Considering the photoionization of the Xe@C 60 endohedral compound, we study in detail the ionization cross sections of various levels of the system at energies higher than the plasmon resonance region. Five classes of single-electron levels are identified depending on their spectral character. Each class engenders distinct oscillations in the cross section, emerging from the interference between active ionization modes specific to that class. Analysis of the cross sections based on their Fourier transforms unravels oscillation frequencies that carry unique fingerprints of the emitting level.

The concept of the nanoflare, used in interpreting the solar X‐ray corona, is extended to RS CVn stars which, unlike the Sun, exhibit non‐thermal quiescent radio spectra. The theoretical synchrotron‐radiation radio spectrum emitted by a regular series of nanoflare‐electron pulses, injected into the coronal magnetic field, is derived: for an electron energy spectrum N(γ)∝γ−s, the spectral power density is given by P(ν)∝ν−s/2. This result is valid for the observation of a series of nanoflares with total time duration ≳ the characteristic electron radiation lifetime, which is the case for electrons trapped in extensive coronal regions such as exist in RS CVn stars on the magnetic‐dipole magnetospheric model. The tenuous coronal plasma allows the electrons to give a radio spectrum unaffected at high frequencies (≳5 GHz) by electron collision loss, while the emission of bremsstrahlung X‐rays by the electrons also occurs with a spectrum that is related to their radio emission. The observation of individual X‐ray bursts, which would provide direct evidence for microflares, is not, however, attainable with current instrumentation.

The echo produced when a light 'pulse' from a stellar source (e.g. a nova or supernova) is reflected by circumstellar or interstellar material can appear as a luminous ring expanding at a rate that can be superluminal, i.e. having an apparent motion within the source, transverse to the observer's line of sight, at a speed greater than that of light. 'Light-echo optics' applied to the star RS Puppis and its nebula suggests that when nebular features in peripheral regions of circumstellar-shell images are observed, superluminal effects are not evident; however, such observations can give the stellar distance from the observer. Light-echo optics for an interstellar plane sheet, inclined to the observer's line of sight, can be applied to Nova GK Persei 1901 and SN 1987A, which show superluminal effects. For SN 1987A, an intense thermal x-ray source should be produced in AD 2003, when the advancing supernova ejecta interact with a circumstellar ring, 250 light days in radius: the arc-shaped x-ray image, while not actually a 'radiation echo', should expand at a superluminal rate for about 75 days from the time of its first appearance, and also for the same time before the completion of its 'circuit' around the ring.