The angular power spectrum of the diffuse gamma-ray emission as measured by the Fermi Large Area Telescope and constraints on its Dark Matter interpretation

The isotropic gamma-ray background arises from the contribution of unresolved sources, including members of confirmed source classes and proposed gamma-ray emitters such as the radiation induced by dark matter annihilation and decay. Clues about the properties of the contributing sources are imprinted in the anisotropy characteristics of the gamma-ray background. We use 81 months of Pass 7 Reprocessed data from the Fermi Large Area Telescope to perform a measurement of the anisotropy angular power spectrum of the gamma-ray background. We analyze energies between 0.5 and 500 GeV, extending the range considered in the previous measurement based on 22 months of data. We also compute, for the first time, the cross-correlation angular power spectrum between different energy bins. We find that the derived angular spectra are compatible with being Poissonian, i.e. constant in multipole. Moreover, the energy dependence of the anisotropy suggests that the signal is due to two populations of sources, contributing, respectively, below and above 2 GeV. Finally, using data from state-of-the-art numerical simulations to model the dark matter distribution, we constrain the contribution from dark matter annihilation and decay in Galactic and extragalactic structures to the measured anisotropy. These constraints are competitive with those that can be derived from the average intensity of the isotropic gamma-ray background. Data are available at https://www-glast.stanford.edu/pub_data/552.

Diffuse $gamma$-ray emission from unresolved BL Lac objects

Blazars represent the most abundant class of high-energy extragalactic $gamma$-ray sources. The subset of blazars known as BL Lac objects is on average closer to Earth and characterized by harder spectra at high energy than the whole sample. The fraction of BL Lacs that is too dim to be detected and resolved by current $gamma$-ray telescopes is therefore expected to contribute to the high-energy isotropic diffuse $gamma$-ray background (IGRB). The IGRB has been recently measured over a wide energy range by the Large Area Telescope (LAT) on board the Gamma-ray Space Telescope ({it Fermi}). We present a new prediction of the diffuse $gamma$-ray flux due to the unresolved BL Lac blazar population. The model is built upon the spectral energy distribution and the luminosity function derived from the fraction of BL Lacs detected (and spectrally characterized) in the $gamma$-ray energy range. We focus our attention on the ${cal O}(100)$ GeV energy range, predicting the emission up to the TeV scale and taking into account the absorption on the extragalactic background light. In order to better shape the BL Lac spectral energy distribution, we combine the {it Fermi}-LAT data with Imaging Atmospheric Cerenkov Telescopes measurements of the most energetic sources. Our analysis is carried on separately for low- and intermediate-synchrotron-peaked BL Lacs on one hand, and high-synchrotron-peaked BL Lacs on the other one: we find in fact statistically different features for the two. The diffuse emission from the sum of both BL Lac classes increases from about 10$%$ of the measured IGRB at 100 MeV to $sim$100$%$ of the data level at 100 GeV. At energies greater than 100 GeV, our predictions naturally explain the IGRB data, accommodating their softening with increasing energy. Uncertainties are estimated to be within of a factor of two of the best-fit flux up to 500 GeV.

Astrophysical interpretation of the anisotropies in the unresolved gamma-ray background

Recently, a new measurement of the auto- and cross-correlation angular power spectrum (APS) of the isotropic gamma-ray background was performed, based on 81 months of data of the Fermi Large-Area Telescope (LAT). Here, we fit, for the first time, the new APS data with a model describing the emission of unresolved blazars. These sources are expected to dominate the anisotropy signal. The model we employ in our analysis reproduces well the blazars resolved by Fermi LAT. When considering the APS obtained by masking the sources in the 3FGL catalogue, we find that unresolved blazars under-produce the measured APS below $sim$1 GeV. Contrary to past results, this suggests the presence of a new contribution to the low-energy APS, with a significance of, at least, 5$sigma$. The excess can be ascribed to a new class of faint gamma-ray emitters. If we consider the APS obtained by masking the sources in the 2FGL catalogue, there is no under-production of the APS below 1 GeV, but the new source class is still preferred over the blazars-only scenario (with a significance larger than 10$sigma$). The properties of the new source class and the level of anisotropies induced in the isotropic gamma-ray background are the same, independent of the APS data used. In particular, the new gamma-ray emitters must have a soft energy spectrum, with a spectral index ranging, approximately, from 2.7 to 3.2. This complicates their interpretation in terms of known sources, since, normally, star-forming and radio galaxies are observed with a harder spectrum. The new source class identified here is also expected to contribute significantly to the intensity of the isotropic gamma-ray background.

Probing dark matter signals in neutrino telescopes through angular power spectrum

The hypothesis of two different components in the high-energy neutrino flux observed with IceCube has been proposed to solve the tension among different data-sets and to account for an excess of neutrino events at 100 TeV. In addition to a standard astrophysical power-law component, the second component might be explained by a different class of astrophysical sources, or more intriguingly, might originate from decaying or annihilating dark matter. These two scenarios can be distinguished thanks to the different expected angular distributions of neutrino events. Neutrino signals from dark matter are indeed expected to have some correlation with the extended galactic dark matter halo. In this paper, we perform angular power spectrum analyses of simulated neutrino sky maps to investigate the two-component hypothesis with a contribution from dark matter. We provide current constraints and expected sensitivity to dark matter parameters for future neutrino telescopes such as IceCube-Gen2 and KM3NeT. The latter is found to be more sensitive than IceCube-Gen2 to look for a dark matter signal at low energies towards the galactic center. Finally, we show that after 10 years of data-taking, they will firmly probe the current best-fit scenario for decaying dark matter by exploiting the angular information only.

Angular power spectrum analysis on current and future high-energy neutrino data

Astrophysical neutrino events have been measured in the last couple of years, which show an isotropic distribution, and the current discussion is their astrophysical origin. We use both isotropic and anisotropic components of the diffuse neutrino data to constrain the contribution of a broad number of extra-galactic source populations to the observed neutrino sky. We simulate up-going muon neutrino events by applying statistical distributions for the flux of extragalactic sources, and by Monte Carlo method we exploit the simulation for current and future IceCube, IceCube-Gen2 and KM3NeT exposures. We aim at constraining source populations by studying their angular patterns, for which we assess the angular power spectrum. We leave the characteristic number of sources ($N_{star}$) as a free parameter, which is roughly the number of neutrino sources over which the measured intensity is divided. With existing two-year IceCube data, we can already constrain very rare, bright sources with $N_{star}lesssim$100. This can be improved to $N_{star}lesssim 10^4$-$10^5$ with IceCube-Gen2 and KM3NeT with ten-year exposure, constraining the contribution of BL Lacs ($N_{star}=6times10^{2}$). On the other hand, we can constrain weak sources with large number densities, like starburst galaxies ($N_{star} = 10^{7}$), if we measure an anisotropic neutrino sky with future observations.