About cosmic gamma ray lines

Abstract

Abstract. Gamma ray lines from cosmic sources convey the action of nuclear reactions in cosmic sites and their impacts on astrophysical objects. Gamma rays at characteristic energies result from nuclear transitions following radioactive decays or highenergy collisions with excitation of nuclei. The gamma-ray line from the annihilation of positrons at 511 keV falls into the same energy window, although of different origin. We present here the concepts of cosmic gamma ray spectrometry and the corresponding instruments and missions, followed by a discussion of recent results and the challenges and open issues for the future. Among the lessons learned are the diffuse radioactive afterglow of massive-star nucleosynthesis in 26 Al and 60 Fe gamma rays, which is now being exploited towards the cycle of matter driven by massive stars and their supernovae; large interstellar cavities and superbubbles have been recognised to be of key importance here. Also, constraints on the complex processes making stars explode as either thermonuclear or core-collapse supernovae are being illuminated by gamma-ray lines, in this case from shortlived radioactivities from 56 Ni and 44 Ti decays. In particular, the three-dimensionality and asphericities that have recently been recognised as important are enlightened in different ways through such gamma-ray line spectroscopy. Finally, the distribution of positron annihilation gamma ray emission with its puzzling bulge-dominated intensity disctribution is measured through spatially-resolved spectra, which indicate that annihilation conditions may differ in different parts of our Galaxy. But it is now understood that a variety of sources may feed positrons into the interstellar medium, and their characteristics largely get lost during slowing down and propagation of positrons before annihilation; a recent microquasar flare was caught as an opportunity to see positrons annihilate at a source.

Low-lying resonances in Si26 relevant for the determination of the astrophysical Al25(p,γ

“…The suggested sites for production of 26 Al are predominately massive Wolf-Rayet (WR) stars and their subsequent CCSN phase [1,8]. However, the observed 60 Fe/ 26 Al ratio of their γ-ray fluxes is significantly smaller than theoretical predictions when compared to CCSN models, supporting the hypothesis that there are other important sources of 26 Al in the Galaxy [3,9].…”

“…The understanding of the nucleosynthesis of Galactic 26 Al is elemental in various research areas and has the potential of being used as, for example, a high-resolution chronometer of events during the time of planetary formation in the Galaxy [6]; a method to estimate the rate of core collapse supernovae (CCSN) explosions [7]; to identify the stellar origin of pre-solar grains with high inferred 26 Al/ 27 Al ratios [3]; or to benchmark models of massive star nucleosynthesis [1,8].…”

“…These values are taken as averages over all the reported findings. The 4 + 4 state has been confirmed at 5517.3 (8) keV [20]. However, the contribution of this state to the astrophysical reaction rate is negligible due to its location just a few keV above the proton-threshold in 26 Si, placing it well below the Gamow window for novae temperatures.…”

511-CAM mission: a pointed 511 keV gamma-ray telescope with a focal plane detector made of stacked transition edge sensor microcalorimeter arrays

“…Harrison et al describe prominent lines at 158, 749, and 812 keV (from the decay of 56 Ni) and at 847 and 511 keV (from the decay of 56 Co) in the spectra of SNe Ia. These two decays power much or all of the emission from SNe Ia, 30 and a NUSTAR follow-up mission could detect several tens of these objects per year. A deeper understanding of SNe Ia explosion mechanisms would greatly benefit many areas of study, given their use as standard candles in astronomy and cosmology.…”

The 511-CAM Mission: A Pointed 511 keV Gamma-Ray Telescope with a Focal Plane Detector Made of Stacked Transition Edge Sensor Microcalorimeter Arrays

“…Diehl et al list prominent γ-ray lines at 68 keV and 78 keV (from the radioactive decay of 44 Ti in core-collapse supernovae), at 122 keV and 158 keV (from decays of 57 Ni and 56 Ni, respectively, in supernova nucleosynthesis), at 478 keV (from the decay of 7 Be in nova nucleosynthesis), and at 812 keV and 847 keV (from the decays of 56 Ni and 56 Co, respectively, in supernova nucleosynthesis) as important diagnostics. 31 A deeper understanding of SNe Ia explosion mechanisms would greatly benefit their use as standard candles in astronomy.…”