X-Ray Luminous Galaxies. II. Chandra and XMM-Newton Observations of the ``Composite'' Galaxy IRAS 20051-1117

We present Chandra and XMM-Newton observations of the composite star-forming/Seyfert galaxy IRAS 20051-1117. The X-ray imaging and spectral properties reveal the presence of an active nucleus. The Chandra image shows a strong nuclear point source [L(2-10keV)~4×10⁴²ergss^-1]. The nuclear X-ray source coincides with a bright, also unresolved, UV source that appears in the XMM-Newton Optical Monitor images. The XMM-Newton and Chandra spectrum is well represented by a power law with a photon index of Γ~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'' active galactic nuclei (AGNs). We find no evidence for short-term variability in the X-ray light curves, while we detect no variations between the XMM-Newton and Chandra observations, which are separated by about 20 days. Optical spectroscopic observations that were performed ~2 months after the XMM-Newton observation show that the optical spectrum is dominated by a star-forming galaxy component, although a weak broad Hα 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.