A Link Between White Dwarf Pulsars and Polars: Multiwavelength Observations of the 9.36-minute Period Variable Gaia22ayj

Abstract

<p>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 greater than or similar to 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/P-center dot approximate to 5.6 x 10(6) yr timescale. It is believed they will eventually become polars, accreting systems in which a magnetic WD (B approximate to 10-240 MG) accretes from a Roche lobe-filling donor spinning in sync with the orbit (greater than or similar to 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/P-center dot = 6.1(-0.2)(+0.3 )x 10(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 (similar to 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 & Aring;. We find an X-ray luminosity of L-X = 2.7(-0.8)(+6.2 )x 10(32)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 mu Jy 3 sigma 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.<br></p>