  We report on a long (100 ks) XMM-Newton observation of the bright Seyfert 1
galaxy Arakelian 120. The source previously showed no signs of intrinsic
reddening in its infrared-ultraviolet continuum and previous observations had
shown no evidence for ionized absorption in either the ultraviolet or X-ray
bands. The new XMM-Newton RGS data place tight limits on the presence of an
ionized X-ray absorber and confirm that the X-ray spectrum of Ark 120 is
essentially unmodified by intervening matter. Thus Ark 120 can be considered a
`bare' Seyfert 1 nucleus. This observation therefore offers a clean view of the
X-ray spectrum of a `normal' Seyfert galaxy free from absorption effects. The
spectrum shows a Doppler broadened iron emission line (FWHM ~ 3*10^4 km/s) and
a smooth, continuous soft excess which appears to peak at an energy ~0.5 keV.
This adds weight to the claim that genuine soft excesses (i.e. those due to a
real steepening of the underlying continuum below ~2 keV) are ubiquitous in
Seyfert 1 spectra. However, the detailed shape of the excess could not be
reproduced by any of the simple models tested (power-laws, blackbodies,
Comptonised blackbodies, accretion disc reflection). This observation therefore
demonstrates both the need to understand the soft excess (as a significant
contributor to the luminosity of most Seyfert 1s) and the inability of the
existing, simple models to explain it.
