X-RAY DECLINE AND REJUVENATION OF A RECURRENT NOVA

The dynamical evolution and nonequilibrium X-ray emission of recurrent-nova remnants have been investigated by using a spherically symmetric hydrodynamic code. We assume that the nova ejecta expand into a wind from a red-giant companion. The wind material is blast shocked, and emits copious X-rays. The blast shock soon breaks out of the wind region which has formed since the previous outburst, and the remnant undergoes nearly unimpeded expansion. The rarefaction and adiabatic cooling of the remnant result in a drastic decline in the X-ray emission. At this stage the recombination exceeds the ionization, and the X-ray spectrum is characterized by strong recombination lines and continua. The blast shock eventually catches up with the relatively slow ejecta of the previous outbursts. The X-ray emission may then be rejuvenated in both luminosity and spectral shape. Observations of such an X-ray decline and rejuvenation may provide a clue to the outburst mechanism of recurrent novae.