Multifrequency behavior of classical novae

Abstract

We review our current understanding of multiwavelength light curves of classical and recurrent novae and show how to determine the WD masses and other binary properties. We pick up PW Vul, U Sco, V745 Sco, RS Oph, and V407 Cyg as representatives of different types of light curves, examine their light curves in detail, discuss physical properties, and clarify the reason of these differences. In the rising phase, the hydrogen-rich envelope expands beyond the size of a close binary in which the companion is embedded deep inside of the photosphere. After the optical maximum, the pseud-photosphere begins to shrink and an optically thin region develops outside of the photosphere. The free-free emission dominates the flux at relatively longer wavelengths (optical and NIR), of which light curves decay along the universal decline law of $F_\nu \propto t^{-1.75}$ (or $t^{-1.55}$). The physical mechanism of super-Eddington phase is presented. In the presence of shock interaction between ejecta and circumstellar matter, it slows down the decay of optical flux as $F_\nu \propto t^{-1.0}$ as seen in the early phase of V407 Cyg. In final stages of outbursts, the wind mass-loss rate sharply drops so the slope of free-free emission decays like $F_\nu \propto t^{-3.5}$. Supersoft X-ray phase begins and continues until hydrogen nuclear burning ends, and after that the nova enters a cooling phase. Hard X-rays may originate from internal shocks between ejecta (or a bow shock between ejecta and the companion). The behaviors of optical-dominant, UV-dominant, and supersoft X-ray source phase have different dependences on the WD mass and envelope chemical composition, so multiwavelength observations are useful to determine the parameters such as the WD mass. Finally, we should note that the very early phase of nova outbursts remains unexplored. An X-ray flash phase is theoretically identified, but not yet successfully detected. Detections of early X-ray flashes give direct information on thermonuclear runaway.