Direct Evidence for Shock-Powered Optical Emission in a Nova
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LETTERS https://doi.org/10.1038/s41550-020-1070-y Direct evidence for shock-powered optical emission in a nova Elias Aydi 1 ✉ , Kirill V . Sokolovsky 1,2 ✉ , Laura Chomiuk1 ✉ , Elad Steinberg3,4, Kwan Lok Li 5,6, Indrek Vurm7, Brian D. Metzger3, Jay Strader1, Koji Mukai8,9, Ondřej Pejcha10, Ken J. Shen11, Gregg A. Wade12, Rainer Kuschnig13, Anthony F. J. Moffat14, Herbert Pablo15, Andrzej Pigulski 16, Adam Popowicz17, Werner Weiss18, Konstanze Zwintz19, Luca Izzo 20, Karen R. Pollard 21, Gerald Handler 22, Stuart D. Ryder 23, Miroslav D. Filipović 24, Rami Z. E. Alsaberi 24, Perica Manojlović24, Raimundo Lopes de Oliveira25,26, Frederick M. Walter27, Patrick J. Vallely28, David A. H. Buckley29, Michael J. I. Brown30, Eamonn J. Harvey31, Adam Kawash1, Alexei Kniazev29,32,33, Christopher S. Kochanek28, Justin Linford 34,35,36, Joanna Mikolajewska22, Paolo Molaro 37, Marina Orio38,39, Kim L. Page 40, Benjamin J. Shappee41 and Jennifer L. Sokoloski3 Classical novae are thermonuclear explosions that occur on common origin in shocks. During the flares, the nova luminos- the surfaces of white dwarf stars in interacting binary sys- ity doubles, implying that the bulk of the luminosity is shock tems1. It has long been thought that the luminosity of clas- powered. Furthermore, we detect concurrent but weak X-ray sical novae is powered by continued nuclear burning on the emission from deeply embedded shocks, confirming that the surface of the white dwarf after the initial runaway2. However, shock power does not appear in the X-ray band and support- recent observations of gigaelectronvolt γ-rays from classi- ing its emergence at longer wavelengths. Our data, spanning cal novae have hinted that shocks internal to the nova ejecta the spectrum from radio to γ-ray, provide direct evidence that may dominate the nova emission. Shocks have also been sug- shocks can power substantial luminosity in classical novae gested to power the luminosity of events as diverse as stel- and other optical transients. 3 4 5 1 −7 −3 lar mergers , supernovae and tidal disruption events , but In a classical nova, the accreted envelope (mass 10 −10 M⊙) observational confirmation has been lacking. Here we report expands and is ejected at velocities of ~500–5,000 km s−1. The result simultaneous space-based optical and γ-ray observations is an optical transient where the luminosity of the system increases of the 2018 nova V906 Carinae (ASASSN-18fv), revealing a by a factor of ~103−106, sometimes making the source visible to the remarkable series of distinct correlated flares in both bands. naked eye6. After the initial ejection of the envelope, residual nuclear The optical and γ-ray flares occur simultaneously, implying a burning continues on the surface of the hot white dwarf, leading 1Center for Data Intensive and Time Domain Astronomy, Department of Physics and Astronomy, Michigan State University, East Lansing, MI, USA. 2Astro Space Center of Lebedev Physical Institute, Moscow, Russia. 3Columbia Astrophysics Laboratory and Department of Physics, Columbia University, New York, NY, US. 4Racah Institute of Physics, Hebrew University, Jerusalem, Israel. 5Department of Physics, UNIST, Ulsan, Korea. 6Institute of Astronomy, National Tsing Hua University, Hsinchu, Taiwan. 7Tartu Observatory, University of Tartu, Tartumaa, Estonia. 8CRESST and X-ray Astrophysics Laboratory, NASA/GSFC, Greenbelt, MD, USA. 9Department of Physics, University of Maryland, Baltimore, MD, USA. 10Institute of Theoretical Physics, Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic. 11Department of Astronomy and Theoretical Astrophysics Center, University of California, Berkeley, Berkeley, CA, US. 12Department of Physics and Space Science, Royal Military College of Canada, Kingston, Ontario, Canada. 13Institute of Communication Networks and Satellite Communications, Graz University of Technology, Graz, Austria. 14Département de physique, Université de Montréal, and Centre de Recherche en Astrophysique du Quéebec, Montréeal, Quebec, Canada. 15AAVSO, Cambridge, MA, USA. 16Instytut Astronomiczny, Uniwersytet Wrocławski, Wrocław, Poland. 17Silesian University of Technology, Institute of Electronics, Gliwice, Poland. 18Institute for Astrophysics, University of Vienna, Vienna, Austria. 19Universität Innsbruck, Institut für Astro- und Teilchenphysik, Innsbruck, Austria. 20DARK, Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark. 21School of Physical and Chemical Sciences, University of Canterbury, Christchurch, New Zealand. 22Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Warsaw, Poland. 23Department of Physics and Astronomy, Macquarie University, Sydney, New South Wales, Australia. 24School of Computing Engineering and Mathematics, Western Sydney University, Penrith, New South Wales, Australia. 25Departamento de Física, Universidade Federal de Sergipe, São Cristóvão, Brazil. 26Observatório Nacional, Rio de Janeiro, Brazil. 27Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY, USA. 28Department of Astronomy, The Ohio State University, Columbus, OH, USA. 29South African Astronomical Observatory, Observatory, South Africa. 30School of Physics and Monash Centre for Astrophysic, Monash University, Clayton, Victoria, Australia. 31Astrophysics Research Institute, Liverpool John Moores University, Liverpool, UK. 32Southern African Large Telescope Foundation, Observatory, South Africa. 33Sternberg Astronomical Institute, Lomonosov Moscow State University, Moscow, Russia. 34Department of Physics and Astronomy, West Virginia University, Morgantown, WV, USA. 35Center for Gravitational Waves and Cosmology, West Virginia University, Morgantown, WV, USA. 36National Radio Astronomy Observatory, Socorro, NM, USA. 37INAF-Osservatorio Astronomico di Trieste, Trieste, Italy. 38INAF–Osservatorio di Padova, Padova, Italy. 39Department of Astronomy, University of Wisconsin, Madison, WI, USA. 40School of Physics and Astronomy, University of Leicester, Leicester, UK. 41Institute for Astronomy, University of Hawai’i, Honolulu, HI, USA. ✉e-mail: [email protected]; [email protected]; [email protected] NATURE ASTronomY | www.nature.com/natureastronomy LETTERS NATURE ASTRONOMY a NuSTAR 5.0 BRITE 5.5 6.0 6.5 7.0 Optical brightness (mag) 7.5 2.5 ) 1 March 2018 14 April 2018 –1 s –2 2.0 b c HD 92063 HD 92063 erg cm 1.5 –9 1.0 Fermi/LAT down time -ray flux (×10 -ray γ 0.5 V906 Car 0 5 10 15 20 25 30 35 40 45 Time since eruption (d) Fig. 1 | Nova V906 Car was discovered in a complex region of the Galaxy near the Carina nebula and the red giant star HD 92063, which was being Fig. 2 | The optical and GeV γ-ray light curves of nova V906 Car are monitored by the BRITE satellite constellation. a, V906 Car pictured in correlated, showing simultaneous flares in both bands. The black dashed outburst and marked by a white arrow. HD 92063 is the brighter nearby lines represent the dates of the post-maximum flares. The green arrow star. b, BRITE image from before the outburst of V906 Car taken on 1 indicates the date of the first NuSTAR X-ray observation. The black solid March 2018, showing only HD 29063. c, BRITE image during the outburst bar indicates the period of Fermi/LAT down time due to technical issues. taken on 14 April 2018, showing both HD 92063 and V906 Car. Credit: A. Fermi entered another observing gap between days 46 and 57. The error Maury and J. Fabrega (a). bars in the BRITE light curve are 1σ uncertainties. The point-to-point scatter of the binned BRITE measurements is ~2 mmag and therefore the size of to a phase of quasi-constant, near-Eddington luminosity powered the error bars is smaller than the symbol size. The error bars in the Fermi by the hot white dwarf2,7. This should manifest as an optical light light curve are 1σ uncertainties. The eruption start is on 2018 March 16.03 curve smoothly declining from maximum light, as the photosphere UT (see Methods for more details). recedes and the peak of the spectral energy distribution moves blue- ward from the optical into the ultraviolet and finally into the soft of the eruption. The GeV γ-ray flux reached 2.1 × 10−9 erg cm−2 s−1 X-ray1. However, some novae show erratic flares around maximum on days 25 and 29, making V906 Car the brightest γ-ray nova to light with a variety of timescales and amplitudes8; these features are date18,19. Current theory suggests that the GeV γ-rays originate from still poorly explored and their origin remains a matter of debate. shocks internal to the nova ejecta—specifically as a fast biconical Proposed explanations include instabilities in the envelope of the wind slams into a slower equatorial torus20,21. The shocks accelerate white dwarf leading to multiple ejection episodes9,10, instabilities in particles to relativistic speeds and γ-rays are produced when these an accretion disk that survived the eruption11 and variations in mass relativistic particles interact with either the surrounding medium or transfer from the secondary to the white dwarf12. seed photons22,23. The optical transient V906 Carinae (ASASSN-18fv) was discov- The exceptional γ-ray brightness of V906 Car allowed us to ered by the All-Sky Automated Survey for Supernovae (ASAS-SN13) obtain the most detailed γ-ray light curve of a nova to date, showing on 2018 March 20.3 ut, and was shortly thereafter spectroscopi- multiple γ-ray peaks. Comparing this with the BRITE light curve, cally confirmed as a classical nova14,15. Serendipitously, V906 Car we see that the γ-ray peaks coincide in time with the optical flares happened to occur in a field being monitored by the BRIght Target (Fig. 2). This correlation implies that the optical and γ-ray emission Explorer (BRITE) nanosatellite constellation16 (Fig. 1), resulting in an in novae share a common origin19. One possibility is that the lumi- unprecedented optical light curve tracking the evolution of the erup- nosity in both bands is driven by shock power—much as has been tion from its start (2018 March 16.13 ut; Fig. 2). The continuous, theorized to occur in type IIn supernovae24. The typical expansion high-cadence BRITE optical light curve (presented with 1.6 h resolu- velocities of nova ejecta (~1,000 km s−1) and timing of the γ-rays tion in Fig.