Revisiting INTEGRAL/SPI observations of 44Ti from Cassiopeia A
Abstract
The 340-year old supernova remnant Cassiopeia A 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 44Ti and 56Ni which originate from the innermost regions of the supernova. 44Ti was first discovered in this remnant, but is not seen consistently in other core-collapse sources. We analyse the observations accumulated with the SPI spectrometer on INTEGRAL, together with an improved instrumental background method, to achieve high spectroscopic resolution which enables interpretation towards a velocity constraint on 44Ti ejecta from the 1.157 MeV γ-ray line of the 44Sc decay. 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 3.8 σ. Measured fluxes are (2.10.4)~10-5~ph~cm-2~s-1 and (3.51.2)~10-5~ph~cm-2~s-1, which corresponds to (1.50.4)~10-4 and (2.40.9)~10-4~M of 44Ti, respectively. The measured Doppler broadening of the lines implies expansion velocities of 4300 and 2200~km~s-1, respectively. Combining our results with previous studies, we determine a more precise estimate of ejected 44Ti of (1.370.19)~10-4~M. The measurements of both 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.
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