Fermi-LAT Stacking Analysis Technique: An Application to Extreme Blazars and Prospects for their CTA Detection

Abstract

We present a likelihood profile stacking technique based on the Fermi -Large Area Telescope (LAT) data to explore the 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 ( ), and we report a cumulative γ -ray detection with more than 32 σ 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.