1994A&A...286..194S

We present broadband linear polarization measurements of the active dK5e flare star, BD +26 730, which show a rapid change in polarization amplitude and direction over a two hour period. We conclude that impact polarization resulting from flare-generated particle beams is the most likely cause of the polarization variations. If this interpretation is correct, this event represents the most direct evidence to date for flare-generated particle beams in a star other than the Sun. With proton beams as the most probable source, energy fluxes of the order of 10^9–10 erg/cm2/s are required to power the observed polarization changes. Using the non-flare polarization level (likely produced by stellar magnetic regions) and simple models, we estimate a lower limit of 7% for the inhomogeneous component of the magnetic region area filling factor. We compare our results with solar flares and suggest future observational and theoretical efforts.