Deriving the contribution of blazars to the Fermi-LAT Extragalactic $\gamma$-ray background at $E>10$ GeV with efficiency corrections and photon statistics

Abstract

The Fermi Large Area Telescope (LAT) Collaboration has recently released the Third Catalog of Hard Fermi-LAT Sources (3FHL), which contains 1556 sources detected above 10 GeV with seven years of Pass 8 data. We investigate the source count distribution of 3FHL sources at Galactic latitudes $|b|>20^{\circ }$, where the sources are mostly blazars. We use two complementary techniques: 1) a source-detection efficiency correction method and 2) an analysis of pixel photon count statistics with the 1-point probability distribution function (1pPDF). With the first method, using realistic Monte Carlo simulations of the $\gamma$-ray sky, we calculate the efficiency of the LAT to detect point sources. This enables us to find the intrinsic source count distribution at photon fluxes down to $7.5\times 10^{-12}$ ph cm${}^{-2}$s${}^{-1}$. With this method we detect a flux break at $(3.5\pm 0.4)\times 10^{-11}\mathrm{ph}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ with a significance of at least $5.4\sigma$. The power-law indexes of the source count distribution above and below the break are $2.09\pm 0.04$ and $1.07\pm 0.27$, respectively. This result is confirmed with the 1pPDF method, which has a sensitivity reach of $\sim 10^{-11}$ ph cm${}^{-2}$s${}^{-1}$. Integrating the derived source count distribution above the sensitivity of our analysis, we find that $(42\pm 8)\%$ of the extragalactic $\gamma$-ray background originates from blazars.

13 pages, 13 figures, 3 table. 2018 ApJ 856 106

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