Al-26 gamma-ray line observations of the galaxy

Observations of Galactic Al-26 with SPI on INTEGRAL have resulted in spectra at high resolution for the inner Galaxy, and for selected regions along the Galactic plane. The massive-star origin of most or all of Galactic Al-26 is confirmed. The indicated spectroscopic signature of Galactic rotation allows interpretation of the Al-26 brightness in terms of a massive-star census for the Galaxy. The result corresponds to a core-collapse supernova rate of 1.9 (+/- 1.1) per century from 2.8 M-circle dot (+/- 0.8 M-circle dot) of 26 Al. The width of the gamma-ray line is consistent with this galaxy-wide massive-star origin, and with normal interstellar kinematics from the source regions with ISM velocities around 100 km s(-1). Future refinements of these measurements promise interesting constraints for specific source regions; a prominent example is the Cygnus region with its rather young population of massive stars, where modest broadening of the gamma-ray line is indicated.