Origin of magnetic fields in magnetic white dwarfs

Determine the physical origin of the powerful magnetic fields observed in magnetic white dwarfs, identifying which mechanisms—such as fossil fields from progenitors, crystallization-driven dynamos, or common-envelope dynamos—are responsible and under what evolutionary conditions they operate.

Background

Magnetic white dwarfs exhibit surface fields spanning 103–109 G, with many objects showing complex, non-dipolar structures. Several formation channels have been proposed, including inheritance of fossil fields from progenitors, dynamo action during crystallization, and dynamos active in common-envelope phases of binary evolution.

Understanding the origin of these fields is crucial for modeling magnetospheric environments that influence weakly interacting slim particle (WISP) searches, including axion–photon conversion signatures in X-ray and polarization observables. Despite multiple hypotheses, a definitive, unifying explanation for the observed magnetic field strengths and geometries in magnetic white dwarfs has not yet been established.

References

Nevertheless, the physical origin of the powerful magnetic fields observed in MWDs remains an open question.

— Lectures on Light Particles and Compact Objects  (2604.00125 - Lella et al., 31 Mar 2026) in Section 5, WISP searches with White Dwarfs (Section label: sec:WDs)

We tentatively suggest that a solar-like $r$-process abundance pattern would be reproduced by AIC only in the presence of an extremely strong magnetic field in the progenitor WD, well above the value adopted here, although it is not clear how such strong field configurations would be established in nature.

— Nucleosynthesis in the Accretion-induced Collapse of magnetised, Rotating White Dwarfs  (2608.21000 - Thümmler et al., 21 Aug 2026) in Section 5, “Conclusions”

The origin of white dwarf magnetic fields is an open question.

— Circular polarimetry of suspect wind-accreting pre-polars II  (2608.25439 - Hakala et al., 26 Aug 2026) in Abstract; Section 1, Introduction