Breakthrough in Gamma-Ray Line Spectroscopy\hfil\break with COMPTEL

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Session 29 -- Gamma Ray Astrophysics
Oral presentation, Wednesday, January 12, 2:15-3:45, Salon III Room (Crystal Gateway)

[29.01] Breakthrough in Gamma-Ray Line Spectroscopy\hfil\break with COMPTEL

H. Bloemen$^2$, K. Bennett$^4$, R. Diehl$^1$, C. Dupraz$^2$, W. Hermsen$^2$,\break A. Iyudin$^1$, G. Lichti$^1$, D. Morris$^3$, J. Ryan$^3$, V. Sch\"onfelder$^1$, G. Stacy$^3$, A.W. Strong$^1$, C. de Vries$^2$, C. Winkler$^4$ (1. MPE Germany \ 2. SRON-Leiden The Netherlands \ 3. UNH \ 4. SSD/ESA The Netherlands)

The COMPTEL telescope aboard the Compton Gamma Ray Observatory has reached a real breakthrough in gamma-ray line spectroscopy with the following three findings:

1. COMPTEL has detected gamma-ray emission from the Orion complex in the 3-7 MeV range that can be identified with the 4.44 and 6.13 MeV nuclear de-excitation lines of $^{12}$C$^\star$ and $^{16}$O$^\star$, respectively, which are predicted to be the strongest gamma-ray lines originating from the interaction of energetic particles. There is good circumstantial evidence for strongly enhanced abundances of C and O in low-energy cosmic rays, rather than high fluxes of cosmic-ray protons and $\alpha$-particles.

2. For the first time, COMPTEL has now detected the $^{44}$Ti nuclear decay line at 1.15 MeV from a supernova remnant, namely Cas-A. Observations of this isotope (with a decay time of about 78 years), provide important constraints for nucleosynthesis models.

3. COMPTEL has for the first time mapped the entire Galactic plane in the light of the 1.8 MeV line from radioactive $^{26}$Al, which has a decay time of about 1 million years. The emission is mainly seen towards the inner Galaxy. Its highly structured appearance indicates that the $^{26}$Al production occurs in rather localized regions, not restricted to the very inner part of the Galaxy. In particular, there is convincing evidence now for emission from the Vela region.

COMPTEL operates in the 0.75 to 30 MeV range with a wide field-of-view of about 1 steradian, a position location accuracy of typically $1^\circ$, and an energy resolution of 5-10\%. It is capable of imaging gamma-ray line emission with a sensitivity of the order $10^{-5}$ photon cm$^{-2}$ s$^{-1}$.

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