Energy-dependent variability of the bare Seyfert 1 galaxy Ark 120

Abstract

We present results from a detailed spectral-timing analysis of a long ∼ 486 ks XMM-Newton observation of the bare Seyfert 1 galaxy Ark 120 which showed alternating diminution and increment in the 0.3−10 keV X-ray flux over four consecutive orbits in 2014. We study the energy-dependent variability of Ark 120 through broad-band X-ray spectroscopy, fractional root-mean-squared (rms) spectral modelling, hardness−intensity diagram and flux−flux analysis. The X-ray (0.3−10 keV) spectra are well fitted by a thermally Comptonized primary continuum with two (blurred and distant) reflection components and an optically thick, warm Comptonization component for the soft X-ray excess emission below ∼ 2 keV. During the first and third observations, the fractional X-ray variability amplitude decreases with energy while for second and fourth observations, X-ray variability spectra are found to be inverted-crescent and crescent shaped respectively. The rms variability spectra are well modelled by two constant reflection components, a soft excess component with variable luminosity and a variable intrinsic continuum with the normalization and spectral slope being correlated. The spectral softening of the source with both the soft excess and UV luminosities favour Comptonization models where the soft excess and primary X-ray emission are produced through Compton up-scattering of the UV and UV/soft X-ray seed photons in the putative warm and hot coronae, respectively. Our analyses imply that the observed energy-dependent variability of Ark 120 is most likely due to variations in the spectral shape and luminosity of the hot corona and to variations in the luminosity of the warm corona, both of which are driven by variations in the seed photon flux.

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

“…Both the difference and the rms spectra clearly confirm that the main source of the variability in the 2014 campaign is due to a soft power-law-like component which primarily varies in flux. Given the relative smoothness of the component and the fact that it can be fitted with a steep power law, this is likely consistent with the cooler Comptonization component detected in paper IV (also see Mallick et al 2017). The soft component appears to dominate the variability on timescales corresponding to frequencies in the range ν ∼ 0.77 − 5 × 10 −5 Hz (corresponding to timescales in the range: 10-130 ks) and also between observations on timescales of ∼days and ∼years.…”

Modelling X-ray RMS spectra II: the ultrafast outflow of PDS 456

“…This restricts the results that can be obtained, and makes comparison between different spectra difficult. The typical analysis of such a spectrum is a qualitative examination by eye, and a more sophisticated analysis will include comparison with a simulated spectrum based on the model fit to the count spectrum (e.g., Vaughan & Fabian 2004;Matzeu et al 2016;Yamasaki et al 2016;Mallick et al 2017;Alston et al 2020).…”

Classifying AGN by X-ray hardness variability

“…McHardy et al (1999) measure softening of spectral slope with intensity in two AGN, MCG 6-30-15 and NGC 5506, drawing an analogue with BHXRB. Two more examples are Emmanoulopoulos et al (2012) which find a harder-when-brighter behavior for NGC 7213, and Mallick et al (2017) who find a softer-whenbrighter behavior for Ark 120. Unlike BHXRBs, due to the longer timescale no single AGN can been observed to transition in a full HR cycle.…”