Max-Planck-Institut für extraterrestrische Physik
- Gamma-Ray Astronomy -
Research Area : Nucleosynthesis
|MPE Gamma-Ray Astronomy Gamma-Ray Astronomy Research Nucleosynthesis|
Astrophysics with Gamma-Ray Lines from RadioactivitiesRadioactive 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.
number of isotopes has lifetimes sufficiently long to not have decayed
transparency of the site - these are relevant for gamma-ray
nucleosynthesis. The different lifetimes of the isotopes imply that the
time" of a gamma-ray measurement varies correspondingly, and the
from several or many nucleosynthesis events is superimposed for
Astrophysics with Gamma-Ray Lines from Nuclear ExcitationAtomic 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 LinesWe 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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