Toward a Unified Light‐Curve Model for Multiwavelength 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☉ 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 A light curves are well reproduced. Supersoft X-rays emerged on day ~250 after the 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 the X-ray flat peak is well reproduced by steady hydrogen shell burning on the WD. The 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 postwind phase on day ~200. The development of hard X-ray flux is also reasonably understood as originating in the shock 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 A.