Abstract
ABSTRACT The transient optical source MASTER OT J072007.30+451611.6 has been recently discovered and proposed as a peculiar polar with an unusually high amplitude of the orbital brightness variation in the optical of $\sim$3 mag. To clarify its nature, we performed multiband time-series optical photometry with 1.5-m class telescopes and spectroscopy with the 10.4-m Gran Telescopio Canarias. We also analysed archival data of different optical surveys and detected the source in X-rays with the Spectrum-RG/eROSITA telescope (extended ROentgen Survey with an Imaging Telescope Array on board Spectrum Roentgen Gamma observatory). We confirm the orbital period of $\approx$1.5 h with the high amplitude of the brightness modulation. Compiling survey data, covering $\sim$19 yr, we find high and low-brightness states of the object at time-scales of years, likely explained by different accretion rates. Our data were obtained in the high-brightness state. Optical spectra with hydrogen and helium emission lines, consisting of broad and narrow components, indicate the presence of an accretion stream without disc. The Doppler tomography shows that the narrow component is mainly emitted from the Lagrangian L$_1$ point, while the broad component is from the region where the accretion stream interacts with the white dwarf magnetosphere. The ratio of equivalent widths of He ii 4686 and H $\beta$ emission lines is $<$0.4, which is curiously low for polars. The X-ray spectrum of the source can be described by the thermal plasma emission model with parameters consistent with values observed for polars.