SDSS J134441.83+204408.3: A highly asynchronous, short-period magnetic cataclysmic variable with a 56 MG field strength

TL;DR

This paper reports the discovery of a highly asynchronous magnetic cataclysmic variable star with a 56 MG magnetic field, challenging existing theories that predict synchronization at such high field strengths.

Contribution

The study provides the first direct measurement of the magnetic field in an asynchronous short-period mCV and discusses its implications for spin-period evolution theories.

Findings

Discovered a highly asynchronous short-period mCV with a 56 MG magnetic field.

Revised the magnetic field strength estimate to 56±2 MG based on cyclotron spectrum modeling.

Challenges existing models predicting synchronization at high magnetic field strengths.

Abstract

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 min). Previously misidentified as a synchronously rotating mCV, J1344 was observed by TESS 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 SDSS spectrum makes it possible to directly measure the field strength in the cyclotron-emitting region; a previously study estimated 65 MG based on its identification of two cyclotron humps, but we revise this to 56$\pm$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.