COMPTEL three - year search for galactic sources of 44 Ti gamma - ray line emission at 1 . 157 MeV

Because of its short lifetime, radioactive 44Ti is a probe of the supernova activity of the Galaxy during the last few centuries. The COMPTEL experiment aboard the Compton Observatory is capable of imaging sources of 44Ti line emission at 1.157MeV with a sensitivity of about 2 10−5 γ 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 years, young objects still obscured by interstellar extinctionmay also show up in the light of decaying 44Ti. Using the first three years of COMPTEL observations, we have carried out a systematic 1.157MeV survey of the Galactic plane in order to search for these conjectural 44Ti sources. We report on the following issues: — of the six Galactic supernovae recorded during the last millenium, only Cassiopeia A is detected by COMPTEL. The updated flux, (3.4±0.9) 10−5 γ cm−2 s−1 (systematic uncertainty <∼ 25%), is lower than previously determined and implies an initial 44Ti mass of 0.9 to 2.4 10−4 M in better agreement with observational and theoretical pictures of the Cas A supernova; — no serendipitous 44Ti source is detected by COMPTEL at the ≥ 3σ level, whereas we would expect 3 detections on statistical grounds. This negative conclusion is consistent at the < 5% confidence levelwith canonical values of theGalactic supernova rate (2.5 to 3 events per century) and 44Ti supernova yields (a few 10−5 to 10−4 M ).