  Peaking at 3.7 mag on 2020 July 11, YZ Ret was the second-brightest nova of
the decade. The nova's moderate proximity (2.7 kpc from Gaia) provided an
opportunity to explore its multi-wavelength properties in great detail. Here we
report on YZ Ret as part of a long-term project to identify the physical
mechanisms responsible for high-energy emission in classical novae. We use
simultaneous Fermi/LAT and NuSTAR observations complemented by XMM-Newton X-ray
grating spectroscopy to probe the physical parameters of the shocked ejecta and
the nova-hosting white dwarf. The XMM-Newton observations revealed a super-soft
X-ray emission which is dominated by emission lines of CV, CVI, NVI, NVII, and
OVIII rather than a blackbody-like continuum, suggesting CO-composition of the
white dwarf in a high-inclination binary system. Fermi/LAT detected YZ Ret for
15 days with the gamma-ray spectrum best described by a power law with an
exponential cut-off at 1.9 +/-0.6 GeV. In stark contrast with theoretical
predictions and in keeping with previous NuSTAR observations of Fermi-detected
classical novae (V5855 Sgr and V906 Car), the 3.5-78 keV X-ray emission is
found to be two orders of magnitude fainter than the GeV emission. The X-ray
emission observed by NuSTAR is consistent with a single-temperature thermal
plasma. We detect no non-thermal tail of the GeV emission expected to extend
down to the NuSTAR band. NuSTAR observations continue to challenge theories of
high-energy emission from shocks in novae.
