Gamma-Rays from 26Al radioactivity have become a tracer of massive stars
in our Galaxy, as these shed nucleosynthesis products through Wolf-Rayet winds and
supernovae. With a radioactive-decay time scale of 1 Myr, ejecta from many stars and
supernovae are generally superimposed. Thus, 26Al gamma-rays reflect current
nucleosynthesis from massive stars in our Galaxy, and is seen from regions distributed
along the plane of the Galaxy and its spiral arms.
Early INTEGRAL / SPI high-resolution spectroscopy had exploited the global signal
from the inner-Galaxy's bright ridge. The 26Al gamma-ray line (E = 1809 keV)
had been found to be rather narrow and in agreement with a standard moderately-turbulent
ISM around typical massive-star groups. Also, encouraged by 26Al line
gamma-ray energy shifts from the inner Galaxy, which were recognized and compatible
with large-scale Galactic rotation, the 26Al flux was interpreted as a global
Galactic quantity. Using model yields and the standard IMF, it is converted into a
measurement of the current core-collapse supernova rate in our Galaxy of 1.9 +/- 1.1
supernovae per century.
Now the exposure along the Galactic plane becomes sufficient for spatially-resolved
spectroscopy in the 26Al line, the signal exceeds 5σ significance in
20-degree wide bins along the bright parts of the Galactic plane. The flux in the bright
inner-Galaxy ridge is determined as
2.9 +/- 0.2x10-4 ph cm-2 s-1rad-1,
and a small (~1.3 +/- 0.2) asymmetry favoring the fourth quadrant is indicated. The
systematic shifts of line centroids from large-scale Galactic rotation are now clearly
established, their detailed comparisons with expected Doppler shifts from (uncertain and
model-dependent) large-scale motions in the inner Galaxy with its bar are under investigation.
The line appears to be narrow with ISM velocities within the 100-km s-1 range in
all samples, except for a hint of increased velocity spread towards the Aquila region. The
latitudinal scale height of 26Al gamma-rays is ~130 pc and significantly exceeds
the thickness of the Galactic disk in molecular gas. For the Cygnus region, a
sufficently-accurate stellar census has been derived in recent years. Employing the spatial
resolution of the SPI gamma-ray telescope, the 26Al flux attributed to Cygnus
complex of massive stars is discriminated against the large-scale Galactic-disk emission
as 3.9 +/- 1.1x10-5 ph cm-2 s-1, with the Cygnus OB2
association as dominating source. This allows for a more detailed consistency check of
observed flux against nucleosynthesis models for massive stars and their supernovae.
References:
Spectral and intensity variations of Galactic 26Al emission,
W. Wang et al., A&A 496, 713 - 724 (2009)
The gamma-ray line emission of the Cygnus region from INTEGRAL/SPI observations,
P. Martin et al.
A&A (in press) (2009)
