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We survey our understanding of classical novae—nonterminal, thermonuclear eruptions on the surfaces of white dwarfs in binary systems. The recent and unexpected discovery of GeV gamma rays from Galactic novae has highlighted the complexity of novae and their value as laboratories for studying shocks and particle acceleration. We review half a century of nova literature through this new lens, and conclude the following: ▪ The basics of the thermonuclear runaway theory of novae are confirmed by observations. The white dwarf sustains surface nuclear burning for some time after runaway, and until recently, it was commonly believed that radiation from this nuclear burning solely determines the nova's bolometric luminosity. ▪ The processes by which novae eject material from the binary system remain poorly understood. Mass loss from novae is complex (sometimes fluctuating in rate, velocity, and morphology) and often prolonged in time over weeks, months, or years. ▪ The complexity of the mass ejection leads to gamma-ray-producing shocks internal to the nova ejecta. When gamma rays are detected (around optical maximum), the shocks are deeply embedded and the surrounding gas is very dense. ▪ Observations of correlated optical and gamma-ray light curves confirm that the shocks are radiative and contribute significantly to the bolometric luminosity of novae. Novae are therefore the closest and most common interaction-powered transients.
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This content will become publicly available on February 3, 2027
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 $$\gamma$$-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 $$\gamma$$-ray production mechanism, we present a systematic study of a sample of recent Galactic novae, comparing their $$\gamma$$-ray properties ($$\gamma$$-ray luminosity and duration) with their outflow velocities, peak V-band magnitudes, and the decline times of their optical light curves ($$t_2$$). We uniformly estimate distances in a luminosity-independent manner, using spectroscopic reddening estimates combined with three-dimensional Galactic dust maps. Across our sample, $$\gamma$$-ray luminosities ($>$100 MeV) vary by three orders of magnitude, spanning $$10^{34}{{-}}10^{37}$$ erg s$$^{-1}$$. Novae with larger velocity of the fast outflow (or larger differential between the fast and slow outflow) have larger $$\gamma$$-ray luminosities, but are detectable for a shorter duration. The optical and $$\gamma$$-ray fluxes are correlated, consistent with substantial thermal emission in the optical from shock-heated gas. Across six novae with $$\gamma$$-ray and infrared light curves, evidence for dust formation appears soon after the end of the detected $$\gamma$$-ray emission. Dusty and non-dusty novae appear to have similar $$\gamma$$-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 $$\gamma$$-ray emission in novae depend heavily on the ejecta properties, and are consistent with expectations for internal shocks.
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- Award ID(s):
- 2205631
- PAR ID:
- 10703057
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Publisher / Repository:
- OUP
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 546
- Issue:
- 3
- ISSN:
- 0035-8711
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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