OSSE Observations of $\gamma$-ray Emissions from the Cassiopeia A Supernova Remnant
Session 66 -- Supernova Remnants
Oral presentation, Thursday, 2, 1994, 10:00-11:30

[66.07] OSSE Observations of $\gamma$-ray Emissions from the Cassiopeia A Supernova Remnant

L.-S. The, D. D. Clayton, M. D. Leising (Department of Physics \& Astronomy, Clemson University), W. N. Johnson, R. L. Kinzer, J. D. Kurfess, M. S. Strickman (E.O. Hulburt Center for Space Research, Naval Research Laboratory), G. V. Jung (Universities Space Research Associates), D. A. Grabelsky, W. R. Purcell, M. P. Ulmer (Department of Physics \& Astronomy, Northwestern University)

Measurement of the amount of $^{44}$Ti ejected by a supernova is essential for confirming the accuracy of nucleosynthesis calculations of this isotope production in incomplete Si burning during the evolution of a massive star, during explosive Si burning, or during an alpha-rich (low-density) freeze-out of nuclear statistical equilibrium. The recent CGRO/COMPTEL discovery of the 1.16 MeV $\gamma$-ray line of $^{44}$Ti from the Cas A supernova remnant (SNR) is highly significant because it implies a relatively high yield of $^{44}$Ti ($\sim$ 2--3 $\times$ 10$^{-4}$ M$_{\sun}$) compared with explosive nucleosynthesis calculations ($\sim$ 1 $\times$ 10$^{-4}$ M$_{\sun}$) and because it demonstrates that unobserved supernovae in the galaxy from the past several hundred years are possibly detectable through their $^{44}$Ti $\gamma$-line emission with CGRO and future missions. Cas A, the youngest known supernova remnant in the Galaxy and a strong radio and X-ray source, was observed by CGRO/OSSE July 16 - August 6, 1992. Its proximity($\sim$ 3 kpc) and its young age ($\sim$ 300 yrs) make Cas A the best candidate among known supernova remnants for detecting $^{44}$Ti $\gamma$-ray lines. In contrast to the COMPTEL result, we find no evidence for $\gamma$-radiation at 67.9 keV, 78.4 keV, or 1.157 MeV, the three strongest $^{44}$Ti $\gamma$-ray lines. From simultaneous fits to the three lines our 99\% confidence upper limit of the $\gamma$-line fluxes is 5.2$\times$10$^{-5} \; \gamma \;$ cm$^{-2} \;$ s$^{-1}$ for each line feature. We also report upper limits for 4.44 MeV $^{12}$C nuclear excitation line, which could be caused by interactions of accelerated particles in the supernova remnant, and for the hard X-ray continuum.