Abstract
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. 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. Recent hybrid PIC simulations show yet lower proton acceleration
efficiencies (consistent with zero) for shocks propagating perpendicular to the
upstream magnetic field, the geometry relevant if the magnetic field in the
nova outflow is dominated by its azimuthal component. However, localized
regions of parallel shocks, created either by global asymmetries or local
inhomogeneities ("clumpiness") in the ejecta, may account for the requisite
proton acceleration. 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.
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