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Max-Planck-Institut für extraterrestrische Physik

- Gamma-Ray Astronomy -

Research Area : Nucleosynthesis


  MPEMPE    Zeiger Gamma-Ray Astronomy GroupGamma-Ray Astronomy   GroupResearch   researchNucleosynthesis


Astrophysics with Gamma-Ray Lines from Radioactivities

Radioactive isotopes are co-produced with stable isotopes at cosmic sites of nucleosynthesis. Such sites can be stellar interiors, supernovae, novae, and interstellar space. Many of those radioactive isotopes decay with emission of characteristic gamma-ray lines, such that a measurement of these lines can be related directly to existence of the parent isotope. Due to the penetrating nature of gamma-rays, such a measurement is therefore more direct than measurements of photospheric or interstellar absorption or emission lines: gamma-rays penetrate an equivalent mass layer of a few grams per cm2 easily, while optical photons do not penetrate. Stellar interiors are still more opaque, but expanding explosive sites such as novae and supernovae are gamma-ray transparent typically a few days to weeks after the explosion, thus allowing a direct gamma-ray view at the nucleosynthesis site.

Only a small number of isotopes has lifetimes sufficiently long to not have decayed already before transparency of the site - these are relevant for gamma-ray measurements of nucleosynthesis. The different lifetimes of the isotopes imply that the "exposure time" of a gamma-ray measurement varies correspondingly, and the emission from several or many nucleosynthesis events is superimposed for long-lived isotopes.

From gamma-ray line measurements, we can infer existence of these trace isotopes at observed sites, and hence learn that the nuclear reactions to produce this isotope must have had operated at the site; the dependence of the nuclear-reaction yield on seed isotope abundances and temperatures of the reaction site allows far-reaching inferences in the context of models for these sites. As an example, the measurement of 56Ni decay gamma-rays determines the total radioactive power of a thermonuclear supernova, and the time profile of the gamma-ray line emission reflects the proceeding expansion of the supernova envelope. Another example, the image of the sky in 26Al gamma-rays reflects the spatial distribution of the integrated nucleosynthesis activity from supernovae, novae, and massive-star wind ejection over the past ~million years, which is long compared to the time between individual nucleosynthesis events, but short compared to the evolutionary time scale of the Galaxy.

Astrophysics with Gamma-Ray Lines from Nuclear Excitation

Atomic nuclei in excited states are produced in energetic collisions in interstellar space and in the vicinity of compact objects. Typical excitation energies of atomic nuclei are in the MeV regime, prominent examples being the C first excited level at 4.4 MeV, or O at 6.1 MeV. In the outer regions of the Sun, such collisions occur prominently during solar flares. In interstellar space, cosmic ray interactions with ambient interstellar gas results in de-excitation gamma-ray lines. These allow us to infer the characteristics of cosmic rays. In accretion disks around black holes or neutron stars, we would hope to measure temperatures very close to the compact object and learn about the inner parts of the accretion flows towards compact objectc. Although plausibly expected and predicted in great detail (e.g. Ramaty, Kozlovsky and Lingenfelter ApJ 1979), these lines have not been convincingly measured from any cosmic site. A tantalizing COMPTEL measurement of C and O lines from the Orion region (Bloemen et al. ApJ 1994) stimulated much theoretical work, but turned out contaminated with instrumental background. The inner Galaxy is the most promising target for a search for nuclear de-excitation gamma-rays.

Astronomy of Gamma-Ray Lines

We measure gamma-ray lines with telescopes carried above the Earth atmosphere, such as the COMPTEL telescope flown on the NASA Compton Gamma-Ray Observatory from 1991-2000, and the SPI telescope  launched on ESA's INTEGRAL mission in 2002.

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last edit 28 Jun 2009 by Roland Diehl
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