The discrepant long-term activities of the polars BY Camelopardalis and AR Ursae Majoris

Abstract

Polars are cataclysmic variables with strongly magnetized white dwarfs (WDs). This analysis of the long-term optical activity of two polars (BY Cam and AR UMa) used the light curves of CCD data from the ZTF and CRTS and photographic data from the DASCH databases. These two polars display remarkably discrepant long-term activities. The high-state activity dominated BY Cam, except for a short and shallow low-state episode. A fit to BY Cam’s light curve shows long-term brightness changes in the high state. We ascribe the variable profiles of the histograms of the residuals of this fit (1-year bins) to the changes in the dominance of the individual accretion modes and accreting regions on the WD with time (hundreds of days). The high-state episodes (no matter how long and bright) of AR UMa occurred from a relatively stable low-state brightness level. The superorbital changes of AR UMa dominated in the high states. Bursts of mass transfer from the donor are likely to contribute to the short high-state episodes.