We present the spectral/timing properties of the bare Seyfert galaxy Ark 120 through a deep \sim 420 ks XMM-Newton campaign plus recent NuSTAR observations and a \sim 6-month Swift monitoring campaign .
We investigate the spectral decomposition through fractional rms , covariance and difference spectra , finding the mid- to long-timescale ( \sim day–year ) variability to be dominated by a relatively smooth , steep component , peaking in the soft X-ray band .
Additionally , we find evidence for variable Fe K emission red-ward of the Fe K \alpha core on long timescales , consistent with previous findings .
We detect a clearly-defined power spectrum which we model with a power law with a slope of \alpha \sim 1.9 .
By extending the power spectrum to lower frequencies through the inclusion of Swift and RXTE data , we find tentative evidence of a high-frequency break , consistent with existing scaling relations .
We also explore frequency-dependent Fourier time lags , detecting a negative ( ‘ soft ’ ) lag for the first time in this source with the 0.3–1 keV band lagging behind the 1–4 keV band with a time delay , \tau , of \sim 900 s. Finally , we analyze the variability in the optical and UV bands using the Optical/UV Monitor on-board XMM-Newton and the UVOT on-board Swift and search for time-dependent correlations between the optical/UV/X-ray bands .
We find tentative evidence for the U-band emission lagging behind the X-rays with a time delay of \tau = 2.4 \pm 1.8 days , which we discuss in the context of disc reprocessing .