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The Seyfert 1 galaxy, Ark 120, is a prototype example of the so-called classof bare nucleus AGN, whereby there is no known evidence for the presence ofionized gas along the direct line of sight. Here deep ($>400$ ks exposure),high resolution X-ray spectroscopy of Ark 120 is presented, from XMM-Newtonobservations which were carried out in March 2014, together with simultaneousChandra/HETG exposures. The high resolution spectra confirmed the lack ofintrinsic absorbing gas associated with Ark 120, with the only X-ray absorptionpresent originating from the ISM of our own Galaxy, with a possible slightenhancement of the Oxygen abundance required with respect to the expected ISMvalues in the Solar neighbourhood. However, the presence of several soft X-rayemission lines are revealed for the first time in the XMM-Newton RGS spectrum,associated to the AGN and arising from the He and H-like ions of N, O, Ne andMg. The He-like line profiles of N, O and Ne appear velocity broadened, withtypical FWHM widths of $\sim5000$ km s$^{-1}$, whereas the H-like profiles areunresolved. From the clean measurement of the He-like triplets, we deduce thatthe broad lines arise from gas of density $n_{\rm e}\sim10^{11}$ cm$^{-3}$,while the photoionization calculations infer that the emitting gas covers atleast 10 percent of $4\pi$ steradian. Thus the broad soft X-ray profiles appearcoincident with an X-ray component of the optical-UV Broad Line Region onsub-pc scales, whereas the narrow profiles originate on larger pc scales,perhaps coincident with the AGN Narrow Line Region. The observations show thatArk 120 is not intrinsically bare and substantial X-ray emitting gas exists outof our direct line of sight towards this AGN.

A deep X-ray view of the bare AGN Ark 120. I. Revealing the Soft X-ray Line Emission