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 56Ni, 57Ni, and 44Ti. 56Ni is the prime explosive nucleosynthesis product of Type Ia supernovae, and its decay determines to a large extent the Type Ia light curves. 56Ni is also a product of core-collapse supernovae, and in fact, γ-ray line emission from its daughter product, 56Co, has been detected from SN1987A by several instruments. The early occurrence of this emission was surprising and indicates that some fraction of 56Ni, 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 44Ti ( 44Ti → 44Sc → 44Ca), which is accompanied by line emission at 68, 78, and 1157 keV. As the decay time of 44Ti is ∼86 yr, one expects this line emission from young supernova remnants. Although the 44Ti 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 44Ti 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 44Ti 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 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 44Ti 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 3 σ 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 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., Δ v < 1000 km s −1) can be almost excluded at the 2 σ level, for an assumed line flux of 1.9 × 10 −5 ph cm −2 s −1.