Figure: Panel A shows the distributed γ-ray emission in photon counts/bin from
the Cygnus X superbubble obtained by Fermi. White contours are the 10-5.6
W/m2/sr contour of the 8-micron intensity. Panel B shows the 8 micrometer map
(in W/m2/sr) of the same region with circles for 7gamma;-Cygni and stellar clusters
(Ackermann et al. Science 334, 1103, 2011).
Measurements of the isotopic abundances of galactic cosmic rays (GCR) by the
ACE/CRIS instrument have been shown to be consistent with a model in which the
source of accelerated material consists of a mix of ~20% massive star outflow
(including Wolf-Rayet star outflow and supernova ejecta) and ~80% normal
interstellar medium material with solar system abundances (ACE News #74, #95 &
#102; Binns et al. Space Science Reviews, 2007). These observations have led
to a model of galactic cosmic ray origin in which associations of massive
stars (OB associations) and their associated superbubbles are both the source
of the material and site of the acceleration for a substantial fraction of
GCRs. In addition, combined measurements of GCR elemental abundances by the
balloon-borne TIGER instrument, HEAO-3, and by the ACE-CRIS instrument from
carbon to strontium (Z = 6 to 38), have shown that the ordering of these
abundances by mass (atomic weight) relative to a reference population is
greatly improved if they are compared to a ~20/80% mix instead of normal solar
system abundances. The element comparisons are completely independent of the
isotope results, but lead to the same conclusion.
Recently, investigators from NASA's Fermi Large Area Telescope (LAT) have
reported distributed γ-ray emission from 1 to 100 GeV in the Cygnus X region
of the sky (Ackerman et al. Science 334, 1103, 2011). They identified a
"cocoon" of freshly-accelerated cosmic rays that extends ~50 parsecs
from the Cygnus OB2 association to γ-Cygni. The Cygnus OB2 association is a
very large cluster of >500 massive stars and γ-Cygni is a young (age ~7000
years) supernova remnant. NGC 6910, an open cluster containing ~75 massive
stars, is also near γ-Cygni. The γ-ray emission is most intense in Cygnus OB2
and γ-Cygni. The hardness of the γ-ray spectrum, created by π0 decay,
indicates that the protons left the accelerator only recently. After
subtracting a modeled background emission spectrum, including point-source
emission from γ-Cygni, most of the remaining distributed emission extends
along a line from Cygnus OB2 to NGC 6910, with peak emission coming from those
stellar clusters. The morphology of the enhanced emission region corresponds
to the edge of the Cygnus superbubble. Although the emission spectrum could
result from either proton or electron acceleration, higher-energy (~1013 eV)
Milagro γ-ray observations of this region are inconsistent with electron
emission, thereby favoring proton acceleration. Either way, the γ-ray spectrum
presents compelling evidence of freshly-accelerated cosmic rays within a
superbubble. Thus, the Fermi observations identify a specific superbubble
where cosmic rays are accelerated, while the ACE, TIGER, and HEAO-3
observations identify OB associations and their superbubbles as a class of
astrophysical objects in which cosmic rays originate (for additional
information, see W. R. Binns, Science 334, 1071, 2011).
The Fermi observations, taken together with ACE, TIGER, and HEAO-3 elemental
and isotope measurements, give a consistent story that OB associations and
their superbubbles are very likely the source of a substantial fraction of
galactic cosmic rays. Continued studies of cosmic-ray composition and γ-ray
observations of OB associations offer a rich potential for understanding the
detailed nature of cosmic ray origin.
This item was contributed by
This item was contributed by W. R. Binns of Washington University, St. Louis.
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Last modified 5 Feb 2012.