Revisiting INTEGRAL/SPI observations of 44Ti from Cassiopeia A

Context. The 340-yr old supernova remnant Cassiopeia A, located at 3.4 kpc distance, is the best-studied young core-collapse supernova remnant. Nucleosynthesis yields in radioactive isotopes have been studied with different methods, in particular for production and ejection of Ti-44 and Ni-56, which originate in the innermost regions of the supernova. Ti-44 was first discovered in this remnant, but is not seen consistently in other core-collapse sources. Aims. We aim to measure radioactive Ti-44 ejected in Cassiopeia A and to place constraints on velocities of these ejecta by determining X-and gamma-ray line-shape parameters of the emission lines. Methods. We analyzed the observations made with the SPI spectrometer on INTEGRAL together with an improved instrumental background method, to achieve a high spectroscopic resolution that enables interpretation for a velocity constraint on Ti-44 ejecta from the 1.157 MeV gamma-ray line of the Sc-44 decay. Results. We observe both the hard X-ray line at 78 keV and the gamma-ray line at 1157 keV from the Ti-44 decay chain at a combined significance of 3.8 sigma. Measured fluxes are (2.1 +/- 0.4) x 10(5) ph cm (2) s (1) and (3.5 +/- 1.2) x 10 (5) ph cm (2) s (1), which corresponds to (1.5 +/- 0.4) x 10(-4) and (2.4 +/- 0.9) x 10(-4) M-circle dot of Ti-44, respectively. The measured Doppler broadening of the lines implies expansion velocities of 4300 and 2200 km s (1), respectively. By combining our results with previous studies, we determine a more precise estimate of ejected Ti-44 of (1.37 +/- 0.19) x 10(-4) M-circle dot. Conclusions. The measurements of the two lines are consistent with previous studies. The flux in the line originating from excited Ca-44 is significantly higher than the flux determined in the lines from Sc-44. Cosmic-ray acceleration within the supernova remnant may be responsible for an additional contribution to this line from nuclear de-excitation following energetic particle collisions in the remnant and swept-up material.