Novae as Tevatrons: prospects for CTA and IceCube

Abstract

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 photo-ionized layer immediately ahead of the shock. Based on the growth rate in this layer 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 higher energies ∼ > 100 GeV accessible to atmosphere Cherenkov telescopes, such as the planned Cherenkov Telescope Array (CTA). Detecting TeV neutrinos with IceCube in hadronic scenarios appears to be more challenging, although the prospects are improved for a particularly nearby event (distance ∼ < kpc) or if the shock power during the earliest, densest phases of the nova outburst is higher than is implied by the observed GeV light curves, due to downscattering of the gamma-rays by electrons within the ejecta. Novae provide ideal nearby laboratories to study magnetic field amplification and the onset of cosmic ray acceleration, because other time-dependent sources (e.g. radio supernovae) typically occur too distant to detect as gamma-ray sources.

Internal shocks from variable outflows in classical novae

“…The observed range of luminosities are obtained if the acceleration efficiency of relativistic particles at the shock is ∼ 0.3%, similar to those expected due to ion acceleration at the corrugated interface of radiative shocks (Steinberg & Metzger 2018) and thus further supporting a hadronic origin for the gamma-ray emission (e.g. Metzger et al 2016;Li et al 2017;Martin et al 2018).…”

Phenomenology of gamma-ray emitting binaries

“…[76,77]). A weak neutrino signal could also be expected in this case [78]. In the alternative ICbased models, internal shocks may also be taking place, but their contribution would be ∼negligible [79].…”

Galactic and extragalactic novae - A Review

“…4. An investigation of the gamma ray emission of non-symbiotic novae and the role of the hadronic and leptonic models has been presented by [62], [63]. The gamma ray emission in novae probes the physical processes related to the relativistic particle acceleration at non relativistic shocks [62].…”

“…The discovery of gamma ray emission of novae in the range 0.1 to 1 GeV has triggered the investigation of the shocks and the relativistic particle acceleration involved in the outburst [63]. The gas upstream of the shock is neutral and shielded by ionization.…”