SDSS J134441.83+204408.3: A Highly Asynchronous Short-period Magnetic Cataclysmic Variable with a 56 MG Field Strength

When the accreting white dwarf in a magnetic cataclysmic variable star (mCV) has a field strength in excess of 10 MG, it is expected to synchronize its rotational frequency to the binary orbit frequency, particularly at small binary separations, due to the steep radial dependence of the magnetic field. We report the discovery of an mCV (SDSS J134441.83+204408.3, hereafter J1344) that defies this expectation by displaying asynchronous rotation (P (spin)/P (orb) = 0.893) in spite of a high surface field strength (B = 56 MG) and a short orbital period (114 minutes). Previously misidentified as a synchronously rotating mCV, J1344 was observed by Transiting Exoplanet Survey Satellite during sector 50, and the resulting power spectrum shows distinct spin and orbital frequencies, along with various sidebands and harmonics. Although there are several other asynchronous mCVs at short orbital periods, the presence of cyclotron humps in J1344's Sloan Digital Sky Survey spectrum makes it possible to directly measure the field strength in the cyclotron-emitting region, and while a previously study estimated 65 MG based on its identification of two cyclotron humps, we revise this to 56 +/- 2 MG based on the detection of a third hump and on our modeling of the cyclotron spectrum. Short-period mCVs with field strengths above 10 MG are normally expected to be synchronous, so the highly asynchronous rotation in J1344 presents an interesting challenge for theoretical studies of spin-period evolution.

Automated summary

In this study, we report the discovery of an accreting white dwarf in a magnetic cataclysmic variable star (mCV) that displays asynchronous rotation despite having a high surface field strength and a short orbital period. This challenges the theoretical understanding of spin-period evolution.