2026ApJ...998..156H

V1674 Her is the fastest (t₂ ∼ 1 day) classical nova in our Galaxy, and its absolute V peak of ${M}_{V,{\rm{\max }}}\sim -10.2$ is 1 mag brighter than typical very fast novae. Such a nova is sometimes called a superbright nova. Using our fully self-consistent nova outburst model combined with the optically thick winds on a 1.35 M_⊙white dwarf (WD) with a mass accretion rate of 1 × 10^–11M_⊙ yr^–1, we have clarified that a strong reverse shock arises 0.3 day after the outburst, which is just after the maximum expansion of the WD photosphere. The shocked shell is optically thick and expanding with a velocity of ∼3500 km s^–1. Its V brightness reaches a maximum of ${M}_{V,{\rm{\max }}}=-10.2$ when the shocked shell expands to R_shell ∼ 300 R_⊙ on day ∼0.7. After that, the shocked shell turns optically thin and becomes fainter than the brightness of free-free emission from the nova wind. In chronological order, the optical brightness of free-free emission reaches a maximum of M_V = -9 on day 0.3. However, it is overtaken on days 0.5-0.7 by the ∼1 mag brighter luminosity of the optically thick shocked shell. The GeV gamma-ray flux reaches a maximum on day 0.4 because the gamma-rays are emitted by the shock that arises on day 0.3. Our model consistently explains both the superbrightness and chronological order that the gamma-ray peak precedes substantially before the optical V peak. We also present a similar light-curve model for another superbright nova, V1500 Cyg.