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Classical novae are among the most frequent transient events in the Milky Way, and key agents of ongoing nucleosynthesis. Despite their large numbers, they have never been observed in soft $gamma$-ray emission. Measurements of their $gamma$-ray signatures would provide both, insights on explosion mechanism as well as nucleosynthesis products. Our goal is to constrain the ejecta masses of $mathrm{^7Be}$ and $mathrm{^{22}Na}$ from classical novae through their $gamma$-ray line emissions at 478 and 1275 keV. We extract posterior distributions on the line fluxes from archival data of the INTEGRAL/SPI spectrometer telescope. We then use a Bayesian hierarchical model to link individual objects and diffuse emission and infer ejecta masses from the whole population of classical novae in the Galaxy. Individual novae are too dim to be detectable in soft $gamma$-rays, and the upper bounds on their flux and ejecta mass uncertainties cover several orders of magnitude. Within the framework of our hierarchical model, we can, nevertheless, infer tight upper bounds on the $mathrm{^{22}Na}$ ejecta masses, given all uncertainties from individual objects as well as diffuse emission, of $<2.0 times 10^{-7},mathrm{M_{odot}}$ (99.85th percentile). In the context of ONe nucleosynthesis, the $mathrm{^{22}Na}$ bounds are consistent with theoretical expectations, and exclude that most ONe novae happen on white dwarfs with masses around $1.35,mathrm{M_{odot}}$. The upper bounds from $mathrm{^{7}Be}$ are uninformative. From the combined ejecta mass estimate of $mathrm{^{22}Na}$ and its $beta^+$-decay, we infer a positron production rate of $<5.5 times 10^{42},mathrm{e^+,s^{-1}}$, which would make at most 10% of the total annihilation rate in the Milky Way.
A general review of the relevance of classical novae for the chemical evolution of the Galaxy, as well as for Galactic radioactivity is presented. A special emphasis is put on the pioneering work done by Jim Truran in this field of research. The impa
The Transient Gamma Ray Spectrometer (TGRS) on board the WIND spacecraft has spent most of the interval 1995-1997 in a high-altitude orbit where gamma-ray backgrounds are low. Its high-resolution Ge spectrometer is thus able to detect weak lines whic
Gamma-ray emission at energies >100MeV has been detected from nine novae using the Fermi-LAT, and it can be explained by particle acceleration at shocks in these systems. Eight out of these nine objects are classical novae in which interaction of the
Detection of X-rays from classical novae, both in outburst and post-outburst, provides unique and crucial information about the explosion mechanism. Soft X-rays reveal the hot white dwarf photosphere, whenever hydrogen (H) nuclear burning is still on
We examine the prospects for producing Nickel-56 from black hole accretion disks, by examining a range of steady state disk models. We focus on relatively slowly accreting disks in the range of 0.05 - 1 solar masses per second, as are thought to be a