2025PASP..137b4202R

White dwarfs (WDs) are the most abundant compact objects, and recent surveys have suggested that over a third of WDs in accreting binaries host a strong (B ≳ 1 MG) magnetic field. However, the origin and evolution of WD magnetism remain under debate. Two WD pulsars, AR Sco and J191213.72-441045.1 (J1912), have been found, which are non-accreting binaries hosting rapidly spinning (1.97 minutes and 5.30 minutes, respectively) magnetic WDs. The WD in AR Sco is slowing down on a $P/\dot{P}\approx 5.6\times 1{0}^{6}$ yr timescale. It is believed they will eventually become polars, accreting systems in which a magnetic WD (B ≈ 10-240 MG) accretes from a Roche lobe-filling donor spinning in sync with the orbit (≳78 minutes). Here, we present multiwavelength data and analysis of Gaia22ayj, which outbursted in 2022 March. We find that Gaia22ayj is a magnetic accreting WD that is rapidly spinning down ($P/\dot{P}=6.{1}_{-0.2}^{+0.3}\times 1{0}^{6}$ yr) like WD pulsars, but shows clear evidence of accretion, like polars. Strong linear polarization (40%) is detected in Gaia22ayj; such high levels have only been seen in the WD pulsar AR Sco and demonstrate the WD is magnetic. High speed photometry reveals a 9.36 minutes period accompanying a high amplitude (∼2 mag) modulation. We associate this with a WD spin or spin-orbit beat period, not an orbital period as was previously suggested. Fast (60 s) optical spectroscopy reveals a broad “hump,” reminiscent of cyclotron emission in polars, between 4000 and 8000 Å. We find an X-ray luminosity of ${L}_{X}=2.{7}_{-0.8}^{+6.2}\times 1{0}^{32}{{\rm{ergs}}}^{-1}$ in the 0.3-8 keV energy range, while two very large array radio campaigns resulted in a non-detection with a F_r < 15.8 μJy 3σ upper limit. The shared properties of both WD pulsars and polars suggest that Gaia22ayj is a missing link between the two classes of magnetic WD binaries.