Detection of the 67.9 and 78.4 keV Lines Associated with the Radioactive Decay of 44Ti in Cassiopeia A

We report the detection of the 44Sc nuclear decay lines at 67.9 and 78.4 keV associated with the nuclear decay of 44Ti in Cassiopeia A. The line emission was observed by the Phoswich Detection System instrument on board BeppoSAX, which recently observed the supernova remnant for over 500 ks. The detection of the line emission with a flux of (2.1 ± 0.7) × 10-5 photons cm-2 s-1 in each line (90% confidence) is at the 5 σ significance level, if we can assume that the 12-300 keV continuum is adequately represented by a single power law. However, as the nature of the continuum is not clear, we investigate various other possibilities. A more conservative estimate of the line flux is made by assuming that a power-law continuum is at least a good approximation to the continuum emission for a narrower 30-100 keV energy range. With this limitation, the measured line flux is (1.9 ± 0.9) × 10-5 photons cm-2 s-1, with the detection still at the 3.4 σ significance level. We suggest that together with the Compton Gamma Ray Observatory/COMPTEL measurement of the 44Ca line at 1157 keV of (3.3 ± 0.6) × 10-5 photons cm-2 s-1, a flux for all three lines of (2.5 ± 1.0) × 10-5 photons cm-2 s-1 for Cas A can be adopted. This implies an initial 44Ti mass of (0.8-2.5) × 10-4 M☉.