UNVEILING THE GAMMA-RAY SOURCE COUNT DISTRIBUTION BELOW THE FERMI DETECTION LIMIT WITH PHOTON STATISTICS

Abstract

The source-count distribution as a function of their flux, , is one of the main quantities characterizing gamma-ray source populations. We employ statistical properties of the Fermi  Large Area Telescope (LAT) photon counts map to measure the composition of the extragalactic gamma-ray sky at high latitudes ( °) between 1 and 10 GeV. We present a new method, generalizing the use of standard pixel-count statistics, to decompose the total observed gamma-ray emission into (a) point-source contributions, (b) the Galactic foreground contribution, and (c) a truly diffuse isotropic background contribution. Using the 6 yr Fermi -LAT data set ( P7REP ), we show that the distribution in the regime of so far undetected point sources can be consistently described with a power law with an index between 1.9 and 2.0. We measure down to an integral flux of improving beyond the 3FGL catalog detection limit by about one order of magnitude. The overall distribution is consistent with a broken power law, with a break at The power-law index for bright sources above the break hardens to for fainter sources below the break. A possible second break of the distribution is constrained to be at fluxes below at 95% confidence level. The high-latitude gamma-ray sky between 1 and 10 GeV is shown to be composed of ∼25% point sources, ∼69.3% diffuse Galactic foreground emission, and ∼6% isotropic diffuse background.