Revisiting INTEGRAL/SPI observations of ⁴⁴Ti from Cassiopeia A

¹ Max-Planck-Institut für extraterrestrische Physik, 85741 Garching, Germany
e-mail: tsiegert@mpe.mpg.de
² Excellence-Cluster “Origin & Structure of Universe”, 85748 Garching, Germany
³ Universitäts-Sternwarte Ludwig-Maximilians-Universität, 81679 München, Germany

Received: 12 February 2015
Accepted: 20 May 2015

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 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 estimate 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.