A deep X-ray view of the bare AGN Ark 120. III. X-ray timing analysis and multiwavelength variability

Abstract

We present the spectral/timing properties of the bare Seyfert galaxy Ark 120 through a deep ~420ks XMM-Newton campaign plus recent NuSTAR observations and a ~6-month Swift monitoring campaign. We investigate the spectral decomposition through fractional rms, covariance and difference spectra, finding the mid- to long-timescale (~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 FeK emission red-ward of the FeK-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 ~ 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 of ~900s. 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 2.4 +/- 1.8 days, which we discuss in the context of disc reprocessing.

Citation

(2017). A deep X-ray view of the bare AGN Ark 120. III. X-ray timing analysis and multiwavelength variability. Monthly Notices of the Royal Astronomical Society, 3237-3258. https://doi.org/10.1093/mnras/stx2889

• Acceptance Date: Nov 8, 2017
• Publication Date: Nov 8, 2017
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Publisher: Oxford University Press
• Pages: 3237-3258
• DOI: https://doi.org/10.1093/mnras/stx2889
• Keywords: accretion, accretion discs, X-rays, galaxies
• Publisher URL: https://doi.org/10.1093/mnras/stx2889