  A spatial clustering signal has been established in Spitzer/IRAC measurements
of the unresolved Cosmic near-Infrared Background (CIB) out to large angular
scales, ~1 deg. This CIB signal, while significantly exceeding the contribution
from the remaining known galaxies, was further found to be coherent at a highly
statistically significant level with the unresolved soft Cosmic X-ray
Background (CXB). This measurement probes the unresolved CXB to very faint
source levels using deep near-IR source subtraction. We study contributions
from extragalactic populations at low to intermediate redshifts to the measured
positive cross-power signal of the CIB fluctuations with the CXB. We model the
X-ray emission from AGN, normal galaxies and hot gas residing in virialized
structures, calculating their CXB contribution including their spatial
coherence with all infrared emitting counterparts. We use a halo model
framework to calculate the auto and cross-power spectra of the unresolved
fluctuations based on the latest constraints of the halo occupation
distribution and the biasing of AGN, galaxies and diffuse emission. At small
angular scales (<1'), the 4.5mic vs 0.5-2 keV coherence can be explained by
shot noise from galaxies and AGN. However, at large angular scales (~10 arcmin)
we find that the net contribution from the modeled populations is only able to
account for ~3% of the measured CIBxCXB cross-power. The discrepancy suggests
that the CIBxCXB signal originates from the same unknown source population
producing the CIB clustering signal out to ~1 deg.
