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We explain the observed multiwavelength photon spectrum of a number of BL Lacobjects detected at very high energy (VHE, $E \gtrsim 30$ GeV), using alepto-hadronic emission model. The one-zone leptonic emission is employed tofit the synchrotron peak. Subsequently, the SSC spectrum is calculated, suchthat it extends up to the highest energy possible for the jet parametersconsidered. The data points beyond this energy, and also in the entire VHErange are well explained using a hadronic emission model. The ultrahigh-energycosmic rays (UHECRs, $E\gtrsim 0.1$ EeV) escaping from the source interact withthe extragalactic background light (EBL) during propagation over cosmologicaldistances to initiate electromagnetic cascade down to $\sim1$ GeV energies. Theresulting photon spectrum peaks at $\sim1$ TeV energies. We consider a randomturbulent extragalactic magnetic field (EGMF) with a Kolmogorov power spectrumto find the survival rate of UHECRs within 0.1 degrees of the direction ofpropagation in which the observer is situated. We restrict ourselves to an RMSvalue of EGMF, $B_{\rm rms}\sim 10^{-5}$ nG, for a significant contribution tothe photon spectral energy distribution (SED) from UHECR interactions. We foundthat UHECR interactions on the EBL and secondary cascade emission can fitgamma-ray data from the BL Lacs we considered at the highest energies. Therequired luminosity in UHECRs and corresponding jet power are below theEddington luminosities of the super-massive black holes in these BL Lacs.

Ultrahigh-energy cosmic ray interactions as the origin of very high energy $\gamma-$rays from BL Lacs