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 $1.2times10^{42}$~erg~s$^{-1}$, while faintest in the 4th with $2.9times10^{41}$~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.

Suzaku Discovery of a Slowly Varying Hard X-ray Continuum from the Type I Seyfert Galaxy NGC 3516

The bright type I Seyfert galaxy NGC 3516 was observed by {it Suzaku} twice, in 2005 October 12--15 and 2009 October 28--November 2, for a gross time coverage of 242 and 544 ksec and a net exposure of 134 and 255 ksec, respectively. The 2--10 keV luminosity was $2.8 times 10^{41}$ erg s$^{-1}$ in 2005, and $1.6 times 10^{41}$ erg s$^{-1}$ in 2009. The 1.4--1.7 keV and 2--10 keV count rates both exhibited peak-to-peak variations by a factor of $sim2$ in 2005, while $sim 4$ in 2009. In either observation, the 15--45 keV count rate was less variable. The 2--10 keV spectrum in 2005 was significantly more convex than that in 2009. Through a count-count-plot technique, the 2--45 keV signals in both data were successfully decomposed in a model-independent way into two distinct broadband components. One is a variable emission with a featureless spectral shape, and the other is a non-varying hard component accompanied by a prominent Fe-K emission line at 6.33 keV (6.40 keV in the rest frame). The former was fitted successfully by an absorbed power-law model, while the latter requires a new hard continuum in addition to a reflection component from distant materials. The spectral and variability differences between the two observations are mainly attributed to long-term changes of this new hard continuum, which was stable on time scales of several hundreds ksec.