Evidence of Magnetic Accretion in an SW Sextantis Star: Discovery of Variable Circular Polarization in LS Pegasi

Abstract

We report on the discovery of variable circular polarization in the SWSextantis star LS Pegasi. The observed modulation has an amplitude of~0.3% and a period of 29.6 minutes, which we assume as the spin periodof the magnetic white dwarf. We also detected periodic flaring in theblue wing of H{β}, with a period of 33.5 minutes. The differencebetween both frequencies is just the orbital frequency, so we relate the33.5 minute modulation to the beat between the orbital and spin period.We propose a new accretion scenario in SW Sex stars, based on the shockof the disk-overflown gas stream against the white dwarf'smagnetosphere, which extends to the corotation radius. From thisgeometry, we estimate a magnetic field strength of B_{1}~5-15MG. Our results indicate that magnetic accretion plays an important rolein SW Sex stars, and we suggest that these systems are probablyintermediate polars with the highest mass accretion rates. Based onobservations made with the William Herschel Telescope, operated on theisland of La Palma by PPARC in the Spanish Observatorio del Roque de LosMuchachos of the Instituto de Astrof{í}sica de Canarias.