Simultaneous five color (UBVRI) polarimetry of EF Eri

Abstract

We present the first observations of the AM Her type object EF Eri where both the polarization and the photometric data are recorded simultaneously in five color bands (UBVRI). The position angle rotates strongly (∼30°) vs. wavelength from U to I, probably due to Faraday rotation or due to fact that the polarized radiation seen in the different wavelength bands comes from different parts of the accretion region. The phase dependence of the position angle requires field and accretion geometry more complicated than a simple centered dipole and a second emitting region producing weaker intermediate pulses in the infrared seems to be present. We derive the value of the inclination of the system i=55°±5°, the colatitude of the active pole β₁,= 38° ±5° and the second emitting region β = 115° ± 5°, both of which are nearly at the same longitude facing the main accretion stream.