For how long are particles accelerated in shells of recurrent novae?

Abstract

ABSTRACT Galactic novae are at present a well established class of γ-ray sources. We wonder for how long the mechanism of acceleration of electrons operates in the shells of novae. In order to put constraints on the time-scale of the electron acceleration, we consider a specific model for the injection and propagation of electrons within the shell of the recurrent nova RS Ophiuchi. We calculate the equilibrium spectra of electrons within the nova shell and the γ-ray fluxes produced by these electrons in the Comptonization of the soft radiation from the red giant within a nova binary system and also radiation from the nova photosphere. We investigate a two-component time-dependent model in which a spherically ejected nova shell propagates freely in the polar region of a nova binary system. However, the shell is significantly decelerated in the dense equatorial region of the binary system. We discuss the conditions under which electrons can produce γ-rays that might be detectable by present and/or future γ-ray observatories. It is concluded that freely expanding shells of novae in the optimal case (strongly magnetized shell and efficiency of acceleration of electrons of the order of 10 per cent) can produce TeV γ-rays within the sensitivity of the Cherenkov Telescope Array within 1–2 yr of explosion only. On the other hand, decelerated shells of novae have a chance to be detected during the whole recurrence period of RS Ophiuchi, i.e. ∼15 yr.