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.
