X-ray evidence of a buried active galactic nucleus in UGC 5101

We present X-ray observations of the ultraluminous infrared galaxy, UGC 5101, thought to contain a buried active galactic nucleus (AGN) based on observations in other wave bands. We detected an absorbed hard component at greater than 3 keV, as well as soft emission in the energy range 0.5 - 2 keV. The soft X-ray component, possibly due to a modestly dust-obscured extended starburst, has an absorption-corrected 0.5 - 2 keV X-ray luminosity of L-X(0.5 - 2 keV) = 1.2 x 10(41) ergs s(-1). The 0.5 - 2 keV X-ray - to - infrared luminosity ratio is a factor of similar to 5 lower than typical values for a normal starburst, suggesting that this extended starburst is unlikely to be energetically dominant in UGC 5101. The most plausible origin of the absorbed hard component is the putative buried AGN. The 6.4 keV Fe Kalpha emission line has a modest equivalent width (similar to400 eV), suggesting that this hard component is direct emission from the AGN, rather than a scattered component. The absorption-corrected 2 - 10 keV X-ray luminosity of the buried AGN was estimated to be L-X(2 - 10 keV) similar to5 x 10(42) ergs s(-1). The intrinsic 2 - 10 keV X-ray luminosity and the 2 - 10 keV X- ray - to - infrared luminosity ratio are both comparable to values measured for Mrk 463, a Seyfert 2 galaxy of similar infrared luminosity.