Gamma-Ray Light Curves and Spectra of Classical Novae

TL;DR

This paper models gamma-ray emissions from classical novae, revealing that early gamma-ray flashes are much fainter than previously thought, impacting detection prospects and aiding retrospective data analysis.

Contribution

It provides refined numerical models of nova gamma-ray spectra and light curves, especially focusing on early emission phases, with updated predictions on detectability.

Findings

The 511-keV-line flash is heavily suppressed, making it undetectable by current instruments.

Expected gamma-ray fluxes are at least one million times fainter than previous estimates.

Time-resolved spectra can enhance retrospective analyses of archival gamma-ray data.

Abstract

The nucleosynthesis in classical novae, in particular that of radioactive isotopes, is directly measurable by its $\gamma$-ray signature. Despite decades of observations, MeV $\gamma$-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 modeling 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 $\gamma$-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 $\gamma$-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 $10^{-9}\,{\rm ph}\,$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.