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The post-common-envelope X-ray binary nucleus of the planetary nebula NGC 2392
Miszalski, Brent; Manick, Rajeev; Van Winckel, Hans; Escorza, Ana
AA(South African Astronomical Observatory, PO Box 9, Observatory, 7935, South Africa 0000-0003-2561-6306), AB(Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium), AC(Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium), AD(Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium)
Publications of the Astronomical Society of Australia, Volume 36, id. e018 (PASA Homepage)
Publication Date:
Astronomy Keywords:
accretion, accretion disks, planetary nebulae: general, planetary nebulae: individual: NGC 2392 (PN G197.8+17.3), techniques: radial velocities, white dwarfs, X-rays: binaries
Abstract Copyright:
2019: Astronomical Society of Australia
Bibliographic Code:


The Chandra X-ray Observatory has detected relatively hard X-ray emission from the central stars of several planetary nebulae (PNe). A subset has no known late-type companions, making it very difficult to isolate which of several competing mechanisms may be producing the X-ray emission. The central star of NGC 2392 is one of the most vexing members, with substantial indirect evidence for a hot white dwarf (WD) companion. Here we report on the results of a radial velocity (RV) monitoring campaign of its central star with the HERMES échelle spectrograph of the Flemish 1.2 m Mercator telescope. We discover a single-lined spectroscopic binary with an orbital period of 1.902208 ± 0.000013 d and an RV semi-amplitude of 9.96 ± 0.13 km s-1. The high degree of nebula ionisation requires a WD companion (M ≳ 0.6M&sun;), which the mass-function supports at orbital inclinations ≲ 7°, in agreement with the nebula orientation of 9°. The hard component of the X-ray spectrum may be explained by the companion accreting mass from the wind of the Roche lobe filling primary, while the softer component may be due to colliding winds. A companion with a stronger wind than the primary could produce the latter and would be consistent with models of the observed diffuse X-ray emission detected in the nebula. The diffuse X-rays may also be powered by the jets of up to 180 km s-1, and active accretion would imply that they may be the first active jets of a post-common-envelope PN, potentially making NGC 2392 an invaluable laboratory to study jet formation physics. The 1.9 d orbital period rules out a double-degenerate merger leading to a Type Ia supernova, and the weak wind of the primary likely also precludes a single-degenerate scenario. We suggest that a hard X-ray spectrum, in the absence of a late-type companion, could be a powerful tool to identify accreting WD companions.
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