What determines the γ-ray luminosities of classical novae?

Abstract

Classical novae in the Milky Way have now been well-established as high-energy GeV γ-ray sources. In novae with main-sequence companions, this emission is believed to result from shocks internal to the nova ejecta, as a later fast wind collides with an earlier slow outflow. To test this model and constrain the γ-ray production mechanism, we present a systematic study of a sample of recent Galactic novae, comparing their γ-ray properties (γ-ray luminosity and duration) with their outflow velocities, peak V-band magnitudes, and the decline times of their optical light curves (t₂). We uniformly estimate distances in a luminosity-independent manner, using spectroscopic reddening estimates combined with three-dimensional Galactic dust maps. Across our sample, γ-ray luminosities (>100 MeV) vary by three orders of magnitude, spanning 10³⁴- 10³⁷erg s⁻¹. Novae with larger velocity of the fast outflow (or larger differential between the fast and slow outflow) have larger γ-ray luminosities, but are detectable for a shorter duration. The optical and γ-ray fluxes are correlated, consistent with substantial thermal emission in the optical from shock-heated gas. Across six novae with γ-ray and infrared light curves, evidence for dust formation appears soon after the end of the detected γ-ray emission. Dusty and non-dusty novae appear to have similar γ-ray luminosities, though novae that have more material processed by the shocks may be more likely to form dust. We find that the properties of the γ-ray emission in novae depend heavily on the ejecta properties, and are consistent with expectations for internal shocks.