A light-curve analysis of the X-ray flash first observed in classical novae

Abstract: An X-ray flash, expected in a very early phase of a nova outburst, was at last detected with the {\it SRG}/eROSITA in the classical nova YZ Reticuli 2020. The observed flash timescale, luminosity, and blackbody temperature substantially constrain the nova model. We present light curve models of the X-ray flash for various white dwarf (WD) masses and mass accretion rates. We have found the WD mass in YZ Ret to be as massive as $M_{\rm WD}\sim 1.3 ~M_\odot$ with mass accretion rates of $\dot M_{\rm acc}\sim 5 \times 10^{-10}- 5\times 10^{-9} ~M_\odot$ yr$^{-1}$ including the case that the mass accretion rate is changing between them, to be consistent with the {\it SRG}/eROSITA observation. The X-ray observation confirms the luminosity to be close to the Eddington limit at the X-ray flash. The occurrence of optically thick winds, with the photospheric radius exceeding $\sim 0.1~R_\odot$, terminated the X-ray flash of YZ Ret by strong absorption. This sets a constrain on the starting time of wind mass loss. A slight contamination of the core material into the hydrogen rich envelope seems to be preferred to explain a very short duration of the X-ray flash.