Revisiting INTEGRAL/SPI observations of 44Ti from Cassiopeia A

TL;DR

This study reanalyzes INTEGRAL/SPI observations of Cassiopeia A to measure $^{44}$Ti ejection, providing refined estimates of its flux, velocity, and implications for supernova nucleosynthesis and cosmic ray acceleration.

Contribution

It offers an improved analysis of $^{44}$Ti gamma-ray lines from Cassiopeia A, refining flux, velocity, and ejected mass estimates with high-resolution spectroscopy.

Findings

Detected $^{44}$Ti lines with 3.8 sigma significance.

Estimated ejected $^{44}$Ti mass as (1.37 ± 0.19) × 10^{-4} M_sun.

Inferred expansion velocities of 4300 and 2200 km/s.

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 $(2.1\pm0.4)~10^{-5}~\mathrm{ph~cm^{-2}~s^{-1}}$ and $(3.5\pm1.2)~10^{-5}~\mathrm{ph~cm^{-2}~s^{-1}}$, which corresponds to $(1.5\pm0.4)~10^{-4}$ and $(2.4\pm0.9)~10^{-4}~\mathrm{M}_{\odot}$ of $^{44}$Ti, respectively. The measured Doppler broadening of the lines implies expansion velocities of $4300$ and $2200~\mathrm{km~s^{-1}}$, respectively. Combining our results with previous studies, we determine a more precise estimate of ejected $^{44}$Ti of $(1.37\pm0.19)~10^{-4}~\mathrm{M}_{\odot}$. 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.