Concluding Remarks

During the 21 years of the exciting history of X-ray observations of novae, results have changed a from very noisy first detection just above the significance limit to high resolution X-ray spectra. The X-ray regime is crucial for the observational coverage of novae, since many phenomena are only observable in the X-ray regime. X-ray observations allow us to study the hot white dwarf and shocks in the expanding envelope, to derive temperatures and luminosities and to carry out plasma diagnostics of the line emitting regions.

However, the picture we have is far from complete and systematic. Only few novae have so far been observed in X-rays and for only one nova, V1974 Cyg, does a fairly complete coverage of the whole outburst cycle exist. All other observations are more or less "snapshots." One of the main characteristics of the X-ray results obtained so far is that each nova was different from the predecessors and nearly each new nova observed in X-rays brought new surprises and, in part, totally unexpected phenomena like X-ray bursts and periodic variations. Many of these phenomena are still unexplained, in particular the variations of the soft X-ray flux defied any generally accepted explanation to this date. A problem of more general kind is that during the early phases the soft flux is not observable because of the high opacity in the expanding envelope.

For the future more, and above all, systematic X-ray observations over the whole outburst are needed. Only X-rays allow us to look directly at those areas in a nova where the essential physical processes are going on, i.e., where the energy is produced. All other wavelength regions can yield only indirect evidence from these regions.

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