Cosmic Gamma-Ray Spectroscopy

TL;DR

This paper reviews gamma-ray spectroscopy in astronomy, discussing instrumentation, data analysis, and astrophysical insights from gamma-ray line observations related to nuclear processes in stars, supernovae, solar flares, and the interstellar medium.

Contribution

It provides a comprehensive overview of gamma-ray instrumentation, data processing techniques, and recent astrophysical findings, highlighting new insights into nuclear processes and positron sources in space.

Findings

Detection of gamma-ray lines from radioactive isotopes in stars and supernovae

Observation of positron annihilation gamma-rays in the Milky Way's bulge

Identification of nuclear de-excitation lines in solar flares

Abstract

Gamma-ray instrumentation for astronomical spectroscopy consists of multiple-interaction detectors in space combined with sophisticated post-processing of detector events on ground. Spectral signatures in the MeV regime originate from transitions in the nuclei of atoms (rather than in their electron shell). Nuclear transitions are stimulated by either radioactive decays or high-energy nuclear collisions such as with cosmic rays. Gamma-ray lines have been detected from radioactive isotopes produced in nuclear burning inside stars and supernovae, and from energetic-particle interactions in solar flares. 56Ni directly reflects the source of supernova light. The paucity of corresponding 44Ti gamma-ray line sources reflects the variety of dynamical conditions herein. 26Al and 60Fe are dispersed in interstellar space from massive-star nucleosynthesis over millions of years. Gamma-rays from their decay are measured in detail by gamma-ray telescopes, astrophysical interpretations reach from massive-star interiors to dynamical processes in the interstellar medium. Nuclear de-excitation gamma-ray lines have been found in solar-flare events, and convey information about energetic-particle production in these events, and their interaction in the solar atmosphere. The annihilation of positrons leads to another type of cosmic gamma-ray source. The characteristic annihilation gamma-rays at 511 keV have been measured long ago in solar flares, and now throughout the interstellar medium of our Milky Way galaxy. But now a puzzle has appeared, as a surprising predominance of the central bulge region was determined. This requires either new positron sources or transport processes not yet known to us. In this paper we discuss instrumentation and data processing for cosmic gamma-ray spectroscopy, and the astrophysical issues and insights from these measurements.