Dr. Roland Diehl (MPE)

University of Florida Astronomy Colloquium - Jan. 14th, 2015

New Insights from Cosmic Gamma-Ray Line Observations

Gamma-ray lines from cosmic sources arise from radioactive decay of unstable isotopes co-produced by nucleosynthesis, from energetic collisions among atomic nuclei which may excite nuclei above their ground level, and from interstellar annihilation of positrons ejected from a variety of candidate sources. Such gamma-ray lines are being measured with ESA's INTEGRAL space mission since its launch twelve years ago, complementing the earlier survey of NASA's Compton Gamma-Ray Observatory. The nuclei seen by those missions in their characteristic gamma-rays are 56Ni, 57Ni, 44Ti, 26Al, and 60Fe from their characteristic sources, positron annihilation has been measured and mapped throughout the Galaxy both in the 511 keV line and positronium continuum. The current NuSTAR mission complements those observations at the low-energy end for 44Ti decay. In this talk we present those observations and discuss their implications.

The 26Al isotope with 1My decay time had been first direct proof of currently- ongoing nucleosynthesis in our Galaxy. This has now become a tool to study the ~My history of specific massive-star groups and associations in nearby regions throughout our Galaxy. Additionally, mapping of the Doppler shifted 26Al line showed that superbubbles around massive-star groups in the Galaxy exhibit a remarkable asymmetry, on average, which has important implications for feedback from massive stars and their supernovae. 60Fe is co-produced by the sources of 26Al, and the isotopic ratio from their nucleosynthesis reflects nucleosynthesis conditions within the complex stellar structure of massive stars. Annihilation gamma-rays from positrons in interstellar space show a puzzling bright and extended source region central to our Galaxy which may be related to special high-energy processes in the central part of our Galaxy, but also may be partly related to nucleosynthesis. 56Ni and 44Ti gamma-ray lines have been used to constrain supernova explosion mechanisms: For the type Ia supernova SN2014J the surprising gamma-ray line signature points to a non-spherical explosion, from 56Co decay lines, following a primary ignition of the white dwarf surface region, revealed by early 56Ni decay lines. Cas A and SN1987A are the two supernovae seen in 44Ti gamma-ray lines, and the radioactivity gamma rays provide independent clues on the kinematics and morphology of the ejecta expanding after core-collapse and explosion.