The vertical disc structure of U Geminorum in outburst

Abstract

We have obtained phase-resolved UV spectroscopy from the International Ultraviolet Explorer, and V-band CCD photometry of U Gem in outburst. Our data show little or no orbital modulation outside eclipse, in contrast to the X-ray light curves during outburst, which show large dips. We find that a consistent model of the optical, UV and X-ray light curves is obtained if one assumes that the accretion disc has considerable vertical structure, as seen in other cataclysmic variables. We also show that the hydrogen column density 25 degrees above the plane of the disc varies from <~10^19 to about ~10^22 cm^-2 (a factor of 10 higher than previously thought). We show that the ratio of soft X-ray photons from the boundary layer to those from the accretion disc corona or wind is 50 :1.