Evidence for GeV 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 $\sim$ 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 $\mathrm{log}_{10}B_{10}>2$, providing evidence in favor of extended emission against the point-source hypothesis, consistent with expectations for pair halos produced in the IGMF with strength $B_{\mathrm{IGMF}}\sim 10^{-17}-10^{-15}\mathrm{G}$.