Unresolved Gamma-Ray Sky through its Angular Power Spectrum
  The gamma-ray sky has been observed with unprecedented accuracy in the last
decade by the Fermi large area telescope (LAT), allowing us to resolve and
understand the high-energy Universe. The nature of the remaining unresolved
emission (unresolved gamma-ray background, UGRB) below the LAT source detection
threshold can be uncovered by characterizing the amplitude and angular scale of
the UGRB fluctuation field. This work presents a measurement of the UGRB
autocorrelation angular power spectrum based on eight years of Fermi LAT Pass 8
data products. The analysis is designed to be robust against contamination from
resolved sources and noise systematics. The sensitivity to subthreshold sources
is greatly enhanced with respect to previous measurements. We find evidence
(with $\sim$3.7$\sigma$ significance) that the scenario in which two classes of
sources contribute to the UGRB signal is favored over a single class. A double
power law with exponential cutoff can explain the anisotropy energy spectrum
well, with photon indices of the two populations being 2.55 $\pm$ 0.23 and 1.86
$\pm$ 0.15.
