| 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. |
|
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%).