Gamma-Ray Line Studies of Nuclei in the Cosmos

TL;DR

Gamma-ray line observations over the past 30 years have advanced our understanding of cosmic nucleosynthesis, revealing radioactive isotopes in space, their distribution, and dynamics, with ongoing data collection from the INTEGRAL observatory.

Contribution

This paper reviews three decades of gamma-ray line studies, highlighting recent high-resolution measurements and their implications for understanding cosmic nucleosynthesis and isotope distribution.

Findings

Detection of gamma-ray lines from 44Ti, 26Al, and 60Fe in supernova remnants.

Mapping of 26Al throughout the Galaxy, revealing star-forming regions.

Positron annihilation signals suggest complex origins, including supernovae.

Abstract

Gamma-ray line studies are capable of identifying radioactive tracer isotopes generated in cosmic nucleosynthesis events. Pioneering measurements were made 30 years ago with HEAO-C1, detecting the first interstellar gamma-ray line from 26Al, then with SMM and numerous balloon experiments, among their results the detection of radioactivity from supernova SN1987A, and with the Compton Observatory and its OSSE and COMPTEL instruments in 1991-2000, which performed sky surveys in 26Al and 511 keV annihilation emission and the detection of the Cas A supernova remnant in 44Ti radioactivity. The SPI high-resolution Ge spectrometer on INTEGRAL was launched in 2002 and continues to collect data on astrophysically-important gamma-ray lines from decays of 44Ti, 26Al, 60Fe, and positron annihilation. 44Ti decay lines from Cas A have been observed with both INTEGRAL telescopes, and constrain the expansion dynamics of the ejecta. The lack of other 44Ti remnants is a mystery. The 26Al gamma-ray line is now measured throughout the Galaxy, tracing the kinematics of interstellar gas near massive stars, and highlighting special regions of interest therein, such as groups of massive stars in Cygnus and even more nearby regions. The detection of 60Fe radioactivity lines at the level of 15% of the 26Al flux presents a challenge both for observers and models. Positron annihilation emission from the nucleosynthesis regions within the Galactic plane appears to be mainly from 26Al and other supernova radioactivity, while the bulge's positron annihilation brightness remains puzzling.