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We present the spectral signatures of the Bethe-Heitler pair production($pe$) process on the spectral energy distribution (SED) of blazars, inscenarios where the hard $\gamma$-ray emission is of photohadronic origin. Ifrelativistic protons interact with the synchrotron blazar photons producing$\gamma$ rays through photopion processes, we show that, besides the $2-20$ PeVneutrino emission, the typical blazar SED should have an emission feature dueto the synchrotron emission of $pe$ secondaries that bridges the gap betweeenthe low-and high-energy humps of the SED, namely in the energy range 40 keV-40MeV. We first present analytical expressions for the photopion and $pe$ lossrates in terms of observable quantities of blazar emission. For the $pe$ lossrate in particular, we derive a new approximate analytical expression for thecase of a power-law photon distribution, which has an excellent accuracy withthe numerically calculated exact one, especially at energies above thethreshold for pair production. We show that for typical blazar parameters, thephotopair synchrotron emission emerges in the hard X-ray/soft $\gamma$-rayenergy range with a characteristic spectral shape and non negligible flux,which may be even comparable to the hard $\gamma$-ray flux produced throughphotopion processes. We argue that the expected "$pe$ bumps" are a naturalconsequence of leptohadronic models, and as such, they may indicate thatblazars with a three-hump SED are possible emitters of high-energy neutrinos.

Bethe-Heitler emission in BL Lacs: filling the gap between X-rays and $\gamma$-rays