Hubble Space Telescope Spectroscopy of the Dwarf Nova RX Andromedae during Outburst Rise and Decline

Abstract

We obtained Hubble Space Telescope far-ultraviolet spectra of the Z Cam-type dwarf nova RX And during the early rise to outburst and the decline from the same outburst. The spectral wavelength range covered was 1149-1435 Å. The rise spectrum is dominated by strong, very broad absorption lines, while the decline spectrum has strong, narrower absorption with weak to moderately strong emission wings due to the presence of disk material. We have carried out a combined model accretion disk and high-gravity model atmosphere analysis of these spectra. The rise spectrum is best fitted to the Lyα region, metal absorption lines, and longward continuum with a model optically thick accretion disk having an accretion rate of 2 × 10-10 M⊙ yr-1, a white dwarf of mass Mwd = 0.8 M⊙, with Teff = 40,000 K, log g = 8.2, and a disk inclination angle of 41°. The hot white dwarf accounts for 62% of the far-UV flux, while the disk accounts for 38%. Our best-fitting multicomponent model of the decline spectrum reveals the presence of a hot (45,000 K) white dwarf contributing 81.4% of the flux during the decline and a remaining accretion disk component with an accretion rate during the decline of 2.85 × 10-10, contributing 18.6% of the flux. We find that the RX And white dwarf was heated by ∼11,000 K during the outburst. Evolutionary simulations with time-variable accretion indicate that boundary layer irradiation has a larger effect on this heating amplitude than compressional heating.