Abstract V1674 Her is the fastest ( t 2 ∼ 1 day) classical nova in our Galaxy, and its absolute V peak of M V , max ∼ − 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 −11 M ⊙ 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 , 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.
Hachisu et al. (Fri,) studied this question.