Long Term X-ray Variability of Narrow Line Seyfert 1 Galaxies

T. Horikawa$^1$, K. Hayashida$^1$, and H. Katayama$^1$ 

$^1$Osaka University, Toyonaka, Osaka, Japan


X-ray variability of AGNs is a subject of extensive study, on
power spectra, energy dependence, delay time between the soft and the 
hard band, {\it etc.}. However, the variability time scales investigated
so far have been limited to minutes to days, i.e., typical data length
of X-ray observations of AGNs, in most cases.  We look into the X-ray 
variability
of AGNs over years, by employing the archival data of previous X-ray missions.
In this paper, we focus on the long term variability of narrow line Seyfert 1
(NLS1) galaxies particularly, of which X-ray properties (steep X-ray spectrum, 
rapid and large
amplitude variability within a day) are distinctive to those of broad 
line Seyfert 1 (BLS1) galaxies.

We have constructed X-ray long term light curves of 15 NLS1 sources so far. 
It is found that half the NLS1 sources show variability exceeding one order of 
magnitudes,
which contrast to the fact that most of the BLS1 sources show at most 
factor of two variabilities over years. We also note that some NLS1 
sources show
monotonically decrease in X-ray flux over years. In this paper, we will show
the long term X-ray light curves of NLS1 sources, hopefully larger sample 
than 15 sources, and will discuss its implications on the nature of the NLS1 sources.