2015A&A...579A.124S

The 340-yr old supernova remnant Cassiopeia A, located at 3.4kpc 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 ⁴⁴Ti and ⁵⁶Ni, which originate in the innermost regions of the supernova. ⁴⁴Ti was first discovered in this remnant, but is not seen consistently in other core-collapse sources. We aim to measure radioactive ⁴⁴Ti ejected in Cassiopeia A and to place constraints on velocities of these ejecta by determining X- and γ-ray line-shape parameters of the emission lines. 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 ⁴⁴Ti ejecta from the 1.157MeV γ-ray line of the ⁴⁴Sc decay. We observe both the hard X-ray line at 78keV and the γ-ray line at 1157keV from the ⁴⁴Ti decay chain at a combined significance of 3.8σ. Measured fluxes are (2.1 ±0.4)x10^–5ph/cm2/s and (3.5 ±1.2)x10^–5ph/cm2/s, which corresponds to (1.5±0.4)x10^–4 and (2.4±0.9)x10^–4M_☉ of ⁴⁴Ti, respectively. The measured Doppler broadening of the lines implies expansion velocities of 4300 and 2200km/s, respectively. By combining our results with previous studies, we determine a more precise estimate of ejected ⁴⁴Ti of (1.37±0.19)x10^–4M_☉. The measurements of the two lines are consistent with previous studies. The flux in the line originating from excited ⁴⁴Ca is significantly higher than the flux determined in the lines from ⁴⁴Sc. 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.