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

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 ∼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 ∼0.01 for the extended emission against the point-source hypothesis. A Bayesian estimation provides Bayes factors log₁₀B₁₀>2 , consistent with expectations for pair halos produced in the intergalactic magnetic fields with strength B_IGMF∼1 0^-17- 1 0^-15 G .