Energy dependent variability of the bare Seyfert 1 galaxy Ark 120

We present results from a detailed spectral-timing analysis of a long ($\sim486$\ks{}, consisting of four observations each with $\sim120$\ks{}) \xmm{} observation of the bare Seyfert~1 galaxy Ark~120 which showed diminution and increment in 0.3$-$10\keV{} X-ray flux alternatively during 2014 March. We study the energy dependent variability of Ark~120 through broadband X-ray spectroscopy, fractional root mean square (rms) spectral modelling, hardness$-$intensity diagram and flux$-$flux analysis. The X-ray (0.3$-$10\keV{}) spectra are well fitted by a power-law continuum with two reflection components and an additional Comptonization model to explain the soft X-ray excess emission below $\sim2$\keV{}. During first and third observations, the X-ray fractional rms variability 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 a soft excess component with variable luminosity and a variable intrinsic power-law continuum with the normalization and spectral slope being correlated. The source hardness decreases with increasing luminosity as expected from a radio-quiet active galaxy. The flux due to more ionized reflection is found to be correlated with the continuum flux on the observed $\sim7.5$~days timescale. Our results imply that the observed energy dependent variability of Ark~120 is likely to be the result of variations in the spectral shape and intrinsic luminosity of the primary X-ray emitting hot corona as well as the luminosity of the soft excess emitting warm corona.

Comments: 14 pages, 15 figures, 6 tables, Submitted to MNRAS

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