Toward a unified light curve model for multi-wavelength observations of V1974 Cygni (Nova Cygni 1992)

Abstract

We present a unified model for optical, ultraviolet (UV), and X-ray light curves of V1974 Cygni (Nova Cygni 1992). Based on an optically thick wind model of nova outbursts, we have calculated light curves and searched for the best fit model that is consistent with optical, UV, and X-ray observations. Our best fit model is a white dwarf (WD) of mass 1.05 M_\sun with a chemical composition of X=0.46, C+N+O=0.15, and Ne = 0.05 by mass weight. Both supersoft X-ray and continuum UV 1455 \AA light curves are well reproduced. Supersoft X-rays emerged on day ~ 250 after outburst, which is naturally explained by our model: our optically thick winds cease on day 245 and supersoft X-rays emerge from self-absorption by the winds. The X-ray flux keeps a constant peak value for ~ 300 days followed by a quick decay on day ~ 600. The duration of X-ray flat peak is well reproduced by a steady hydrogen shell burning on the WD. Optical light curve is also explained by the same model if we introduce free-free emission from optically thin ejecta. A t^{-1.5} slope of the observed optical and infrared fluxes is very close to the slope of our modeled free-free light curve during the optically thick wind phase. Once the wind stops, optical and infrared fluxes should follow a t^{-3} slope, derived from a constant mass of expanding ejecta. An abrupt transition from a t^{-1.5} slope to a t^{-3} slope at day ~ 200 is naturally explained by the change from the wind phase to the post-wind phase on day ~ 200. The development of hard X-ray flux is also reasonably understood as shock-origin between the wind and the companion star. The distance to V1974 Cyg is estimated to be ~ 1.7 kpc with E(B-V)= 0.32 from the light curve fitting for the continuum UV 1455 \AA.