Keitaro Takahashi, Masaki Mori, Kiyotomo Ichiki, Susumu Inoue, Hajime Takami
Lower bounds are derived on the amplitude B of intergalactic magnetic fields (IGMFs) in the region between our Galaxy and the blazar Mrk 421, from constraints on the delayed GeV flux of pair echos that are emitted by secondary electrons and positrons produced in gamma-gamma interactions between primary TeV gamma-rays and the cosmic infrared background. The distribution of galaxies mapped by the Sloan Digital Sky Survey shows that this region is dominated by a large intergalactic void. We utilize data from long-term, simultaneous GeV-TeV observations by the Fermi Large Area Telescope and the ARGO-YBJ experiment extending over 600 days. For an assumed value of B, we evaluate the daily GeV flux of the pair echo expected from the TeV data, select the dates where this exceeds the Fermi 2-sigma sensitivity, compute the probability that this flux is excluded by the Fermi data for each date, and then combine the probabilities using the inverse normal method. Consequently, we exclude B < 10^(-20.5) G for a field coherence length of 1 kpc at about 4-sigma level. This is much more significant than the 2-sigma bounds we obtained previously from observations of Mrk 501 involving Cherenkov telescopes, by virtue of more extensive data and improved statistical analysis. Compared with most other studies of IGMF bounds, the evidence we present here for a non-zero IGMF is more robust as it does not rely on unproven assumptions on the primary TeV emission during unobserved periods.
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http://arxiv.org/abs/1303.3069
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