title: Search for GeV γ-Ray Pair Halos Around Low Redshift Blazars.
abstract: 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 log_{10}B_{10}>2, consistent with expectations for pair halos produced in the intergalactic magnetic fields with strength B_{IGMF}~10^{-17}-10^{-15} G.
authors.affiliation: Department of Physics and McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, Missouri 63130, USA.
authors.forename: Wenlei
authors.initials: W
authors.lastname: Chen
authors.affiliation: Department of Physics and McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, Missouri 63130, USA.
authors.forename: James H
authors.initials: JH
authors.lastname: Buckley
authors.affiliation: Department of Physics and McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, Missouri 63130, USA.
authors.forename: Francesc
authors.initials: F
authors.lastname: Ferrer