OSSE Observations of gamma -ray Emissions from the Cassiopeia A Supernova Remnant

Astronomy and Astrophysics – Astronomy

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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 ( ~ 2--3 times 10(-4) M_&sun;) compared with explosive nucleosynthesis calculations ( ~ 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( ~ 3 kpc) and its young age ( ~ 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 5.2times 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.

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