![]() However, information about the geometry, structure and physical processes that dominate these systems can be indirectly inferred via alternative methods. Given the compact nature of AGN and the great distances at which we observe them, their central regions cannot be directly resolved with the current generation of observatories. This thermal UV emission is then thought to be responsible for producing the X-ray continuum, which is commonly power law in shape, via inverse-Compton scattering of soft thermal photons via an optically thin ‘corona’ of hot ( T ∼ 10 9 K) electrons, usually within a few tens of r g from the black hole (Haardt & Maraschi 1993). The location of the UV-emitting material is typically 10–1 000 r g 1 from the central black hole, depending on the properties of the accretion flow. Normally, peaking at ultraviolet (UV) wavelengths, the dominant energy output of Seyfert galaxies is generally considered to arise from thermal emission from material in the inner parts of a geometrically thin, optically thick accretion disc surrounding the supermassive black hole (SMBH Shakura & Sunyaev 1973). We find tentative evidence for the U-band emission lagging behind the X-rays with a time delay of τ = 2.4 ± 1.8 d, which we discuss in the context of disc reprocessing.Īccretion, accretion discs, galaxies: active, galaxies: Seyfert, X-rays: galaxies 1 INTRODUCTIONĪctive galactic nuclei (AGN) are routinely observed to emit strongly across the entire electromagnetic spectrum with the broad-band spectral energy distribution comprising both thermal and non-thermal emission components (Shang, Brotherton & Wills 2011). Finally, we analyse the variability in the optical and ultraviolet (UV) bands using the Optical/UV Monitor onboard XMM–Newton and the Ultra-Violet/Optical Telescope onboard Swift and search for time-dependent correlations between the optical/UV/X-ray bands. 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, τ, of ∼900 s. By extending the power spectrum to lower frequencies through the inclusion of Swift and Rossi X-ray Timing Explorer data, we find tentative evidence of a high-frequency break, consistent with existing scaling relations. We detect a clearly defined power spectrum which we model with a power law with a slope of α ∼ 1.9. ![]() Additionally, we find evidence for variable Fe K emission redward of the Fe Kα core on long time-scales, consistent with previous findings. ![]() We investigate the spectral decomposition through fractional rms, covariance and difference spectra, finding the mid- to long-time-scale (∼day–year) variability to be dominated by a relatively smooth, steep component, peaking in the soft X-ray band. ![]() We present the spectral/timing properties of the bare Seyfert galaxy Ark 120 through a deep ∼420 ks XMM–Newton campaign plus recent NuSTAR observations and a ∼6-month Swift monitoring campaign. ![]()
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