Lower Limits on the Anisotropy of the Extragalactic Gamma-Ray Background implied by the 2FGL and 1FHL Catalogs

Abstract

In principle, the angular anisotropy in the extragalactic gamma-ray background (EGRB) places severe constraints upon putative populations of unresolved gamma-ray point sources. Existing estimates of the EGRB anisotropy have been constructed by excising known point sources, e.g., taken from the First or 2 Year Fermi-LAT Source Catalog (1FGL or 2FGL, respectively) and statistically analyzing the residual gamma-ray sky maps. We perform an independent check of the EGRB anisotropy limits by comparing the values obtained from the 1FGL-masked sky maps to the signal implied by sources that lie below the 1FGL detection threshold in the more sensitive 2FGL and 1FHL (First Fermi-LAT catalog of >10 GeV sources). As such, our analysis provides an internal consistency check of implications for source counts and spectral index distributions of gamma-ray bright active galactic nuclei obtained from Fermi-LAT data. Based on this, we find evidence for substantially larger anisotropies than those previously reported at energies above 5 GeV, where BL Lac objects are likely to provide the bulk of their contribution to the EGRB. This uncertainty in the EGRB anisotropy cautions against using it as an independent constraint for the high-redshift gamma-ray universe. Moreover, this would suggest that contrary to previous claims, smooth extensions of the resolved point-source population may be able to simultaneously explain both the isotropic and anisotropic components of the EGRB.

18 pages, 14 figures, published version

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• The Spectrum of the Universe
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• Deep observation of the NGC 1275 region with MAGIC: search of diffuse gamma-ray emission from cosmic rays in the Perseus cluster
• The Effect of Nonlinear Landau Damping on Ultrarelativistic Beam Plasma Instabilities
• Unveiling the Gamma-ray Source Count Distribution Below the Fermi Detection Limit with Photon Statistics
• The Linear Instability of Dilute Ultrarelativistic $e^{\pm }$ Pair Beams
• Using rotation measure grids to detect cosmological magnetic fields -- a Bayesian approach
• The Second Radio Synchrotron Background Workshop: Conference Summary and Report
• Angular Power Spectra with Finite Counts
• Patchy blazar heating: diversifying the thermal history of the intergalactic medium
• Bow Ties in the Sky I: The Angular Structure of Inverse Compton Gamma-ray Halos in the Fermi Sky
• Bow Ties in the Sky II: Searching for Gamma-ray Halos in the Fermi Sky Using Anisotropy
• Constraints on the Intergalactic Magnetic Field from Bow Ties in the Gamma-ray Sky
• Measurement of the Anisotropy Power Spectrum of the Radio Synchrotron Background
• Diffuse Sources, Clustering and the Excess Anisotropy of the Radio Synchrotron Background