MASTER OT J132104.04+560957.8: A Polar with Absorption-Emission Line Reversals

We present time-resolved photometry and spectroscopy of the recently classified polar MASTER OT J132104.04+560957.8. The spectrum shows a smooth, nonthermal continuum at the time of maximum light, without any individually discernible cyclotron harmonics. Using homogenous cyclotron modeling, we interpret this as cyclotron radiation whose individual harmonics have blended together, and on this basis, we loosely constrain the magnetic-field strength to be less than ∼30 MG. In addition, for about one-tenth of the orbital period, the Balmer and He i emission lines transition into absorption features, with He ii developing an absorption core. We use our observations of this phenomenon to test theoretical models of the accretion curtain and conclude that the H and He i lines are produced throughout the curtain, in contravention of theoretical predictions of separate H and He i line-forming regions. Moreover, a significant amount of He ii emission originates within the accretion curtain, implying that the curtain is significantly hotter than expected from theory. Finally, we comment on the object's long-term photometry, including evidence that it recently transitioned into a prolonged, exceptionally stable high state following a potentially decades-long low state.