2.1 Gamma-ray emission from
the interstellar medium
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Observations with Fermi LAT
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Nucleonic gamma-ray production
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Cosmic-ray distribution and energy
density in the interstellar medium
How to detect photons with
E>20 MeV
Photo effect
~E­3
Pair production
~const
~E­1
Compton
Tracker: 1.5 X0, Calorimeter: 8.6 X0
Tray of a tracker module
Atwood et al. 2009
Conversion & Track detection
Picture: D. Smith
All-sky map Fermi-LAT
(E>100 MeV)
What is the origin?
Electrons (Synchrotron, Bremsstrahlung, inverse Compton?)
Nuclei?
Haslam et al. 1982
For comparison: Radio
All­sky view at 408 Mhz
[10 yrs of observation with Effelsberg, Parkes etc.
pp interaction
Origin of E>100 MeV gammarays
Atomic + Molecular Gas
PP scattering: p+p­>π0+X, π0->γγ
Bremsstrahlung: e+p -> e + p +γ
Inverse Compton: e+γir ->e+γ
Density of cosmic rays
Comparison model&data
Fermi results (Porter 09)
Summary: Gamma-ray emission
from the interstellar medium
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Gamma-rays produced mostly by Cosmic-ray – Gas
interactions
Minor contributions from other radiation processes
(Bremsstrahlung, inverse Compton emission)
Cosmic-rays present in the Galactic disk
Small density variation of cosmic-rays (energy
density globally similar to local environment)
Energy spectrum of cosmic-rays in the disk:
dN/dE~E-2.7
Open questions
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Energy density of cosmic-rays in the
halo?
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Variation of the ratio HI/CO?
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Contribution of unresolved sources?
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Remaining variations of cosmic-ray
densities – where do they come from?