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

Le résumé fourni par la source

In the recent years, the wealth of data on potential double white dwarf merger remnants made available by time-domain surveys like the Zwicky Transient Facility (ZTF), ATLAS, Kepler and TESS, together with spectroscopic counterparts such as SDSS V and DESI, has been essential in advancing our knowledge of ompact binary evolution. In particular, the emergence of peculiar systems offers invaluable insight into the limitations of our current best theories. The periodically variable white dwarf ZTF J200832.79+444939.67, hereafter ZTF J2008+4449, is possibly one of the most exotic discovered so far: a likely merger remnant showing signs of accretion from circumstellar material without a stellar or substellar companion. The nature of ZTF J2008+4449 as a merger remnant is supported by its physical properties: hot (35, 500±300 K) and massive (1.12±0.03 M⊙), the white dwarf is rapidly rotating on 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 (1.8 ± 0.1) × 10^12 s/s, indicating that the white dwarf is spinning down. This, together with the detection of soft X-rays emissions, are tell-tale signs of accretion or, more generally, interaction with circumstellar material; in the absence of a companion, we believe this likely belongs to the fallback of gravitationally bound merger ejecta. We also detect Balmer emission whose unusual variability on the spin period of the white dwarf, showing Doppler shifts as high as ≈ 2, 000 km/s, is consistent with the trapping of a toroidal half-arc of ionised gas in the white dwarf’s magnetosphere.

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