X-ray and Optical Correlation of Type I Seyfert NGC 3516 Studied with
  Suzaku and Japanese Ground-Based Telescopes
  From 2013 April to 2014 April, we performed an X-ray and optical simultaneous
monitoring of the type 1.5 Seyfert galaxy NGC 3516. It employed Suzaku, and 5
Japanese ground-based telescopes, the Pirka, Kiso Schmidt, Nayuta, MITSuME, and
the Kanata telescopes. The Suzaku observations were conducted seven times with
various intervals ranging from days, weeks, to months, with an exposure of
$\sim50$ ksec each. The optical $B$-band observations not only covered those of
Suzaku almost simultaneously, but also followed the source as frequently as
possible. As a result, NGC 3516 was found in its faint phase with the 2-10 keV
flux of $0.21-2.70 \times 10^{-11}$ erg s$^{-1}$ cm$^{-2}$. The 2-45 keV X-ray
spectra were composed of a dominant variable hard power-law continuum with a
photon index of $\sim1.7$, and a non-relativistic reflection component with a
prominent Fe-K$\alpha$ emission line. Producing the $B$-band light curve by
differential image photometry, we found that the $B$-band flux changed by
$\sim2.7 \times 10^{-11}$ erg s$^{-1}$ cm$^{-2}$, which is comparable to the
X-ray variation, and detected a significant flux correlation between the hard
power-law component in X-rays and the $B$-band radiation, for the first time in
NGC 3516. By examining their correlation, we found that the X-ray flux preceded
that of $B$ band by $2.0^{+0.7}_{-0.6}$ days ($1\sigma$ error). Although this
result supports the X-ray reprocessing model, the derived lag is too large to
be explained by the standard view which assumes a "lamppost"-type X-ray
illuminator located near a standard accretion disk. Our results are better
explained by assuming a hot accretion flow and a truncated disk.
