2016MNRAS.457.1786M

The discovery of novae as sources of ∼0.1-1 GeV gamma-rays highlights the key role of shocks and relativistic particle acceleration in these transient systems. Although there is evidence for a spectral cut-off above energies ∼1-100 GeV at particular epochs in some novae, the maximum particle energy achieved in these accelerators has remained an open question. The high densities of the nova ejecta (∼10 orders of magnitude larger than in supernova remnants) render the gas far upstream of the shock neutral and shielded from ionizing radiation. The amplification of the magnetic field needed for diffusive shock acceleration requires ionized gas, thus confining the acceleration process to a narrow photoionized layer immediately ahead of the shock. Based on the growth rate of the hybrid non-resonant cosmic ray current-driven instability (considering also ion-neutral damping), we quantify the maximum particle energy, E_max, across the range of shock velocities and upstream densities of interest. We find values of E_max ∼ 10 GeV-10 TeV, which are broadly consistent with the inferred spectral cut-offs, but which could also in principle lead to emission extending to >= 100 GeV accessible to atmosphere Cherenkov telescopes, such as the Cherenkov Telescope Array (CTA). Detecting TeV neutrinos with IceCube is more challenging, although the prospects are improved for a nearby event (<= kpc) or if the shock power during the earliest, densest phases of the outburst is higher than implied by the GeV light curves, due to downscattering of the gamma-rays within the ejecta.