COMPTEL 3-YEAR SEARCH FOR GALACTIC SOURCES OF TI-44 GAMMA-RAY LINE EMISSION AT 1.157 MEV

Because of its short lifetime, radioactive Ti-44 is a probe of the sup ernova activity of the Galaxy during the last few centuries. The COMPT EL experiment aboard the Compton Observatory is capable of imaging sou rces of Ti-44 line emission at 1.157 MeV with a sensitivity of about 2 10(-5) gamma cm(-2) s(-1). The latest known supernova remnant in the Milky Way, Cassiopeia A, has already been detected by COMPTEL (Iyudin et al. 1994). Although no further event has been observed for 300 year s, young objects still obscured by interstellar extinction may also sh ow up in the light of decaying Ti-44. Using the first three years of C OMPTEL observations, we have carried out a systematic 1.157 MeV survey of the Galactic plane in order to search for these conjectural Ti-44 sources. We report on the following issues: -of the six Galactic super novae recorded during the last millenium, only Cassiopeia A is detecte d by COMPTEL. The updated flux, (3.4+/-0.9) 10(-5) gamma cm(-2) s(-1) (systematic uncertainty s less than or similar to 25%), is lower than previously determined and implies an initial Ti-44 mass of 0.9 to 2.4 10(-4) M-circle dot in better agreement with observational and theoret ical pictures of the Cas A supernova; -no serendipitous Ti-44 source i s detected by COMPTEL at the greater than or equal to 3 sigma level, w hereas we would expect 3 detections on statistical grounds. This negat ive conclusion is consistent at the < 5% confidence level with canonic al values of the Galactic supernova rate (2.5 to 3 events per century) and Ti-44 supernova yields (a few 10(-5) to 10(-4) M-circle dot).