Extreme ultraviolet light curves of UZ Fornacis: Evidence for accretion stream absorption and vertical extent of the accretion SPOT

Abstract

We report on two pointed observations of UZ For carried out by the imaging photometers aboard the Extreme Ultraviolet Explorer (EUVE), one as part of the EUVE Right Angle Program and one as an off-axis source during a guest observation. Both observations lasted approximately 3 days and covered a total of 72 orbits of the UZ For binary providing multiple coverage of all the orbital phases of UZ For. The resulting high signal-to-noise phase-folded light curve strongly constrains the emission and absorption geometry of UZ For. We have detected a narrow absorption dip that we attribute to the accretion stream at the location of the stagnation region many white dwarf radii away from the accretion spot and have also detected a broad dip caused by absorption much closer to the white dwarf surface. Both absorption effects are variable in time and phase. Based on the timescales of M-star eclipse ingress and egress, the angular spot size is constrained to be less than 5 deg; thus the ratio of spot area to white dwarf surface area is less than or equal to 0.0005. To explain the light curve phase duration given this small angular spot size, the extreme-ultraviolet (EUV) accretion spot must be raised vertically by approximately 5% of the white dwarf radius.