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Session 93 - GRIS.
Oral session, Wednesday, January 15
The Gamma Ray Imaging Spectrometer (GRIS) balloon experiment observed the 1809 keV line from the Galactic center region on a flight from Alice Springs, Australia on 24 October 1995. This line results from the decay of radioactive ^26Al. GRIS is a high resolution spectrometer capable of measuring the line profile. It was flown in a new configuration with a 100\times75^\circ FOV to measure the integrated emission from the central region of our galaxy. The observed flux of 4.8\pm 0.7\times 10^-4 ph s^-1 cm^-2 rad^-1 is in good agreement with previous observations. Surprisingly, the observed width of the line was 5.4\pm1.4 keV, which is much larger than the expected width from Galactic rotation (<1.8 keV). The high dispersion velocity (>450 km s^-1) this width implies has important consequences. It clearly favors supernova and Wolf-Rayet models which produce the ^26Al at high velocity over AGB star models that do not. The fact that the ^26Al has not come to rest in the interstellar medium after 10^6 years is very hard to square with our current understanding of the production and propogation of ^26Al in the interstellar medium.
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