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 $M_{\mathrm{WD}}\sim 1.3M_{\odot }$ with mass accretion rates of ${\dot{M}}_{\mathrm{acc}}\sim 5\times 10^{-10}-5\times 10^{-9}{M}_{\odot }$ 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 $\sim 0.1R_{\odot }$, 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.

minor typos corrected: 8 pages, 1 table, 3 figures

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