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The Fermi LAT discovery that classical novae produce >100 MeV gamma-raysestablishes that shocks and relativistic particle acceleration are key featuresof these events. These shocks are likely to be radiative due to the highdensities of the nova ejecta at early times coincident with the gamma-rayemission. Thermal X-rays radiated behind the shock are absorbed by neutral gasand reprocessed into optical emission, similar to Type IIn (interacting)supernovae. Gamma-rays are produced by collisions between relativistic protonswith the nova ejecta (hadronic scenario) or Inverse Compton/bremsstrahlungemission from relativistic electrons (leptonic scenario), where in bothscenarios the efficiency for converting relativistic particle energy into LATgamma-rays is at most a few tens of per cent. The ratio of gamma-ray andoptical luminosities, L_gam/L_opt, thus sets a lower limit on the fraction ofthe shock power used to accelerate relativistic particles, e_nth. The measuredvalues 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 forthe gamma-ray emission are disfavored given the low electron accelerationefficiency, e_nth ~ 1e-4-1e-3, inferred from observations of Galactic cosmicrays 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 ispowered by shocks in V1324 Sco and V339 Del, respectively. Such high fractionschallenge standard models that instead attribute all nova optical emission tothe direct outwards transport of thermal energy released near the white dwarfsurface.

Gamma-ray novae as probes of relativistic particle acceleration at non-relativistic shocks