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 ${}^{44}$Ti and ${}^{56}$Ni which originate from the innermost regions of the supernova. ${}^{44}$Ti 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 ${}^{44}$Ti ejecta from the 1.157 MeV $\gamma$-ray line of the ${}^{44}$Sc decay. We observe both the hard X-ray line at 78 keV and the $\gamma$-ray line at 1157 keV from the ${}^{44}$Ti decay chain, at a combined significance of 3.8 $\sigma$. Measured fluxes are $(2.1\pm 0.4)10^{-5}\mathrm{ph}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$ and $(3.5\pm 1.2)10^{-5}\mathrm{ph}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$, which corresponds to $(1.5\pm 0.4)10^{-4}$ and $(2.4\pm 0.9)10^{-4}{\mathrm{M}}_{\odot }$ of ${}^{44}$Ti, respectively. The measured Doppler broadening of the lines implies expansion velocities of $4300$ and $2200\mathrm{km}{\mathrm{s}}^{-1}$, respectively. Combining our results with previous studies, we determine a more precise estimate of ejected ${}^{44}$Ti of $(1.37\pm 0.19)10^{-4}{\mathrm{M}}_{\odot }$. The measurements of both lines are consistent with previous studies. The flux in the line originating from excited ${}^{44}$Ca is significantly higher than the flux determined in the lines from ${}^{44}$Sc. 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.

arXiv commentAccepted by A&A, 8 pages, 6 figures, removed typos after language editing

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