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   72115
   We request moderately long (100 ksec) observations of the two
   prototypical ultraluminous infrared galaxies (ULIRG) Mrk 231 and Arp
   220 (infrared luminosities greater than 10^12 solar luminosities). The
   primary goal is to search for heavily obscured (i.e., Compton-thick),
   X-ray luminous AGN in these two galaxies, by looking for the spectral
   signatures of reflection-dominated X-ray sources. Detection of such AGN
   (an example of which is seen in the ULIRG NGC 6240) would have major
   implications for the dominant energy source in this important class of
   galaxies.
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