Gamma-ray emission from nova outbursts

TL;DR

This paper reviews gamma-ray emission mechanisms in novae, discussing observational evidence from Fermi satellite detections and exploring radioactive decay and particle acceleration as sources of gamma-rays.

Contribution

It provides a comprehensive discussion of gamma-ray production scenarios in novae, including recent Fermi observations challenging existing models.

Findings

Detection of GeV gamma-rays from novae by Fermi.

Challenges to traditional models posed by recent non-symbiotic nova detections.

Discussion of radioactive decay and particle acceleration as gamma-ray sources.

Abstract

Classical novae produce radioactive nuclei which are emitters of gamma-rays in the MeV range. Some examples are the lines at 478 and 1275 keV (from 7Be and 22Na) and the positron-electron annihilation emission, with the 511 keV line and a continuum. Gamma-ray spectra and light curves are potential unique tools to trace the corresponding isotopes and to give insights on the properties of the expanding envelope. Another possible origin of gamma-rays is the acceleration of particles up to very high energies, so that either neutral pions or inverse Compton processes produce gamma-rays of energies larger than 100 MeV. MeV photons during nova explosions have not been detected yet, although several attempts have been made in the last decades; on the other hand, GeV photons from novae have been detected with the Fermi satellite in V407 Cyg, a nova in a symbiotic binary, where the companion is a red giant with a wind, instead of a main sequence star as in the cataclysmic variables hosting classical novae. Two more novae have been detected recently (summer 2012) by Fermi, apparently in non symbiotic binaries, thus challenging our understanding of the emission mechanism. Both scenarios (radioactivities and acceleration) of gamma-ray production in novae are discussed.