Gamma-ray observations of explosive nucleosynthesis products

Abstract

In this review I discuss the various γ-ray emission lines that can be expected and, in some cases have been observed, from radioactive explosive nucleosynthesis products. The most important γ-ray lines result from the decay chains of 56 Ni, 57 Ni, and 44 Ti. 56 Ni is the prime explosive nucleosynthesis product of Type Ia supernovae, and its decay determines to a large extent the Type Ia light curves.56 Ni is also a product of core-collapse supernovae, and in fact, γ-ray line emission from its daughter product, 56 Co, has been detected from SN1987A by several instruments. The early occurrence of this emission was surprising and indicates that some fraction of 56 Ni, which is synthesized in the innermost supernova layers, must have mixed with the outermost supernova ejecta.Special attention is given to the γ-ray line emission of the decay chain of 44 Ti ( 44 Ti→ 44 Sc→ 44 Ca), which is accompanied by line emission at 68 keV, 78 keV, and 1157 keV. As the decay time of 44 Ti is ∼ 86 yr, one expects this line emission from young supernova remnants. Although the 44 Ti yield (typically 10 −5 − 10 −4 M⊙) is not very high, its production is very sensitive to the energetics and asymmetries of the supernova explosion, and to the mass cut, which defines the mass of the stellar remnant. This makes 44 Ti an ideal tool to study the inner layers of the supernova explosion. This is of particular interest in light of observational evidence for asymmetric supernova explosions.The γ-ray line emission from 44 Ti has so far only been detected from the supernova remnant Cas A. I discuss these detections, which were made by COMPTEL (the 1157 keV line) and BeppoSAX (the 68 keV and 78 keV lines), which, combined, give a flux of (2.6 ± 0.4 ± 0.5) × 10 −5 ph cm −2 s −1 per line, suggesting a 44 Ti yield of (1.5 ± 1.0) × 10 −4 M⊙. Moreover, I present some preliminary results of Cas A observations by INTEGRAL, which so far has yielded a 3σ detection of the 68 keV line with the ISGRI instrument with a flux that is consistent with the BeppoSAX detections. Future observations by INTEGRAL-ISGRI will be able to constrain the continuum flux above 90 keV, as the uncertainty about the continuum shape, is the main source of systematic error for the 68 keV and 78 keV line flux measurements. Moreover, with the INTEGRAL-SPI instrument it will be possible to measure or constrain the line broadening of the 1157 keV line. A preliminary analysis of the available data indicates that narrow line emission (i.e. ∆v < 1000 km s −1 ) can be almost excluded at the 2σ level, for an assumed line flux of 1.9 × 10 −5 ph cm −2 s −1 .

Are $\mathsf{^{44}}$Ti-producing supernovae exceptional?

“…Yet, Cas A seems an established 44 Ti detection in COMPTEL (Schönfelder et al 2000) and Beppo-Sax (Vink et al 2001) and INTEGRAL/IBIS (Vink 2005) measurements, while OSSE and RXTE (Rothschild & Lingenfelter 2003) measurements of Cas A were not sufficiently sensitive. There exist no wellunderstood supernova remnants other than Cas A where the 44 Ti production issue can be tested.…”

INTEGRAL survey of the Cassiopeia region in hard X rays

“…In this paper these sources will not be addressed in detail; we refer to Vink (2005) and Renaud et al (2004) for more detailed studies. This paper presents results on compact hard X-ray sources detected in the deep 1.6-Ms observation of the Cassiopeia region.…”

Ca40(α,γ)Ti44Reaction in the Energy Regime of Supernova Nucleosynthesis

“…Cas A is believed to be the remnant of a core-collapse SN whose progenitor mass was in the range 22-25 M ⊙ (M ⊙ denotes a solar mass) [4]. Using known values of the distance and age of the remnant, half-life of 44 Ti and the combined γ flux from all observations, an initial 44 Ti yield of 160± 60 µM ⊙ is implied [3]. This value is larger by a factor of 2-10 than 44 Ti yields calculated in current models (e.g.…”

“…Stellar production of 44 Ti determines the abundance of stable 44 Ca and contributes to that of 48 Ti (fed by 48 Cr on the α-chain). Live 44 Ti has been directly observed from a point source identified as Cassiopeia A (Cas A) by γ-and X-ray telescopes (CGRO, RXTE, BeppoSAX) and very recently by the INTEGRAL mission (see [2,3]). Cas A is believed to be the remnant of a core-collapse SN whose progenitor mass was in the range 22-25 M ⊙ (M ⊙ denotes a solar mass) [4].…”