Neutrino spectra expected for Mrk 421 for proton-proton interactions and different primary proton spectra characterized by the indicated proton cut-off energy, E_max,p, and by the proton spectral index, γ_p. The Fermi integrated flux is used for normalization. The black line is the envelope of power law sensitivities of IC-40. Only neutrino fluxes not exceeding the envelope sensitivity are shown.