Top panel: photon–photon absorption correction (exp[τ_γγ]) for the source M 87 at 18.5 Mpc, based on the EBL model by FR2017. Bottom panel: observed (open red and continuous line) and absorption-corrected (black line) spectral data are reported. The 50-h 5σ and 100-h 2σ sensitivity limits for CTA, and the HAWC 5-year limit are shown. The blue dotted line and the red dashed line indicate the SWGO and LHAASO 5-year 5σ limits, respectively.