Revisiting INTEGRAL/SPI observations of⁴⁴Ti 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 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 in the innermost regions of the supernova. 44 Ti was first discovered in this remnant, but is not seen consistently in other core-collapse sources. Aims. We aim to measure radioactive 44 Ti 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 44 Ti ejecta from the 1.157 MeV γ-ray line of the 44 Sc decay. Results. We observe both the hard X-ray line at 78 keV and the γ-ray line at 1157 keV from the 44 Ti 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 44 Ti, 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 estimate of ejected 44 Ti 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 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.

THE DISTRIBUTION OF RADIOACTIVE 44Ti IN CASSIOPEIA A

“…Upper limits consistent with the detections were obtained using the OSSE instrument on CGRO , RXTE (Rothschild & Lingenfelter 2003), and the SPI instrument on INTEGRAL (Martin et al 2009). A comparison of the total yield from all of these observations demonstrates that all measurements of the initial 44 Ti mass using the 68 and 78 keV lines are consistent with an initial 44 Ti mass of (1.37±0.19) ×10 −4 M e (Siegert et al 2015). However, only NuSTAR is capable of spatially resolving the remnant and has the energy resolution to search for Doppler broadening of the 67.87 and 78.32 keV decay lines of 44 Ti.…”

INTEGRAL/SPI γ-ray line spectroscopy

“…Each line is now modelled as a convolution of a Gaussian line profile "G(E)" with amplitude "A 0 ", centred at energy "E 0 " and with a width parameter "σ", with a one-sided exponential "T (E)", the latter representing the degradation of charge collection with time between annealings as characterised with degradation parameter "τ" (Kretschmer 2011;Siegert 2017):…”

About cosmic gamma ray lines

“…Also, ejecta which were believed to originate from the innermost regions close to the mass cut between ejecta and the newly forming neutron star [35], were interpreted and related to deviations from spherical symmetry: Only with such models, it was possible to get larger amounts of 44 Ti ejected, as they had been inferred from the detection of the 44 Ti gamma-ray line at 1156 keV with COMPTEL [36] and constraints on SN1987A and the 44 Ca content of the solar system [37]. More measurements on the prominent core collapse events SN1987A and Cas A were needed and made, thereafter [38,39,40,41], essentially confirming this issue. The NuSTAR hard X-ray mirror telescope finally has been able to image the low-energy lines at 68 and 78 keV from 44 Ti decay [32] (Fig.…”

“…and Doppler broadening adds information about inner ejecta kinematics [40,41,32], and is now being compared to elaborate, 3-dimensional, simulations that become available [42,43,44,45].…”