  We present results from observations of SN 1979C with XMM-Newton in X-rays
and in the UV, archival X-ray and HST data, and follow-up ground-based optical
imaging. The XMM-Newton MOS spectrum shows two-temperature thermal plasma
emission characteristics of both the forward (4.1 keV) and reverse shock (0.78
keV) with no intrinsic absorption. The long-term X-ray lightcurve of SN 1979C
shows no sign of a decline over 16-23 yrs after its outburst. The high X-ray
luminosity (8 x 10^38 ergs/s) is caused by the interaction of the shock with
dense circumstellar matter, likely deposited by a strong stellar wind from the
progenitor with a high mass-loss rate of 1.5 x 10^-4 M_sun/yr. The X-ray data
support a strongly decelerated shock and show a mass-loss rate history which is
consistent with a constant progenitor mass-loss rate and wind velocity over the
past >16,000 yrs in the evolution of the progenitor. We find a CSM density
profile of rho_CSM \propto r^-s with index s<1.7 and high CSM densities (>10^4
cm^-3) out to large radii from the site of the explosion (> 4 x 10^17 cm).
Using XMM-Newton Optical Monitor data we further detect optical/UV emission
from SN 1979C, with B, U, and UVW1-band lum. of 5,7, and 9 x 10^36 ergs/s. The
young stellar cluster in the vicinity of the SN, as imaged by the HST and
follow-up ground-based optical imaging, can only provide a fraction of the
total observed flux, so that a significant contribution to the output likely
arises from the strong interaction of SN 1979C with dense CSM.
