Far-UV and extreme-UV observations of SS Cyg and U Gem from Voyagers 1 and 2

Abstract

Recent studies of dwarf novae in the UV and X-ray regions have suggested that much of the energy produced in a dwarf nova is emitted in the far-UV and extreme-UV regions (100–1,200 Å). We report here on observations of SS Cyg and U Gem obtained with the Voyager UV spectrometers (UVS) in the 500–1,200 Å region. SS Cyg has been detected in quiescence at a flux level of 9 × 10⁻¹³erg cm⁻² s⁻¹Å⁻¹at 1,000 Å. U Gem was not detected in quiescence. No emission shortward of the Lyman limit (912 Å) is detected in either star. Observational upper limits to the extreme-UV flux in the 540–740 Å band from SS Cyg and U Gem correspond to 2 × 10⁻¹³erg cm⁻² s⁻¹Å⁻¹. A far-UV (λ_eff= 1,050 Å) light curve for an outburst in SS Cyg is presented. The slope of the far-UV flux distribution during the rise to maximum is observed to be correlated with the UV brightness. The nature of this correlation supports the suggestion that the outburst begins at the outer edge of the accretion disk and moves inward. The far-UV flux distributions of SS Cyg and U Gem in outburst were found to be remarkably similar, both exhibit a substantially flatter (F_λ∝ λ ^−0.5) distribution than that observed in the IUE–UV (F_λ∝ λ^−2.3). This result is contrary to current disk models which predict a continued rise in flux in the far-UV. Evidence for a wind in the Lyman lines of hydrogen is seen in SS Cyg. No evidence of a wind was observed in U Gem.