Irregular Mass Transfer in the Polars VV Puppis and V393 Pavonis during the Low State

Abstract

The polars VV Pup and V393 Pav were observed with XMM-Newton during states of low accretion rate with peak X-ray luminosities of ~1 x 10^30 and ~1 x 10^31 erg/s, respectively. In both polars, accretion onto the white dwarf was extremely irregular, and the accretion rate varied by more than 1 order of magnitude on timescales of ~1 hr. Our observations suggest that this type of irregular accretion is a common phenomenon in polars during the low state. The likely cause of the accretion rate fluctuations are coronal mass ejections or solar flares on the companion star that intermittently increase the mass transfer into the accretion stream. Our findings demonstrate that the companion stars in cataclysmic variables possess highly active atmospheres.