is a radioactive isotope with a lifetime of 1.05 million years. During the decay to the stable isotope 26Mg it emits almost always a 1.8086 MeV gamma ray photon. In roughly 85% of the decays, the disintegration of 26Al is accompanied by the emission of a positron. Eventually, this positron will encounter electrons of the environment, and annihilate under the emission of 2 photons of 511 keV or 3 photons with energies below 511 keV. Additional gamma ray lines occur during the decay of 26Al at 1.1297 MeV (2%) and 2.938 MeV (0.2%).
The 1.809 MeV gamma ray line of 26Al provides a valuable tracer of recent nucleosynthetic activity in our Galaxy. As a by-product of nucleosynthesis, 26Al has been proposed to be produced during core collapse supernovae, during nova outbursts, by Asymptotic Giant Branch (AGB) stars, and by massive stars, in particular during the Wolf-Rayet (WR) phase. In addition to stellar production, 26Al can also be produced by spallation reactions of high energy cosmic rays, although at substantially lower rates.
The COMPTEL telescope aboard the Compton Gamma Ray Observatory (CGRO) satellite provided the first imaging survey in the 1.809 MeV gamma ray line, and established a first map of the observed emission (Knödlseder et al. 1994; Diehl et al. 1995; Oberlack et al. 1996). The image on the left shows the COMPTEL 1.8 MeV all sky map based on 5 years of data. The image has been obtained using a maximum entropy image reconstruction algorithm (MEM). The total number of 1.809 MeV photons registered by COMPTEL during the 5 years period amounts to roughly 50,000 events. In the same time about 4 million instrumental background events were registered by the instrument at the line energy. Consequently, the accuracy of the image reconstruction is severely limited by photon statistics and an extremely low signal to noise ratio.
The 1.8 MeV emission in the MEM map appears irregular and lumpy along the galactic plane with peculiar emission features towards Cygnus, Vela, and Carina. Such irregular emission is indeed expected if massive stars are at the origin of galactic 26Al, reflecting the spiral structure of our Galaxy.
It has gradually become clear, however, that the observed emission clumping reflects at least partially the statistical fluctuations of the data, leading to considerable difficulties for the interpretation of the sky maps. For this reason, a new image reconstruction algorithm has been developed (Knödlseder et al. 1999). This algorithm, called Multiresolution Regularized Expectation Maximization (MREM), explicitly accounts for spatial correlations in the reconstructed image by means of wavelets. MREM is based on the iterative Richardson-Lucy scheme to which a wavelet thresholding step has been added. The iterative MREM scheme converges to a solution that only shows significant emission structures (by significant structure we mean structure that will not change much under perturbation of the data).
Apparently, MREM leads to a much smoother image than MEM, showing however the same peculiar emission features that have been already identified earlier from the analysis of the MEM maps: 
(1) an asymmetric galactic ridge emission reaching from l=45° to l=240°, 
(2) a bright localised emission feature in the Cygnus region around (l,b)=(80°,0°),  and
(3) two emission spots at l=317° and l=332° situated in the galactic plane.
The comparison of both 1.8 MeV all sky maps illustrates that astrophysical answers through image reconstruction are limited and alternative strategies have to be pursued. Testing specific astrophysical hypothesis is such an alternative strategy. On the one hand, COMPTEL 1.8 MeV data have been compared to 3 dimensional geometrical models of the Galaxy in order to establish characteristic parameters of the 26Al source distribution (Diehl et al. 1995; Knödlseder et al. 1996; Diehl et al. 1997). On the other hand, 1.8 MeV data have been correlated with plausible tracers of 26Al nucleosynthesis obtained from other fields of astronomy (Diehl et al. 1995; Diehl et al. 1997). Using a multi-wavelength correlation study, Knödlseder et al. 1999 discovered an extremely close correlation between 53 GHz microwave free-free emission and 1.8 MeV gamma ray line emission. For comparison with the COMPTEL 1.8 MeV all sky maps, the DMR free-free emission map is shown to the left. It obeys the same characteristics as the 1.8 MeV data: an asymmetric galactic ridge emission and a pronounced emission feature in Cygnus.
Within the statistics of the present data, the DMR free-free emission map provides an overall satisfactory fit of COMPTEL 1.8 MeV data. This close correlation suggests that very massive stars are at the origin of galactic 26Al, making Wolf-Rayet stars and core collapse supernovae the most likely source of 26Al (Knödlseder 1999). Small 26Al contributions from Population II objects may well exist, but they are not required by the present data.

Jürgen Knödlseder, 5-11-1999