%0 Journal Article
%A Paliya, Vaidehi
%A Domínguez Díaz, Alberto
%A Ajello, Marco
%A Franckowiak, Anna
%A Hartmann, Dieter
%T Fermi-LAT stacking analysis technique: an application to extreme blazars and prospects for their CTA detection
%D 2019
%@ 2041-8205
%U https://hdl.handle.net/20.500.14352/88603
%X We present a likelihood profile stacking technique based on the Fermi-Large Area Telescope (LAT) data to explorethe γ-ray characteristics of Fermi-LAT undetected astrophysical populations. The pipeline is applied to a sample of γ-ray unresolved extreme blazars, i.e., sources with the highest synchrotron peak frequencies (nu(peak)(Syn) >= 10(17) Hz),  and we report a cumulative γ-ray detection with more than 32σ confidence for 2 degrees of freedom. Comparing the generated stacked γ-ray spectrum with the sensitivity limits of the upcoming Cherenkov Telescope Array (CTA), we find that the Fermi-LAT undetected population of such extreme blazars, on average, may remain well below the CTA detection threshold due to their faintness and extragalactic background light (EBL) absorption. However, γ-ray detected blazars belonging to the same class are promising candidates for CTA observations. The EBL-corrected stacked spectra of these sources do not show any softening up to 1 TeV. This finding suggests the inverse Compton peak of extreme blazars lies above 1 TeV, thus indicating a hard intrinsic TeV spectrum. Our analysis also predicts that at 100 GeV, at least ∼10% of the diffuse extragalactic γ-ray background originates from the γ-ray undetected extreme blazars. These results highlight the effectiveness of the developed stacking technique to explore the uncharted territory of γ-ray undetected astrophysical objects.
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