  Current Cherenkov telescopes have identified a population of
ultra-high-frequency peaked BL Lac objects (UHBLs), also known as extreme
blazars, that exhibit exceptionally hard TeV spectra, including 1ES 0229+200,
1ES 0347-121, RGB J0710+591, 1ES 1101-232, and 1ES 1218+304. Although one-zone
synchrotron-self-Compton (SSC) models have been generally successful in
interpreting the high-energy emission observed in other BL Lac objects, they
are problematic for UHBLs, necessitating very large Doppler factors and/or
extremely high minimum Lorentz factors of the emitting leptonic population. In
this context, we have investigated alternative scenarios where hadronic
emission processes are important, using a newly developed (lepto-)hadronic
numerical code to systematically explore the physical parameters of the
emission region that reproduces the observed spectra while avoiding the extreme
values encountered in pure SSC models. Assuming a fixed Doppler factor
$\delta=30$, two principal parameter regimes are identified, where the
high-energy emission is due to: 1) proton-synchrotron radiation, with magnetic
fields $B \sim 1-100\ \textrm{G}$ and maximum proton energies $E_{p;max}
\lesssim 10^{19}$ eV; and 2) synchrotron emission from p-$\gamma$-induced
cascades as well as SSC emission from primary leptons, with $B \sim 0.1-1\
\textrm{G}$ and $E_{p;max} \lesssim 10^{17}$ eV. This can be realized with
plausible, sub-Eddington values for the total (kinetic plus magnetic) power of
the emitting plasma, in contrast to hadronic interpretations for other blazar
classes that often warrant highly super-Eddington values.

High Energy Astrophysical Phenomena