals that the average
radio-loudness is stronger for low-accreting sources, while it decreases when
the accretion rate is close to the Eddington limit. Our results indicate that
the general properties of AGN accretion states are similar to those of X-ray
binaries. This suggests that the AGN radio dichotomy of radio-loud and
radio-quiet sources can be explained by the evolution of the accretion states.
