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We report X-ray constraints for 20 of 52 high-z ULIRGs identified in theSpitzer xFLS to constrain their obscuration. Notably, decomposition ofSpitzer-IRS spectra for the 52 objects already indicates that most are weak-PAHULIRGs dominated by hot-dust continua, characteristic of AGN. Given theirredshifts, they have AGN bolometric luminosities of ~1e45-1e47 erg/s comparableto powerful QSOs. This, coupled with their high IR-to-optical ratios and oftensignificant silicate absorption, strongly argues in favor of these mid-IRobjects being heavily obscured QSOs. At X-ray energies, we marginally detecttwo ULIRGs, while the rest have only upper limits. Using the IRS-derived 5.8umAGN continuum luminosity as a proxy for the expected X-ray luminosities, wefind that all of the observed sources must individually be highly obscured,while X-ray stacking limits on the undetected sources suggest that themajority, if not all, are likely to be at least mildly Compton-thick (NH>1e24cm-2). With a space density of ~1.4e-7 Mpc-3 at z~2, such objects imply a lowerlimit on the obscured AGN fraction (i.e., the ratio of AGN above and belowNH=1e22 cm-2) of >~1.7:1 even among luminous QSOs. Our findings, which arebased on extensive multi-wavelength constraints including Spitzer IRS spectra,should aid in the interpretation of similar objects from larger or deepermid-IR surveys, where considerable uncertainty about the source propertiesremains and comparable follow-up is not yet feasible.

X-ray Constraints on the AGN Properties in Spitzer-IRS identified z~2 Ultraluminous Infrared Galaxies