CGRO Observations of Nova Herculis 1991 and Nova Cygni 1992
Session 11 -- Novae
Display presentation, Monday, 9, 1995, 9:20am - 6:30pm

## [11.10] CGRO Observations of Nova Herculis 1991 and Nova Cygni 1992

C.R.Shrader (NASA/GSFC), S.G.Starrfield (ASU), J.W.Truran (UChicago), M.D.Leising (Clemson), S.N.Shore (UIndiana SB)

We report on observations of two bright, nearby Oxygen-Neon- Magnesium (ONeMg) novae using the OSSE and COMPTEL instruments onboard the Compton Gamma Ray Observatory. The goal of our observations was to measure the $^{22}Na$ (1275 keV, $\tau_{1/2}=2.6$ yrs) emission-line. Nova Her 1991 and Nova Cyg 1992 were the among the nearest ONeMg novae ever observed, and this subclass of novae are believed to be the best candidates for direct observation of nuclear emission line features. Nova Cyg 1992 was observed with OSSE at several epochs, including a two-week observation during April 1992 shortly after the outburst. This observation resulted in upper limits on the $^{22}Na$ and $^{7}Be$ (478 keV, $\tau_{1/2}=53$ days) line flux that have been reported elsewhere. We note however that concurrent optical and UV observations suggest that the ejecta were still very optically thick at that time. This suggests that follow-up observations at later epoch would represent the optimal chances for detection of $^{22}Na$ line emission. Two weeks of total integration time on Nova Cyg 1992 with the full OSSE detector was obtained during September and October of 1993, approximately 1.5 years post outburst ( $0.6 ^{22}Na$ half-lives). Additionally, results from our analysis of off-axis COMPTEL coverage of the source at three distinct epochs within a year of the outburst will be presented. We observed Nova Her 1991 with OSSE approximately three years post-outburst and identified three epochs within about a year post-outburst for which useful COMPTEL off-axis coverage exists. Results of each of these observations will be presented and considered in the context of model calculations. Implications, such as possible constraints on the mass of the ejected material and the fractional yield of $^{22}Na$ will be discussed. In addition, we reflect briefly on possible implications of this ONeMg novae on the galactic distribution of $^{26}Al$ 1800-keV line emission which has been by established through COMPTEL observations reported elsewhere.