Binary orbits as the driver of γ-ray emission and mass ejection in classical novae

Abstract

Classical novae are the most common astrophysical thermonuclear explosions, occurring on the surfaces of white dwarf stars accreting gas from companions in binary star systems. Novae typically expel about 10(-4) solar masses of material at velocities exceeding 1,000 kilometres per second. However, the mechanism of mass ejection in novae is poorly understood, and could be dominated by the impulsive flash of thermonuclear energy, prolonged optically thick winds or binary interaction with the nova envelope. Classical novae are now routinely detected at gigaelectronvolt γ-ray wavelengths, suggesting that relativistic particles are accelerated by strong shocks in the ejecta. Here we report high-resolution radio imaging of the γ-ray-emitting nova V959 Mon. We find that its ejecta were shaped by the motion of the binary system: some gas was expelled rapidly along the poles as a wind from the white dwarf, while denser material drifted out along the equatorial plane, propelled by orbital motion. At the interface between the equatorial and polar regions, we observe synchrotron emission indicative of shocks and relativistic particle acceleration, thereby pinpointing the location of γ-ray production. Binary shaping of the nova ejecta and associated internal shocks are expected to be widespread among novae, explaining why many novae are γ-ray emitters.

Fermi-Lat Gamma-Ray Detections of Classical Novae V1369 Centauri 2013 and V5668 Sagittarii 2015

“…The best-fit values of the slopes and cutoff energies (GeV) in the hadronic (s p , E cp ) and leptonic (s e , E ce ) models (Table 2) discoveries. This delay could indicate the timescale for particle acceleration in the ejecta or γ-ray absorption at early times when the ejecta are most dense (Abdo et al 2010;Ackermann et al 2014;Chomiuk et al 2014;Metzger et al 2015). These explanations could have distinct signatures in early, timeresolved LAT spectra, with, e.g., the latter scenario predicting progressively harder γ-ray spectra due to increased transmission of higher-energy photons, but the limited statistics in these cases prevent such an analysis.…”

“…While observationally based scenarios have been developed for symbiotic systems (Tatischeff & Hernanz 2007), and V407 Cyg in particular (Abdo et al 2010;Chomiuk et al 2012;Nelson et al 2012;Orlando & Drake 2012;Martin & Dubus 2013;Pan et al 2015), understanding the particle acceleration revealed in the classical novae is still an open problem. Thus, future Fermi-LAT observations of novae (where deep pointed observations are imperative to study the brightest γ-rayemitting novae in detail) can be exploited with multiwavelength observations and modeling to test scenarios for how, when, and where the putative shocks are generated, e.g., in internal shocks or strong turbulence driven in the ejecta (Ackermann et al 2014) or in wind-wind interactions (Chomiuk et al 2014), and can implicate the underlying γ-ray emission mechanism (see, e.g., Metzger et al 2015).…”

Forming equatorial rings around dying stars

“…Another interesting object is the classical nova V959 Mon, for which Chomiuk et al (2014) argue for equatorial outflow and a faster polar outflow. A clear bi-lobed morphology appears ∼ 100 days past the detection of γ-ray emission.…”

“…A clear bi-lobed morphology appears ∼ 100 days past the detection of γ-ray emission. Chomiuk et al (2014) mention that a fast central wind can be focused to the polar directions by the dense equatorial gas. We note that it is quite possible that two opposite jets were launched by the WD few months after outburst, as has been suggested by Retter (2004) for nova V1494 Aql.…”

Rise and fall of the dust shell of the classical nova V339 Delphini

“…The first γ -ray detected nova with a main-sequence star, V595 Monocerotis, provided a vital clue in the bipolar morphology of the ejecta (Ribeiro, Munari & Valisa 2013;Shore et al 2013;Linford et al 2015). The bipolar morphology was interpreted as originating from the interaction of the ejecta with the motion of the binary system, allowing gas to be expelled freely in the polar directions, while within the equatorial plane material flowed more slowly; this set up a system of shocks, observed as synchrotron emission at radio frequencies, at the interface between the equatorial and polar regions where the γ -ray production was proposed to occur (Chomiuk et al 2014).…”