Observational Evidence for Expansion in the SSS spectra of Novae

For several novae, a bright X-ray source with a spectrum resembling the class of Super Soft X-ray Sources (SSS) has been observed a few weeks to months after outburst. Novae are powered by explosive nuclear burning on the surface of a white dwarf, and enough energy is produced to power a radiatively driven wind. Owing to the evolution of the opacity of the ejecta, the observable spectrum gradually shifts from optical to soft X-rays (SSS phase). It has sometimes been assumed that at the beginning of the SSS phase no more mass loss occurs. However, high-resolution X-ray spectra of some novae have shown highly blue-shifted absorption lines, indicating a significant expansion. In this paper, I show that all novae that have been observed with X-ray gratings during their SSS phase show significant blue shifts. I argue that all models that attempt to explain the X-ray bright SSS phase have to accommodate the continued expansion of the ejecta.