  We present Chandra and XMM-Newton observations of the composite
star-forming/Seyfert galaxy IRAS20051-1117. The X-ray imaging and spectral
properties reveal the presence of an active nucleus. The Chandra image shows a
strong nuclear point source (L(2-10 keV) ~ 4x10^{42} erg s-1). The nuclear
X-ray source coincides with a bright, alsoun-resolved, UV source which appears
in the xmm Optical Monitor images. The xmm and chandra spectrum is well
represented by a power-law with a photon index of ~1.7-1.9 and a thermal
component with a temperature of 0.2 keV. We also detect an Fe line at 6.4 keV
with an equivalent width of ~0.3 keV, typical of the iron lines that have been
detected in the X-ray spectra of "classical" AGN. We find no evidence for
short-term variability in the X-ray light curves, while we detect no variations
between the xmm and chandra observations which are separated by about 20 days.
Optical spectroscopic observations which were performed ~2.5 months after the
xmm observation show that the optical spectrum is dominated by a star-forming
galaxy component, although a weak broad Halpha component is present, in
agreement with the results from past observations. The lack of strong AGN
signatures in the optical spectrum of the source can be explained by the
dilution of the nuclear AGN emission by the nuclear star-forming component and
the strong emission of the underlying, bright host galaxy.
