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

Abstract

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.