New Insights from Cosmic Gamma-Ray Line Observations - Roland Diehl - 1er juillet 2015 (Site Buffon)

Roland Diehl -Max Planck Institut für extraterrestrische Physik

Mercredi 1er juillet 2015 à 14 h 30

IMPMC - Site Buffon - 61 rue Buffon, Paris 5^e (entrer par le 43 rue Buffon, plan Vigipirates oblige)

Salle des élèves - 2^e étage

Résumé

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 in 2002, and complementing the earlier survey of NASA’s Compton Gamma‐Ray Observatory with precision spectroscopy. The nuclei seen by those missions in their characteristic gamma‐rays are mainly ⁵⁶Ni, ⁵⁷Ni, ⁴⁴Ti, ²⁶Al, and ⁶⁰Fe, each from their characteristic sources; also 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 with an imaging X‐ray telescope. In this talk we present those observations and discuss their implications. The ²⁶Al isotope with1My 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. Both those isotopes also have been inferred to have existed in the early solar system, and, moreover, ⁶⁰Fe has been found in ocean crust samples. This illustrates that those radio‐isotopes are telling a nucleosynthesis story on scales of the Galaxy as well as in specific times where we have the opportunity of measurements. 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 how core-collapse and explosion might have happened.