Constraints on the kinematics of the 44Ti ejecta of Cassiopeia A from INTEGRAL/SPI

Context. The medium-lived 44Ti isotope is synthesised by explosive Si-burning in core-collapse supernovae. It is extremely sensitive to the dynamics of the explosion and therefore can be used to indirectly probe the explosion scenario. The young supernova remnant Cassiopeia A is to date the only source of gamma-ray lines from 44Ti decay. The emission flux has been measured by CGRO/COMPTEL, BeppoSAX/PDS and INTEGRAL/IBIS. Aims. The high-resolution spectrometer SPI on-board the INTEGRAL satellite can provide spectrometric information about the emission. The line profiles reflect the kinematics of the 44Ti in Cassiopeia A and can thus place constraints on its nucleosynthesis and potentially on the associated explosion process. Methods. Using 4 years of data from INTEGRAL/SPI, we have searched for the gamma-ray signatures from the decay of the 44Ti isotope. The overwhelming instrumental background noise required an accurate modelling and a solid assessment of the systematic errors in the analysis. Results. Due to the strong variability of the instrumental background noise, it has not been possible to extract the two lines at 67.9 and 78.4 keV. Regarding the high-energy line at 1157.0 keV, no significant signal is seen in the 1140–1170 keV band, thereby suggesting that the line signal from Cassiopeia A is broadened by the Doppler effect. From our spectrum, we derive a ∼500 km s−1 lower limit at 2σ on the expansion velocity of the 44Ti ejecta. Conclusions. Our result does not allow us to constrain the location of 44Ti since the velocities involved throughout the remnant, derived from optical and X-ray studies, are all far above our lower limit.