2013A&A...559A..50N

Super-soft-source (SSS) X-ray spectra are blackbody-like spectra with effective temperatures ∼3-7x10⁵K and luminosities of 10^35–38erg/s. Grating spectra of SSS and novae in outburst that show SSS type spectra display atmospheric absorption lines. Radiation transport atmosphere models can be used to derive physical parameters. Blue-shifted absorption lines suggest that hydrostatic equilibrium is an insufficient assumption, and more sophisticated models are required. In this paper, we bypass the complications of spectral models and concentrate on the data in a comparative, qualitative study. We inspect all available X-ray grating SSS spectra to determine systematic, model-independent trends. We collected all grating spectra of conventional SSS like Cal83 and Cal87 plus observations of novae during their SSS phase. We used comparative plots of spectra of different systems to find common and different features. The results were interpreted in the context of system parameters obtained from the literature. We find two distinct types of SSS spectra that we name SSa and SSe. Their main observational characteristics are either clearly visible absorption lines or emission lines, respectively, while both types contain atmospheric continuum emission. SSa spectra are highly structured with no spectral model currently able to reproduce all details. The emission lines clearly seen in SSe may also be present in SSa, hidden within the forest of complex atmospheric absorption and emission features. This suggests that SSe are in fact obscured SSa systems. Similarities between SSe and SSa with obscured and unobscured AGN, respectively, support this interpretation. We find all known or suspected high-inclination systems to emit permanently in an SSe state. Some sources are found to transition between SSa and SSe states, becoming SSe when fainter. SSS spectra are subject to various occultation processes. In persistent SSS spectra such as Cal87, the accretion disc blocks the central hot source when viewed edge on. In novae during their SSS phase, the accretion disc may have been destroyed during the initial explosion but could have reformed by the time of the SSS phase. In addition, clumpy ejecta may lead to temporary obscuration events. The emission lines stem from reprocessed emission in the accretion disc, its wind or further out in clumpy ejecta, while Thomson scattering allows continuum emission to be visible also during total obscuration of the central hot source.