A half ring of ionized circumstellar material trapped in the magnetosphere of a white dwarf merger remnant

A new class of white dwarf merger remnants with X-ray emission

¹ Institute of Science and Technology Austria Am Campus 1 3400 Klosterneuburg, Austria
² Division of Physics, Mathematics and Astronomy, California Institute of Technology Pasadena CA 91125, USA
³ Center for Astrophysics – Harvard & Smithsonian 60 Garden St. Cambridge MA 02138, USA
⁴ Mathematical Sciences Institute, The Australian National University Hanna Neumann Building 145 ACT 2601 Canberra, Australia
⁵ International Center for Radio Astronomy Research, Curtin University GPO Box U1987 Perth WA 6845, Australia
⁶ Department of Physics and Astronomy, University of British Columbia Vancouver BC V6T 1Z1, Canada
⁷ Department of Astronomy & Institute for Astrophysical Research, Boston University 725 Commonwealth Ave. Boston MA 02215, USA
⁸ TAPIR, Mailcode 350-17, California Institute of Technology Pasadena CA 91125, USA
⁹ Anton Pannekoek Institute for Astronomy, University of Amsterdam NL-1090 GE Amsterdam, The Netherlands
¹⁰ Department of Physics, Massachusetts Institute of Technology Cambridge MA 02139, USA
¹¹ Department of Physics, University of Warwick Gibbet Hill Road Coventry CV4 7AL, UK
¹² Centre for Exoplanets and Habitability, University of Warwick Gibbet Hill Road Coventry CV4 7AL, UK
¹³ University of Washington, Department of Astronomy Box 351580 Seattle WA 98195, USA
¹⁴ Max-Planck-Institut für Astrophysik Karl-Schwarzschild-Str 1 D-85748 Garching, Germany
¹⁵ IPAC, California Institute of Technology 1200 E. California Blvd Pasadena CA 91125, USA
¹⁶ Caltech Optical Observatories, California Institute of Technology Pasadena CA 91125, USA
¹⁷ DIRAC Institute, Department of Astronomy, University of Washington 3910 15th Avenue NE Seattle WA 98195, USA

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Received: 15 July 2025
Accepted: 11 October 2025

Abstract

Many white dwarfs are observed in compact double white dwarf binaries, and through the emission of gravitational waves, a large fraction are destined to merge. The merger remnants that do not explode in a Type Ia supernova are expected to initially be rapidly rotating and highly magnetized. In this work, we present our discovery of the variable white dwarf ZTF J200832.79+444939.67, hereafter ZTF J2008+4449, as a likely merger remnant showing signs of circumstellar material without a stellar or substellar companion. The nature of ZTF J2008+4449 as a merger remnant is supported by its physical properties: it is hot (35 500 ± 300 K) and massive (1.12 ± 0.03 M_⊙), rapidly rotating with a period of ≈6.6 minutes, and likely possesses exceptionally strong magnetic fields (∼400−600 MG) at its surface. Remarkably, we detect a significant period derivative of (1.80 ± 0.09)×10⁻¹² s/s, indicating that the white dwarf is spinning down, and a soft X-ray emission that is inconsistent with photospheric emission. As the presence of a mass-transferring stellar or brown dwarf companion is excluded by infrared photometry, the detected spin-down and X-ray emission could be tell-tale signs of a magnetically driven wind or of interaction with circumstellar material, possibly originating from the fallback of gravitationally bound merger ejecta or from the tidal disruption of a planetary object. We also detect Balmer emission, which requires the presence of ionized hydrogen in the vicinity of the white dwarf, showing Doppler shifts as high as ≈2000 km s⁻¹. The unusual variability of the Balmer emission on the spin period of the white dwarf is consistent with the trapping of a half ring of ionized gas in the magnetosphere of the white dwarf.