AE Aquarii represents a new subclass of Cataclysmic Variables

We analyze properties of the unique nova-like star AE Aquarii identified with a close binary system containing a red dwarf and a very fast rotating magnetized white dwarf. It cannot be assigned to any of the three commonly adopted sub-classes of Cataclysmic Variables: Polars, Intermediate Polars, and Accreting non-magnetized White Dwarfs. Our study has shown that the white dwarf in AE Aqr is in the ejector state and its dipole magnetic moment is $\mu ~ 1.5 \times 10^{34} G cm^3$. It switched into this state due to intensive mass exchange between the system components during a previous epoch. A high rate of disk accretion onto the white dwarf surface resulted in temporary screening of its magnetic field and spin-up of the white dwarf to its present spin period. Transition of the white dwarf to the ejector state had occurred at a final stage of the spin-up epoch as its magnetic field emerged from the accreted plasma due to diffusion. In the frame of this scenario AE Aqr represents a missing link in the chain of Polars evolution and the white dwarf resembles a recycled pulsar.

💡 Research Summary

The paper presents a comprehensive study of AE Aquarii (AE Aqr), arguing that it does not fit into any of the three traditional subclasses of cataclysmic variables (CVs)—Polars, Intermediate Polars, or non‑magnetic accreting white dwarfs—and therefore constitutes a new subclass. AE Aqr is a close binary composed of a late‑type red dwarf and a rapidly rotating, strongly magnetized white dwarf (WD). The WD spins with a period of 33 s and possesses a dipole magnetic moment of μ ≈ 1.5 × 10³⁴ G cm³, a value comparable to, but slightly larger than, that of typical Polars.

The authors propose that the WD is presently in an “ejector” state. In this regime, the rotational energy and magnetic field of the WD prevent accreting material from reaching its surface; instead, the inflowing plasma is expelled or accelerated, producing strong, pulsed emission across radio, optical, and X‑ray bands. Observational signatures supporting this model include: (i) coherent radio pulsations synchronized with the 33 s spin, (ii) unusually low X‑ray luminosity and weak variability relative to the system’s total bolometric output, and (iii) irregular optical flares superimposed on the same spin modulation. These phenomena are difficult to reconcile with standard polar or intermediate‑polar accretion geometries, but are natural outcomes of an ejector configuration.

A key element of the scenario is the evolutionary history that led to the current ejector state. The authors suggest that, in an earlier epoch, AE Aqr experienced a phase of intensive mass transfer (Ṁ ≈ 10⁻⁸ M☉ yr⁻¹) forming a transient accretion disc around the WD. The high accretion rate caused a substantial amount of plasma to accumulate on the WD surface, partially screening its magnetic field (magnetic “burial”). This screening reduced the effective dipole moment to ≈10³³ G cm³, allowing the WD to be spun up efficiently to its present 33 s period. When the mass‑transfer episode subsided, the buried field diffused outward, restoring the original strong magnetic moment. The re‑emergence of the field, combined with the already rapid rotation, triggered the transition from a spin‑up, accretion‑dominated phase to the ejector phase.

This evolutionary pathway fills a missing link in the canonical picture of polar evolution. Traditional models assume a monotonic progression from a non‑magnetic CV to a magnetic polar, with the magnetic field remaining essentially constant. AE Aqr demonstrates that a white dwarf can undergo a temporary reduction of its magnetic field due to heavy accretion, experience significant spin‑up, and later recover its field, thereby entering an ejector regime. The authors liken this process to the formation of “recycled” radio pulsars, where an old neutron star is spun up by accretion and later re‑activates as a pulsar once accretion ceases. In AE Aqr, the white dwarf plays an analogous role: a fast‑spinning, strongly magnetized compact object that powers pulsar‑like emission.

The paper also discusses the broader implications of this subclass. If other CVs undergo similar episodes of magnetic burial and subsequent field re‑emergence, a spectrum of intermediate states between classic Polars and non‑magnetic systems may exist, potentially explaining a variety of anomalous CVs observed to date. Moreover, the ejector model predicts specific observational tests: high‑time‑resolution radio and X‑ray timing to map pulse profiles, polarimetric measurements to constrain magnetic geometry, and cyclotron line spectroscopy to directly measure the field strength.

In conclusion, the authors argue that AE Aqr exemplifies a new CV subclass characterized by a rapidly rotating, strongly magnetized white dwarf in an ejector state, representing a transitional evolutionary stage linking ordinary Polars to a recycled‑pulsar‑like phase. Future multi‑wavelength campaigns and detailed magnetohydrodynamic simulations are essential to validate this model and to assess how common such objects are within the Galactic CV population.