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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 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 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-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 motion6,7. 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 novae8, explaining why many novae are γ-ray emitters5.

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Figure 1: Radio light curves and spectra of V959 Mon.
Figure 2: Radio imaging of V959 Mon.
Figure 3: Simple illustration of the 2012 outburst of V959 Mon.


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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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Authors and Affiliations



L.C. wrote the text. L.C., J.D.L., J.Y., T.J.O., Z.P., A.J.M., C.C.C., R.J.B., T.N., Y.Z., J.W. and G.B.T. obtained and reduced the data. All authors contributed to the interpretation of the data and commented on the final manuscript.

Corresponding author

Correspondence to Laura Chomiuk.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Radio/millimetre spectral evolution of V959 Mon.

Measurements and 1σ uncertainties from select epochs are shown as black points. Power-law or broken power-law fits are overplotted as red lines (the function is chosen to minimize the reduced χ2 value). The best-fit spectral indices are listed in each panel, along with the break frequency (νb) in the case of broken power-law fits.

Extended Data Figure 2 Spectral index map from 2012 October 3 VLBA observations.

The spectral index is measured by comparing images at 1.6 and 5 GHz. Overlaid contours are from the 1.6 GHz Stokes I map. Contour levels are −0.08, 0.08, 0.13, 0.16, 0.23, 0.32 and 0.45 mJy per beam.

Extended Data Figure 3 The expansion of V959 Mon as a function of time.

Semi-major axis (a) and semi-minor axis (b), both in units of milliarcseconds. Measurements at four distinct frequencies are plotted in different colours (see key). Error bars from JMFIT (1σ) are so small that they are not visible. Linear fits are made to each frequency separately, and are plotted as coloured lines.

Extended Data Figure 4 Model fit to the radio/millimetre light curve of V959 Mon.

A simple model of thermal expanding ejecta roughly describes the light curve evolution at day 200 and later, and implies an ejected mass of few 10−5 solar masses. Error bars denote 1σ uncertainty.

Extended Data Table 1 VLA observations of V959 Mon
Extended Data Table 2 Millimetre observations of V959 Mon
Extended Data Table 3 VLBI components of V959 Mon

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Chomiuk, L., Linford, J., Yang, J. et al. Binary orbits as the driver of γ-ray emission and mass ejection in classical novae. Nature 514, 339–342 (2014).

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