A prediction of the γ-ray flux from nova herculis 1991

Hydrodynamic simulations of thermonuclear runaways in the accreted hydrogen-rich envelopes on 1.0 M_⊙, 1.25 M_⊙, and 1.35 M_⊙ white dwarfs predict ²²Na and ²⁶Al γ-ray emission from ONeMg novae in outburst. We present and discuss the ultraviolet observations made with the International Ultraviolet Explorer satellite of Nova Her 1991, the most recent bright ONeMg nova. A distance of 3.4 kpc and reddening of E(B-V) = 0.6 to Nova Her 1991 are determined by comparing its UV and optical outburst behavior with those of the extragalactic ONeMg nova LMC 1990 No. 1. Her 1991 was also detected by ROSAT five days into its outburst and we argue that a small fraction of the X-rays may be the product of Comptonized 1.2 and 0.5 MeV emission from the decay of ²²Na. If the mass of the ejected shell is indeed as high as has been estimated by Woodward et al., ∼10^-4 M_⊙, then our results predict that ²²Na γ-ray emission from the ejecta of Nova Her 1991 should still be detectable by GRO in the spring of 1992. The detection level is expected to be ∼5 σ in a 2 week observation. Such a detection would strongly guide and constrain the hydrodynamic and nucleosynthesis simulations of nova outbursts on massive white dwarfs.