Fermi spectrum of Mrk 501 for the 30 day period during the flare (shared area, 68% confidence level of normalization and slope of the spectrum). Data points in the TeV band show VERITAS measurements (Abdo et al. 2011) of the spectrum during the first 3 days of the flare (solid lines) and in the quiescent state (dashed lines). Model curves show emission from electromagnetic cascade initiated by 100 TeV γ-rays in the intergalactic medium. The solid thick red curve is for intergalactic magnetic field 3 × 10^-17 G with a correlation length of 1 Mpc. Dashed curves are for B = 10^-16 G (upper curve) and B = 10^-17 G (lower curve). Grey data points show the spectrum of the historically brightest source state in 1997 measured by CAT telescope (Aharonian et al. 1996).