Long-term Spectral Variability of the Seyfert I Galaxies observed with ASCA

K.~Misaki$^1$, H.~Kunieda$^1$, and Y.~Haba$^2$

$^1$ISAS, Sagamihara, Kanagawa, Japan
$^2$Nagoya University, Chikusa, Nagoya, Japan


Most of AGN are variable objects, so investigation of spectral
variability provides us with crucial keys to reveal emission
mechanisms of AGN.  We examine a time behavior of the iron line and a
spectral variability of the power-law continuum of Seyfert 1 galaxies
observed by {\it ASCA}.  According to the diskline model, physical
conditions in the vicinity of the black hole might be reflected in the
line profile.
The long observations of NGC~5548 were performed twice in 1996 and
1998. So it is suitable to study the spectral feature and time
variability of Seyfert 1 galaxies in detail.  There is no significant
positive correlation between the iron line intensity and the central
luminosity as is expected from the simple reflection model.  
The 2--10~keV flux of Mrk~841 keeps the same level between two
occasions with half an year interval, the apparent photon index
changed from 1.8 to 1.6~. In accordance with this spectral hardening,
the red-wing of the iron line seems to disappear. 
These spectral changes suggest that not only luminosity but a
continuum slope is a key parameter.
Possible scenarios are discussed to accommodate both the spectral 
change of the continuum and the line profile behavior.