Suzaku Studies of the Central Engine in the Typical Type I Seyfert NGC
  3227: Detection of Multiple Primary X-ray Continua with Distinct Properties
  The type I Seyfert galaxy NGC 3227 was observed by Suzaku six times in 2008,
with intervals of $\sim1$ week and net exposures of $\sim50$ ksec each. Among
the six observations, the source varied by nearly an order of magnitude, being
brightest in the 1st observation with a 2-10 keV luminosity of
$1.2\times10^{42}$~erg~s$^{-1}$, while faintest in the 4th with
$2.9\times10^{41}$~erg~s$^{-1}$. As it became fainter, the continuum in a 2-45
keV band became harder, while a narrow Fe-K$\alpha$ emission line, detected on
all occasions at 6.4 keV of the source rest frame, remained approximately
constant in the photon flux. Through a method of variability-assisted
broad-band spectroscopy (e.g., Noda et al. 2013), the 2-45 keV spectrum of NGC
3227 was decomposed into three distinct components. One is a relatively soft
power-law continuum with a photon index of $\sim 2.3$, weakly absorbed and
highly variable on time scales of $\sim5$ ksec; it was observed only when the
source was above a threshold luminosity of $\sim6.6 \times10^{41}$ erg s$^{-1}$
(in 2-10 keV), and was responsible for further source brightening beyond.
Another is a harder and more absorbed continuum with a photon index of $\sim
1.6$, which persisted through the six observations and varied slowly on time
scales of a few weeks by a factor of $\sim2$. This component, carrying a major
fraction of the broad-band emission when the source is below the threshold
luminosity, is considered as an additional primary emission. The last one is a
reflection component with the narrow iron line, produced at large distances
from the central black hole.
