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We propose a theoretical explanation of absorption/emission line systems inclassical novae based on a fully self-consistent nova explosion model. We foundthat a reverse shock is formed far outside the photosphere ($\gtrsim 10^{13}$cm) because later-ejected mass with a faster velocity collides withearlier-ejected matter. Optically thick winds blow continuously at a rate of$\sim 10^{-4} ~M_\odot$ yr$^{-1}$ near the optical maximum, but its velocitydecreases toward the optical maximum and increases afterward, so that the shockarises only after the optical maximum. The nova ejecta is divided by the shockinto three parts, the outermost expanding gas (earliest wind before maximum),shocked shell, and inner fast wind, which respectively contribute topre-maximum, principal, and diffuse-enhanced absorption/emission line systems.A large part of nova ejecta is eventually confined to the shocked shell. Theappearance of the principal system is consistent with the emergence of a shock.This shock is strong enough to explain thermal hard X-ray emissions. Theshocked layer has a high temperature of $k T_{\rm sh} \sim 1$ keV $\times((v_{\rm wind} - v_{\rm shock})/{\rm 1000 ~km~s}^{-1})^2 ={\rm 1 ~keV}\times((v_{\rm d} - v_{\rm p})/{\rm 1000 ~km~s}^{-1})^2$, where $v_{\rm d} - v_{\rmp}$ is the velocity difference between the diffuse-enhanced ($v_{\rm d}$) andprincipal ($v_{\rm p}$) systems. We compare a 1.3 $M_\odot$ white dwarf modelwith the observational properties of the GeV gamma-ray detected classical novaV5856 Sgr (ASASSN-16ma) and discuss what kind of novae can produce GeVgamma-ray emissions.

A strong shock during a nova outburst: an origin of multiple velocity systems in optical spectra and of high-energy emissions