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ABSTRACT The nucleosynthesis in classical novae, in particular that of radioactive isotopes, is directly measurable by its γ-ray signature. Despite decades of observations, MeV γ-rays from novae have never been detected – neither individually at the time of the explosion, nor as a result of radioactive decay, nor the diffuse Galactic emission from the nova population. Thanks to recent developments in modelling of instrumental background for MeV telescopes such as INTEGRAL/SPI and Fermi/GBM, the prospects to finally detect these elusive transients are greatly enhanced. This demands for updated and refined models of γ-ray spectra and light curves of classical novae. In this work, we develop numerical models of nova explosions using sub- and near-Chandrasekhar CO white dwarfs as the progenitor. We study the parameter dependence of the explosions, their thermodynamics and energetics, as well as their chemical abundance patterns. We use a Monte Carlo radiative transfer code to compute γ-ray light curves and spectra, with a focus on the early time evolution. We compare our results to previous studies and find that the expected 511-keV-line flash at the time of the explosion is heavily suppressed, showing a maximum flux of only $10^{-9}\, \mathrm{ph\, cm^{-2}\, s^{-1}}$ and thus making it at least one million times fainter than estimated before. This finding would render it impossible for current MeV instruments to detect novae within the first day after the outburst. Nevertheless, our time-resolved spectra can be used for retrospective analyses of archival data, thereby improving the sensitivity of the instruments. 

ABSTRACT Reticulum II (Ret II) is a satellite galaxy of the Milky Way (MW) and presents a prime target to investigate the nature of dark matter (DM) because of its high mass-to-light ratio. We evaluate a dedicated INTEGRAL observation campaign data set to obtain γ-ray fluxes from Ret II and compare those with expectations from DM. Ret II is not detected in the γ-ray band 25–8000 keV, and we derive a flux limit of ${\lesssim}10^{-8}\, \mathrm{erg\, cm^{-2}\, s^{-1}}$. The previously reported 511 keV line is not seen, and we find a flux limit of ${\lesssim}1.7 \times 10^{-4}\, \mathrm{ph\, cm^{-2}\, s^{-1}}$. We construct spectral models for primordial black hole (PBH) evaporation and annihilation/decay of particle DM, and subsequent annihilation of e+s produced in these processes. We exclude that the totality of DM in Ret II is made of a monochromatic distribution of PBHs of masses ${\lesssim}8 \times 10^{15}\, \mathrm{g}$. Our limits on the velocity-averaged DM annihilation cross section into e+e− are $\langle \sigma v \rangle \lesssim 5 \times 10^{-28} \left(m_{\rm DM} / \mathrm{MeV} \right)^{2.5}\, \mathrm{cm^3\, s^{-1}}$. We conclude that analysing isolated targets in the MeV γ-ray band can set strong bounds on DM properties without multi-year data sets of the entire MW, and encourage follow-up observations of Ret II and other dwarf galaxies.