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 \sim 50 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 \sim 1.7 , 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 \sim 2.7 \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 .