Multiwavelength monitoring of the dwarf nova VW Hydri – III. IUE observations⋆

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Abstract
We present International Ultraviolet Explorer (IUE) observations of VW Hyi obtained between 1984 September 21 and November 10 during quiescence, three ordinary outbursts and one superoutburst. During quiescence the ultraviolet fluxes fall slowly but systematically. During the rise to outburst peak the ultraviolet flux always lags behind the optical flux, but the magnitude of the lag is variable. The flux distributions measured during the decline from different ordinary outbursts and from the superoutburst all have approximately the same slope. The superoutburst starts with an initial rise of the fluxes to the level of an ordinary maximum. Subsequently the fluxes at the short wavelengths decline markedly before the rise is resumed to the superoutburst maximum, which lies about 0.85 mag above the ordinary maximum. Variations in the flux are seen on the time-scale of hours during both quiescence and superoutburst, but the dependence of these variations in the orbital or superhump phases is not clear. A high-resolution spectrum obtained during superoutburst maximum shows a blueshift of about 400km s–1 in the centres of the ultraviolet resonance lines. Low-resolution spectra show rapid variability in the C IV line profiles.

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