1997A&A...324..683D

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 2x10^–5γ/cm²/s. The latest known supernova remnant in the Milky Way, Cassiopeia A, has already been detected by COMPTEL (Iyudin et al. 1994A&A...284L...1I). Although no further event has been observed for 300-years, young objects still obscured by interstellar extinction may 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)x10^–5γ/cm²/s (systematic uncertainty ≲25%), is lower than previously determined and implies an initial ⁴⁴Ti mass of 0.9 to 2.4x10^–4M_☉ 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 level with canonical values of the Galactic supernova rate (2.5 to 3 events per century) and ⁴⁴Ti supernova yields (a few 10^–5 to 10^–4M_☉).