Accretion disk-corona models: X-ray variability amplitude 
and spectral variability

Kawaguchi T.$^1$

$^1$Department of Astronomy, Kyoto University, Japan

We present two topics related to the X-ray variability 
in accretion disk-coronae of AGNs from theoretical point of views.
[1] It is numerically shown that the magnetic turbulence 
in an accretion flow approaches towards a saturating level 
where the mean magnetic pressure $\simeq$ 10\% of gas pressure.
We analyzed the simulated MHD flow at the quasi-steady state, 
and found the clumpy, fractal spatial structure and 
temporal power spectral density in a power-law fashion.
Assuming that the thermal energy in the flow is proportional to 
mean X-ray flux and that magnetic fields in each clump 
provide sporadic energy release, 
the fractional variability amplitude at certain timescale 
[e.g., $10 \, R_{\rm Sch}/V_{\rm Kepler}(10R_{\rm Sch}) 
\, \propto M_{\rm BH}$] does not depend on $M_{\rm BH}$.
That can be the basic physics behind the $M_{\rm BH}$-
determination method by Hayashida et al.
[2] We constructed an accretion disk-corona model which accounts for the
spectral energy distribution of AGNs from optical to hard X-ray 
simultaneously. 
The model exhibits different spectral slopes in X-ray 
($\alpha \sim$ 1.5 below 2keV and $\sim$ 0.5 above, 
where $F_{\nu} \propto \nu^{- \alpha}$) as the results of
different emission mechanisms and different sites; the former slope is
due to unsaturated Comptonization from the innermost zone and the
latter is due to a combination of the Comptonization and bremsstrahlung
from the entire corona ($\leq$ 300 $R_{\rm Sch}$).
The X-ray spectral variability expected from this model will
be discussed.