Revisiting INTEGRAL/SPI observations of $^44$Ti from Cassiopeia A

Abstract

Context. The 340-yr old supernova remnant Cassiopeia A, located at 3.4 kpc distance, is the best-studied young core-collapse super- nova 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. Aims. 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. 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 44Ti ejecta from the 1.157 MeV γ-ray line of the 44Sc decay. Results. We observe both the hard X-ray line at 78 keV and the γ-ray line at 1157 keV from the 44Ti decay chain at a combined significance of 3.8σ. Measured fluxes are (2.1 ± 0.4) × 10−5 ph cm−2 s−1 and (3.5 ± 1.2) × 10−5 ph cm−2 s−1, which corresponds to (1.5 ± 0.4) × 10−4 and (2.4 ± 0.9) × 10−4 M⊙ of 44Ti, 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 esti- mate of ejected 44Ti of (1.37 ± 0.19) × 10−4 M⊙. Conclusions. 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.