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We perform an X-ray spectral analysis of the brightest and cleanest bare AGNknown so far, Ark 120, in order to determine the process(es) at work in thevicinity of the SMBH. We present spectral analysis of data from an extensivecampaign observing Ark 120 in X-rays with XMM-Newton (4$\times$120 ks, 2014March 18-24), and NuSTAR (65.5 ks, 2014 March 22). During this very deep X-raycampaign, the source was caught in a high flux state similar to the earlier2003 XMM-Newton observation, and about twice as bright as the lower-fluxobservation in 2013. The spectral analysis confirms the "softer when brighter"behaviour of Ark 120. The four XMM-Newton/pn spectra are characterized by thepresence of a prominent soft X-ray excess and a significant FeK$\alpha$complex. The continuum is very similar above about 3 keV, while significantvariability is present for the soft X-ray excess. We find that relativisticreflection from a constant-density, flat accretion disk cannot simultaneouslyproduce the soft excess, broad FeK$\alpha$ complex, and hard X-ray excess.Instead, Comptonization reproduces the broadband (0.3-79 keV) continuum well,together with a contribution from a mildly relativistic disk reflectionspectrum. During this 2014 observational campaign, the soft X-ray spectrum ofArk 120 below $\sim$0.5 keV was found to be dominated by Comptonization of seedphotons from the disk by a warm ($kT_{\rm e}$$\sim$0.5 keV), optically-thickcorona ($\tau$$\sim$9). Above this energy, the X-ray spectrum becomes dominatedby Comptonization from electrons in a hot optically thin corona, while thebroad FeK$\alpha$ line and the mild Compton hump result from reflection off thedisk at several tens of gravitational radii.

A deep X-ray view of the bare AGN Ark120. IV. XMM-Newton and NuSTAR spectra dominated by two temperature (warm, hot) Comptonization processes