Circumstellar interaction in the extreme white dwarf merger remnant ZTF J1901+1458

Double-degenerate white dwarf (WD) merger remnants can exhibit extreme magnetic fields exceeding 10 8 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 ( T eff = 27,445 +680 −1390 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, L X = (1.44 ± 0.13)×10 27 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 . 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.