Search for GeV γ-Ray Pair Halos Around Low Redshift Blazars

We report on the results of a search for gamma-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 similar to 1 GeV photons around 24 stacked isolated high-synchrotron-peaked BL Lacs with redshift z < 0.5 shows an excess over that of pointlike sources. A frequentist test yields a p value of p similar to 0.01 for the extended emission against the point-source hypothesis. A Bayesian estimation provides Bayes factors log(10) B-10 > 2, consistent with expectations for pair halos produced in the intergalactic magnetic fields with strength B-IGMF similar to 10(-17)-10(-15) G.