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 motion6,7. 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.
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Acknowledgements The National Radio Astronomy Observatory (NRAO) is a facility of the US National Science Foundation (NSF) operated under cooperative agreement by Associated Universities, Inc. The EVN is a joint facility of European, Chinese, South African and other radio astronomy institutes funded by their respective national research councils. The EVN and e-VLBI research infrastructures were supported by the European Commission Seventh Framework Programme (FP/2007-2013) under grant agreements nos 283393 (RadioNet3) and RI-261525 (NEXPReS). e-MERLIN is operated by The University of Manchester at Jodrell Bank Observatory on behalf of the Science and Technology Facilities Council. The SMA is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics. Support for CARMA construction came from the Moore Foundation, the Norris Foundation, the McDonnell Foundation, the Associates of the California Institute of Technology, the University of Chicago, the states of California, Illinois and Maryland, and the NSF. Ongoing CARMA development and operations are supported by the NSF and by the CARMA partner universities. L.C. is a Jansky Fellow of the NRAO. This research received funding from NASA programmes DPR S-15633-Y and 10-FERMI10-C4-0060 (C.C.C.), NASA award NNX13AO91G (T.N.), NSF award AST-1211778 (J.L.S. and J.W.), the South African SKA Project (V.A.R.M.R.) and the Alexander von Humboldt Foundation (N.R.).
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