Nova SMC 2016 has been the most luminous nova known in the direction of the Magellanic Clouds. It turned into a very luminous supersoft X-ray source between days 16 and 28 after the optical maximum. We observed it with Chandra, the HRC-S camera, and the Low Energy Transmission Grating on 2016 November and 2017 January (days 39 and 88 after optical maximum), and with XMM-Newton on 2016 December (day 75). We detected the compact white dwarf (WD) spectrum as a luminous supersoft X-ray continuum with deep absorption features of carbon, nitrogen, magnesium, calcium, probably argon, and sulfur on day 39, and oxygen, nitrogen, and carbon on days 75 and 88. The spectral features attributed to the WD atmosphere are all blueshifted, by about 1800 km s-1 on day 39 and up to 2100 km s-1 in the following observations. Spectral lines attributed to low-ionization potential transitions in the interstellar medium are also observed. Assuming the distance to the Small Magellanic Cloud, the bolometric luminosity exceeded the Eddington level for at least three months. A preliminary analysis with atmospheric models indicates an effective temperature of around 700,000 K on day 39, peaking at the later dates in the 850,000-900,000 K range, as expected for a ≃1.25 M o WD. We suggest a possible classification as an oxygen-neon WD, but more precise modeling is needed to accurately determine the abundances. The X-ray light curves show a large, aperiodic flux variability, which is not associated with spectral variability. We detected red noise, but did not find periodic or quasiperiodic modulations.
Bibliographical noteFunding Information:
1Department of Astronomy, University of Wisconsin, 475 N. Charter St., Madison WI 53706, USA; firstname.lastname@example.org 2INAF-Osservatorio Astronomico di Padova, vicolo Osservatorio, 5, I-35122 Padova, Italy 3XMM-Newton Science Operations Center, European Space Astronomy Center, Camino Bajo del Castillo s/n, Urb. Villafranca del Castillo, Villanueva de la Caada, E-28692 Madrid, Spain 4Advanced Technologies Research Institute, Faculty of Materials Science and Technology in Trnava, Slovak University of Technology in Bratislava, Bottova 25, 917 24 Trnava, Slovakia 5ESO, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany 6via Pietro Pomponazzi, 33, I-35124 Padova, Italy 7South African Astronomical Observatory, P.O. Box 9, 7935 Observatory, South Africa 8Department of Physics, Technion, Haifa, Israel 9Astronomy Department, University of Cape Town, 7701 Rondebosch, South Africa 10Dipartimento di Fisica e Astronomia, vicolo Osservatorio, 3, I-35122 Padova, Italy 11INAF-Osservatorio Astronomico di Capodimonte, Salita Moiarello, 16, I-80131 Napoli, Italy 12Institut de Ciències de l’ Espai (ICE-CSIC), Campus UAB, c/ Can Magrans s/n, E-08193, Bellaterra, Spain 13Institut d’Estudis Espacials de Catalunya, c/Gran Capità 2-4, Ed. Nexus-201, E-08034, Barcelona, Spain 14Department of Astronomy, San Diego State University, San Diego, CA 92182, USA 15X-ray and Observational Astronomy Group, Department of Physics & Astronomy, University of Leicester, LE1 7RH, UK 16Institute for Astronomy and Astrophysics, Kepler Center for Astro and Particle Physics, Eberhard Karls University, Sand.1., D-72076 Tübingen, Germany 17Departament de Física, EEBE, Universitat Politècnica de Catalunya, BarcelonaTech., Av. d’ Eduard Maristany 10-14, E-08019, Barcelona, Spain 18School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA 19Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 USA 20Minnesota Institute for Astrophysics, University of Minnesota, 116 Church Street, SE, Minneapolis, MN 55455, USA 21via dei Tadi, 5, I-35139 Padova, Italy
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- X-rays: stars
- novae, cataclysmic variables
- stars: abundances
- stars: dwarf novae
- stars: individual (N SMC 2016a)