Context :

Aims : [ Abstract Truncated ] We perform an X-ray spectral analysis of the brightest and cleanest bare AGN known so far , Ark 120 , in order to determine the process ( es ) at work in the vicinity of the SMBH .

Methods : We present spectral analysis of data from an extensive campaign observing Ark 120 in X-rays with XMM-Newton ( 4 \times 120 ks , 2014 March 18–24 ) , and NuSTAR ( 65.5 ks , 2014 March 22 ) .

Results : During this very deep X-ray campaign , the source was caught in a high flux state similar to the earlier 2003 XMM-Newton observation , and about twice as bright as the lower-flux observation in 2013 .
The spectral analysis confirms the “ softer when brighter ” behaviour of Ark 120 .
The four XMM-Newton /pn spectra are characterized by the presence of a prominent soft X-ray excess and a significant Fe K \alpha complex .
The continuum is very similar above about 3 keV , while significant variability is present for the soft X-ray excess .
We find that relativistic reflection from a constant-density , flat accretion disk can not simultaneously produce the soft excess , broad Fe K \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 reflection spectrum .

Conclusions : During this 2014 observational campaign , the soft X-ray spectrum of Ark 120 below \sim 0.5 keV was found to be dominated by Comptonization of seed photons from the disk by a warm ( kT _ { e } \sim 0.5 keV ) , optically-thick corona ( \tau \sim 9 ) .
Above this energy , the X-ray spectrum becomes dominated by Comptonization from electrons in a hot optically thin corona , while the broad Fe K \alpha line and the mild Compton hump result from reflection off the disk at several tens of gravitational radii .