A deep X-ray view of the bare AGN Ark 120. I. Revealing the Soft X-ray
  Line Emission
  The Seyfert 1 galaxy, Ark 120, is a prototype example of the so-called class
of bare nucleus AGN, whereby there is no known evidence for the presence of
ionized gas along the direct line of sight. Here deep ($>400$ ks exposure),
high resolution X-ray spectroscopy of Ark 120 is presented, from XMM-Newton
observations which were carried out in March 2014, together with simultaneous
Chandra/HETG exposures. The high resolution spectra confirmed the lack of
intrinsic absorbing gas associated with Ark 120, with the only X-ray absorption
present originating from the ISM of our own Galaxy, with a possible slight
enhancement of the Oxygen abundance required with respect to the expected ISM
values in the Solar neighbourhood. However, the presence of several soft X-ray
emission 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 and
Mg. The He-like line profiles of N, O and Ne appear velocity broadened, with
typical FWHM widths of $\sim5000$ km s$^{-1}$, whereas the H-like profiles are
unresolved. From the clean measurement of the He-like triplets, we deduce that
the broad lines arise from gas of density $n_{\rm e}\sim10^{11}$ cm$^{-3}$,
while the photoionization calculations infer that the emitting gas covers at
least 10 percent of $4\pi$ steradian. Thus the broad soft X-ray profiles appear
coincident with an X-ray component of the optical-UV Broad Line Region on
sub-pc scales, whereas the narrow profiles originate on larger pc scales,
perhaps coincident with the AGN Narrow Line Region. The observations show that
Ark 120 is not intrinsically bare and substantial X-ray emitting gas exists out
of our direct line of sight towards this AGN.
