Observed γ-ray spectrum of Mkn 501 during a quiescent phase (Acciari et al. 2011). The three panels report the received γ-ray spectrum for different assumed intrinsic spectra (from top to bottom: a simple power law with energy index α = 0.52; a power law with energy index α = 0.52 and an exponential cut-off with e-folding energy E_cut = 50 TeV; a cut-offed power law with α = 0.52 and E_cut = 15 TeV) and values of E_LIV (color code as in Fig. 2). The HAWC and CTA sensitivity curves (as in Fig. 3) are displayed for an exposure time of one year and 50 h, respectively.