Asymmetries in core-collapse supernovae from maps of radioactive 44Ti in Cassiopeia A

Abstract

Asymmetry is required by most numerical simulations of stellar core-collapse explosions, but the form it takes differs significantly among models. The spatial distribution of radioactive (44)Ti, synthesized in an exploding star near the boundary between material falling back onto the collapsing core and that ejected into the surrounding medium, directly probes the explosion asymmetries. Cassiopeia A is a young, nearby, core-collapse remnant from which (44)Ti emission has previously been detected but not imaged. Asymmetries in the explosion have been indirectly inferred from a high ratio of observed (44)Ti emission to estimated (56)Ni emission, from optical light echoes, and from jet-like features seen in the X-ray and optical ejecta. Here we report spatial maps and spectral properties of the (44)Ti in Cassiopeia A. This may explain the unexpected lack of correlation between the (44)Ti and iron X-ray emission, the latter being visible only in shock-heated material. The observed spatial distribution rules out symmetric explosions even with a high level of convective mixing, as well as highly asymmetric bipolar explosions resulting from a fast-rotating progenitor. Instead, these observations provide strong evidence for the development of low-mode convective instabilities in core-collapse supernovae.

Revisiting INTEGRAL/SPI observations of44Ti from Cassiopeia A

“…The measured expansion velocities, however, are similar to those from a Gaussian fit, and systematic differences are smaller than uncertainties from statistics. When the results of The et al (1996) obtained with OSSE on CGRO with the COMPTEL result by Iyudin et al (1997), the BeppoSAX result (Vink et al 2001), the INTEGRAL/IBIS result (Renaud et al 2006), and the NuStar result (Grefenstette et al 2014) are combined with our work through a weighted mean (weighted by the inverse variance of each measurement), the 44 Ti mass is (1.37 ± 0.19) × 10 −4 M .…”

“…This is significantly higher than the yields predicted for typical supernova models, which suggested that Cas A represents a rare subclass of 44 Ti ejecting supernovae, and explosion asymmetries have been thought to cause such anomalies (Nagataki et al 1998;The et al 2006). The 44 Ti image recently obtained by the NuStar hard X-ray telescope (Grefenstette et al 2014) has shown that Cas A ejecta carrying 44 Ti appear in clumps, re-affirming that Cas A did not explode as a spherically symmetric supernova. Other observations had shown this before from X-ray measurements, for example, and especially from optical/IR light echo spectra, revealing this asymmetry in the supernova photosphere.…”

“…Later measurements focused on the equally bright lines from the first stage in the 44 Ti decay chain, the de-excitation of 44 Sc. In particular, the studies with INTEGRAL/IBIS (Renaud et al 2006) and NuStar (Grefenstette et al 2014) settled the amount of 44 Ti to 1.4 × 10 −4 M (IBIS: 1.6 +0.6 −0.3 × 10 −4 M ; NuStar: (1.25 ± 0.3) × 10 −4 M ). This is significantly higher than the yields predicted for typical supernova models, which suggested that Cas A represents a rare subclass of 44 Ti ejecting supernovae, and explosion asymmetries have been thought to cause such anomalies (Nagataki et al 1998;The et al 2006).…”

Frequency-dependent effects of gravitational lensing within plasma

“…The environments around these objects are expected to be complex, and some models explain magnetar-like behaviour through accretion from fall-back material on to the surface of the object (Ouyed, Leahy & Niebergal 2007). Interest in the quark nova scenario has grown considerably due in part to the NuSTAR observations of the Cassiopeia A supernova remnant (Grefenstette et al 2014) which detected a discrepancy in the distribution of titanium-44 and iron-56 abundances that can be explained by the quark nova scenario (Ouyed, Leahy & Koning 2015). Considerations of more realistic dispersion relations coupled with GR may be relevant in the dynamic environment around an energetic, highly compact quark star.…”

The e-ASTROGAM mission

“…Measuring nucleosynthesis products such as 56 Ni, 56 Co and 44 Ti is one of the more direct ways to extract information on the inner processes triggering the explosion near the newly forming compact stellar remnant (e.g., [70]) -other observables are indirect, and mostly reflect interactions within the envelope, or with circumstellar, pre-explosively ejected, or ambient gas. e-ASTROGAM will detect the signatures of 56 Ni and 56 Co decay from several CCSNe in nearby galaxies.…”