TESS photometry of the asynchronous polar CD Ind: A short period analog of BY Cam

We present a time-series analysis of the asynchronous polar CD Ind, using fast cadence TESS photometry. A similar analysis is performed using ground based photometry of BY Cam. These asynchronous polars show remarkably similar light curves and enigmatic timing characteristics. A pair of competing models for the assignment of the white dwarf spin period have been presented for both binaries. TESS allows for the breaking of this degeneracy by providing continuous coverage over several beat-cycles. The CD Ind light curve displays a super-orbital period of 7.2 days due to beat-phased modulation of the accretion rate onto a permanently visible spot. This (main) accretion region is active for 3/4 of the beat-cycle. The dominant photometric signal is at 109.654(4) min and is identified as a side-band related to the white dwarf spin and the binary orbit, analogous to that found in BY Cam. We obtain the white dwarf spin period of CD Ind as 110.820(5) min and an orbital period of 111.952(8) min. The TESS light curve supports a pole-switching scenario for CD Ind, with a total of 4 alternating, and oppositely positioned accretion regions. This inclination of CD Ind is estimated as i = 65°±10°. One accretion region remains in view of the observer at all times. In addition, two pulsed accretion spots each accrete for just less than 1/2 of the beat-cycle. These alternating spots are orthogonal to the main accretion region (which is always in view), and are found to be roughly 180° apart in longitude. Finally, since the mean flux drops by a factor of 2 for about 1/4 of the beat-cycle and the dominant accretion region turns off during this time, we hypothesize the existence of a fourth (permanently hidden) accretion region opposite to the one permanently in view. Photometric models for the 2- and 4-pole accretion configurations are developed and only the 4-pole model is consistent with the observed periodogram for CD Ind. We conclude that complex magnetic fields affect accretion flow onto these white dwarf stars. In particular, the magnetic field of the white dwarf in both of these binaries is inconsistent with either a centered or an offset dipole configuration.