Classical novae are runaway thermonuclear burning events on the surfaces of accreting white dwarfs in close binary star systems, sometimes appearing as new naked-eye sources in the night sky1. The standard model of novae predicts that their optical luminosity derives from energy released near the hot white dwarf, which is reprocessed through the ejected material2,3,4,5. Recent studies using the Fermi Large Area Telescope have shown that many classical novae are accompanied by gigaelectronvolt γ-ray emission6,7. This emission likely originates from strong shocks, providing new insights into the properties of nova outflows and allowing them to be used as laboratories for the study of the unknown efficiency of particle acceleration in shocks. Here, we report γ-ray and optical observations of the Milky Way nova ASASSN-16ma, which is among the brightest novae ever detected in γ-rays. The γ-ray and optical light curves show a remarkable correlation, implying that the majority of the optical light comes from reprocessed emission from shocks rather than the white dwarf8. The ratio of γ-ray to optical flux in ASASSN-16ma directly constrains the acceleration efficiency of non-thermal particles to be around 0.005, favouring hadronic models for the γ-ray emission9. The need to accelerate particles up to energies exceeding 100 gigaelectronvolts provides compelling evidence for magnetic field amplification in the shocks.
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We acknowledge the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this research. We thank the Fermi Science Support Center, supported by the Flight Operations Team, for scheduling the Fermi target-of-opportunity observations. We also acknowledge the use of public data from the Fermi and Swift data archives. We thank Las Cumbres Observatory and its staff for their continued support of ASAS-SN. ASAS-SN is funded in part by the Gordon and Betty Moore Foundation through grant GBMF5490 to the Ohio State University. ASAS-SN is supported by National Science Foundation grant AST-1515927. Development of ASAS-SN has been supported by National Science Foundation grant AST-0908816, the Center for Cosmology and Astro Particle Physics at the Ohio State University, the Mt. Cuba Astronomical Foundation and G. Skestos. We also acknowledge useful discussions with J. Linford, A. Mioduszewski, K. Mukai, J. Sokoloski, K. Stanek and J. Weston, which greatly improved the quality of the paper. This work was partially supported by Fermi GI grant NNX14AQ36G. I.V. acknowledges support from the Estonian Research Council grant PUT1112. J.S. acknowledges support from the Packard Foundation. L.C. acknowledges a Cottrell Scholar Award from the Research Corporation for Science Advancement. B.J.S. is supported by the National Aeronautics and Space Administration through Hubble Fellowship grant HST-HF-51348.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy for the National Aeronautics and Space Administration, under contract NAS 5-26555. T.W.-S.H. is supported by the Department of Energy’s Computational Science Graduate Fellowship, grant number DE-FG02-97ER25308. C.S.K. and T.W.-S.H. are supported by National Science Foundation grants AST-1515876 and AST-1515927.
Electronic supplementary material
Supplementary Text, Supplementary References, Supplementary Tables 1–3 and Supplementary Figures 1–6.