  The source-subtracted cosmic infrared background (CIB) fluctuations uncovered
in deep Spitzer data cannot be explained by known galaxy populations and appear
strongly coherent with unresolved cosmic X-ray background (CXB). This suggests
that the source-subtracted CIB contains emissions from significantly abundant
accreting black holes (BHs). We show that theoretically such populations would
have the angular power spectrum which is largely independent of the epochs
occupied by these sources, provided they are at z>~ 4, offering an important
test of the origin of the new populations. Using the current measurements we
reconstruct the underlying soft X-ray CXB from the new sources and show that
its fluctuations, while consistent with a high-z origin, have an amplitude that
cannot be reached in direct measurements with the foreseeable X-ray space
missions. This necessitates application of the methods developed by the authors
to future IR and X-ray datasets, which must cover large areas of the sky in
order to measure the signal with high precision. The LIBRAE project within
ESA's Euclid mission will probe source-subtracted CIB over ~1/2 the sky at
three near-IR bands, and its cross-power with unresolved CXB can be measured
then from the concurrent eROSITA mission covering the same areas of the sky. We
discuss the required methodology for this measurement and evaluate its
projected S/N to show the unique potential of this experimental configuration
to accurately probe the CXB from the new BH sources and help identify their
epochs.
