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

Classical novae are the most common astrophysical thermonuclear explosions, occurring on the surfaces of white dwarf stars accreting gas from companions in binary star systems1. Novae typically expel about 10^-4 solarmasses ofmaterial at velocities exceeding 1,000 kilometres per second.However, the mechanismofmass ejection innovae is poorly understood, and could be dominated by the impulsive flash of thermonuclear energy², prolonged optically thick winds³ or binary interaction with the nova envelope⁴. Classical novae are now routinely detected at gigaelectronvolt γ-ray wavelengths⁵, suggesting that relativistic particles are accelerated by strong shocks in the ejecta. Here we report high-resolution radio imaging of the γ-rayemitting nova V959 Mon. Wefind that its ejecta were shaped by the motion of the binary system: somegas 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^6,7. At the interface between the equatorial and polar regions, we observe synchrotronemission indicative of shocks and relativistic particle acceleration, thereby pinpointingthe locationof γ-ray production. Binary shaping of the nova ejecta and associated internal shocks are expected to be widespread among novae⁸, explaining why many novae are γ-ray emitters⁵.