X‐Ray Luminous Galaxies. II.ChandraandXMM‐NewtonObservations of the “Composite” Galaxy IRAS 20051−1117

We present Chandra and XMM-Newton observations of the compositestar-forming/Seyfert galaxy IRAS20051-1117. The X-ray imaging and spectralproperties reveal the presence of an active nucleus. The Chandra image shows astrong nuclear point source (L(2-10 keV) ~ 4x10^{42} erg s-1). The nuclearX-ray source coincides with a bright, alsoun-resolved, UV source which appearsin the xmm Optical Monitor images. The xmm and chandra spectrum is wellrepresented by a power-law with a photon index of ~1.7-1.9 and a thermalcomponent with a temperature of 0.2 keV. We also detect an Fe line at 6.4 keVwith an equivalent width of ~0.3 keV, typical of the iron lines that have beendetected in the X-ray spectra of "classical" AGN. We find no evidence forshort-term variability in the X-ray light curves, while we detect no variationsbetween the xmm and chandra observations which are separated by about 20 days.Optical spectroscopic observations which were performed ~2.5 months after thexmm observation show that the optical spectrum is dominated by a star-forminggalaxy component, although a weak broad Halpha component is present, inagreement with the results from past observations. The lack of strong AGNsignatures in the optical spectrum of the source can be explained by thedilution of the nuclear AGN emission by the nuclear star-forming component andthe strong emission of the underlying, bright host galaxy.Comment: To appear in ApJ, 20 page