Optical Study of the Polar BM CrB in a Low State

Abstract We have performed spectroscopic and photometric studies of the poorly explored polar BM CrB. Based on ZTF survey data, we have revealed three brightness states of the polar and evidence of the transition from one-pole to two-pole accretion as the average brightness of the system increases. We show that there are a change in the longitude of the main accretion spot (by $${\approx}17^{\circ}$$ ) and an increase in its extent (by $${\approx}10^{\circ}$$ ) when passing from the low state to the high one. Zeeman H $$\alpha$$ absorptions formed in a magnetic field of strength $$B=15.5\pm 1$$ MG are present in the spectra. The cool halo extending from the accretion spot to $${\approx}{1/4}$$ of the white dwarf radius can be the source of these absorptions. Our modeling of the behavior of the H $$\alpha$$ emission line shows that the main source of the emission is the segment of the accretion stream near the Lagrange point L $${}_{1}$$ that is periodically eclipsed by the donor star. The spectra exhibit a cyclotron component forming in the accretion spot. Their modeling by a simple accretion spot model gives constraints on the magnetic field strength, $$B=15$$ –40 MG, and the temperature, $$T_{e}\gtrsim 15$$ keV.