Evidence for multiple shocks from the $γ$-ray emission of RS Ophiuchi

Abstract: In August of 2021, Fermi-LAT, H.E.S.S., and MAGIC detected GeV and TeV $\gamma$-ray emission from an outburst of recurrent nova RS Ophiuchi. This detection represents the first very high energy $\gamma$-rays observed from a nova, and opens a new window to study particle acceleration. Both H.E.S.S. and MAGIC described the observed $\gamma$-rays as arising from a single, external shock. In this paper, we perform detailed, multi-zone modeling of RS Ophiuchi's 2021 outburst including a self-consistent prescription for particle acceleration and magnetic field amplification. We demonstrate that, contrary to previous work, a single shock cannot simultaneously explain RS Ophiuchi's GeV and TeV emission, particularly the spectral shape and distinct light curve peaks. Instead, we put forward a model involving multiple shocks that reproduces the observed $\gamma$-ray spectrum and temporal evolution. The simultaneous appearance of multiple distinct velocity components in the nova optical spectrum over the first several days of the outburst supports the presence of distinct shocks, which may arise either from the strong latitudinal dependence of the density of the external circumbinary medium (e.g., in the binary equatorial plane versus the poles) or due to internal collisions within the white dwarf ejecta (as powers the $\gamma$-ray emission in classical novae).