August
2026
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2026A&A...712A.230D
Authors
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Desai, Aayush
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Caiazzo, Ilaria
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Vennes, Stephane
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Kawka, Adela
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Cunningham, Tim
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Kotiwale, Gauri
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Cristea, Andrei A.
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Raymond, John C.
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Camisassa, Maria
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Althaus, Leandro G.
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Hermes, J. J.
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Traulsen, Iris
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Fuller, James
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Heyl, Jeremy
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van Roestel, Jan
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Burdge, Kevin B.
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Rodriguez, Antonio C.
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Pelisoli, Ingrid
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Gänsicke, Boris T.
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Szkody, Paula
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Maheshwari, Sumit K.
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Vanderbosch, Zachary P.
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Drake, Andrew
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Ferrario, Lilia
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Wickramasinghe, Dayal
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Justham, Stephen
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Pakmor, Ruediger
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El-Badry, Kareem
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Prince, Thomas
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Kulkarni, S. R.
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Graham, Matthew J.
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Rusholme, Ben
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Laher, Russ R.
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Purdum, Josiah
Abstract
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Double-degenerate white dwarf (WD) merger remnants can exhibit extreme magnetic fields exceeding 108 G and rapid rotation, but their spectral energy distributions and high-energy emission mechanisms remain poorly characterised. ZTF J1901+1458 stands out as the most compact and strongly magnetised object discovered in this class to date. Intriguingly, recent Chandra observations have revealed that the white dwarf is also a source of soft X-ray emission that is too bright and hard to be of photospheric origin. We analysed new phase-resolved ultraviolet (UV) spectroscopy from the Hubble Space Telescope, together with optical and near-infrared photometry and spectroscopy, using new magnetic atmosphere models to determine its effective temperature, radius, mass, average surface magnetic-field strength, and cooling age. The spectral break at ≈3000 Å, observed in several highly magnetised WDs, is well reproduced by our new models, which account for the effect of magnetic opacities on the atmospheric structure. Our best-fit parameters for the WD yield a cooler effective temperature ( Teff = 27,445+680-1390 T eff = 27 , 445 - 1390 + 680 K) and a larger radius than previously reported. Furthermore, the near-infrared data exclude the presence of a stellar or brown dwarf companion hotter than ≈700 K. We jointly analysed published Chandra/Advanced CCD Imaging Spectrometer Imaging array (ACIS-I) data and new XMM-Newton/European Photon Imaging Camera (EPIC) X-ray spectra. The faint X-ray emission, LX = (1.44 ± 0.13)×1027 erg s-1, is highly pulsed at the rotation period of the WD, and the soft spectrum can be modelled by a power-law model with photon index Γ = 2.43+0.17-0.15 Γ = 2 . 43 - 0.15 + 0.17 . We suggest that the X-rays are powered by accretion or by interaction between the WD magnetosphere and circumstellar material. A rapidly rotating magnetic field could power a weak wind along open field lines and extract material from the surface of the WD. Alternatively, low-level accretion of fallback material from the past merger event or the tidal disruption of a planetary body could supply the circumstellar material.
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