We present one-dimensional hydrodynamical simulations including radiative losses, of internal shocks in the outflows from classical novae, to explore the role of shocks in powering multiwavelength emission from radio to gamma-ray wavelengths. Observations support a picture in which the initial phases of some novae generate a slow, equatorially focused outflow (directly from the outer Lagrange point, or from a circumbinary disc), which then transitions to, or is overtaken by, a faster more isotropic outflow from the white dwarf which collides and shocks the slower flow, powering gamma-ray and optical emission through reprocessing by the ejecta. However, the common occurrence of multiple peaks in nova light curves suggests that the outflow’s acceleration need not be monotonic, but instead can involve successive transitions between ‘fast’ and ‘slow’ modes. Such a time-fluctuating outflow velocity naturally can reproduce several observed properties of nova, such as correlated gamma-ray and optical flares, expansion of the photosphere coincident with (though lagging slightly) the peak flare luminosity, and complex time evolution of spectral lines (including accelerating, decelerating, and merging velocity components). While the shocks are still deeply embedded during the gamma-ray emission, the onset of ∼keV X-ray and ∼10 GHz radio synchrotron emission is typically delayed until themore »
New Insights into Classical Novae
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 more »
- Publication Date:
- NSF-PAR ID:
- 10319574
- Journal Name:
- Annual Review of Astronomy and Astrophysics
- Volume:
- 59
- Issue:
- 1
- ISSN:
- 0066-4146
- Sponsoring Org:
- National Science Foundation
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ABSTRACT -
ABSTRACT Shocks in γ-ray emitting classical novae are expected to produce bright thermal and non-thermal X-rays. We test this prediction with simultaneous NuSTAR and Fermi/LAT observations of nova V906 Car, which exhibited the brightest GeV γ-ray emission to date. The nova is detected in hard X-rays while it is still γ-ray bright, but contrary to simple theoretical expectations, the detected 3.5–78 keV emission of V906 Car is much weaker than the simultaneously observed >100 MeV emission. No non-thermal X-ray emission is detected, and our deep limits imply that the γ-rays are likely hadronic. After correcting for substantial absorption (NH ≈ 2 × 1023 cm−2), the thermal X-ray luminosity (from a 9 keV optically thin plasma) is just ∼2 per cent of the γ-ray luminosity. We consider possible explanations for the low thermal X-ray luminosity, including the X-rays being suppressed by corrugated, radiative shock fronts or the X-rays from the γ-ray producing shock are hidden behind an even larger absorbing column (NH > 1025 cm−2). Adding XMM–Newton and Swift/XRT observations to our analysis, we find that the evolution of the intrinsic X-ray absorption requires the nova shell to be expelled 24 d after the outburst onset. The X-ray spectra show that the ejecta are enhanced in nitrogen and oxygen, and the nova occurredmore »
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Accepted Manuscript1.0
- Publisher's Version of Record
- https://doi.org/10.1146/annurev-astro-112420-114502
