  The Fermi LAT discovery that classical novae produce >100 MeV gamma-rays
establishes that shocks and relativistic particle acceleration are key features
of these events. These shocks are likely to be radiative due to the high
densities of the nova ejecta at early times coincident with the gamma-ray
emission. Thermal X-rays radiated behind the shock are absorbed by neutral gas
and reprocessed into optical emission, similar to Type IIn (interacting)
supernovae. Gamma-rays are produced by collisions between relativistic protons
with the nova ejecta (hadronic scenario) or Inverse Compton/bremsstrahlung
emission from relativistic electrons (leptonic scenario), where in both
scenarios the efficiency for converting relativistic particle energy into LAT
gamma-rays is at most a few tens of per cent. The ratio of gamma-ray and
optical luminosities, L_gam/L_opt, thus sets a lower limit on the fraction of
the shock power used to accelerate relativistic particles, e_nth. The measured
values of L_gam/L_opt for two classical novae, V1324 Sco and V339 Del,
constrains e_nth > 1e-2 and > 1e-3, respectively. Inverse Compton models for
the gamma-ray emission are disfavored given the low electron acceleration
efficiency, e_nth ~ 1e-4-1e-3, inferred from observations of Galactic cosmic
rays and particle-in-cell (PIC) numerical simulations. A fraction >
100(0.01/e_nth) and > 10(0.01/e_nth) per cent of the optical luminosity is
powered by shocks in V1324 Sco and V339 Del, respectively. Such high fractions
challenge standard models that instead attribute all nova optical emission to
the direct outwards transport of thermal energy released near the white dwarf
surface.
