Evidence for GeV Pair Halos around Low Redshift Blazars

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We report on the results of a search for γ-ray pair halos with a stacking analysis of low-redshift blazars using data from the Fermi Large Area Telescope. For this analysis we used a number of a-priori selection criteria, including the spatial and spectral properties of the Fermi sources. The angular distribution of 1GeV photons around 24 stacked isolated high-synchrotron-peaked BL Lacs with redshift z<0.5 shows an excess over that of point-like sources. A statistical analysis yields a Bayes factor of log₁₀B₁₀>2, providing evidence in favor of extended emission against the point-source hypothesis, consistent with expectations for pair halos produced in the IGMF with strength BIGMF 10⁻¹⁷-10⁻¹⁵G.