1. Diffuse Emission and Unidentified Sources
Plane of the Milky Way (diffuse emission)
3C279 (quasar)
Geminga (pulsar)
Crab (SNR)
PSR (pulsar)
EGRET, E > 100 MeV
PKS (quasar)
3C454.3 (quasar)
Vela (pulsar)
More than half of all gamma-ray sources are still unidentified!

2. Diffuse g-Ray Emission
Dominant diffuse g-ray production mechanisms:
Interactions of cosmic-rays (highly relativistic particles in space) with the interstellar medium (ISM) and/or the interstellar radiation field.
pcr pISM → p p + p0 ; p0 → 2 g
Bremsstrahlung of cosmic-ray electrons
Compton scattering of cosmic-ray electrons off the interstellar radiation field (infrared/optical light from stars)
But also: Sum of the contributions from many faint, individual, unresolved sources

3. (ultra-high energy cosmic rays = UHECRs)
Charged high-energy particles (electrons, protons, heavier nuclei), but also photons (g-rays) in space
F(E) ~ E-2.7
Galactic origin (supernovae)
~ 1015 eV
Energies:
MeV - >1020 eV
(ultra-high energy cosmic rays = UHECRs)
F(E) ~ E-3.1
Extragalactic origin (AGN, GRBs?)

4. Ultra-High-Energy Cosmic Rays
The Greisen-Zatsepin-Kuzmin cutoff
The Universe is pervaded with a thermal “afterglow” of the Big Band: The Cosmic Microwave Background (CMB) radiation:
Blackbody at T ≈ 2.7 K.
UHECR nuclei with energies E > 3x1019 eV interact with the CMB:
(pg → pp0 or pg → np+)
Protons with E > 3x1019 eV from cosmological distances lose much of their energy on their way to us
UHECRs at E > 3x1019 eV must come from within ~ 100 Mpc.
The resulting cutoff (“GZK cutoff”) in the UHECR spectrum has recently been measured by the Auger collaboration.

5. Spectrum of Diffuse g-Ray Emission
p0 decay
Bremsstrahlung
Compton scattering
Unresolved extragalactic sources

6. The Structure of the Milky Way
75,000 light years
Disk
Most gas and dust is concentrated in the nuclear bulge and the spiral arms.
Nuclear Bulge
Sun
Halo
Open Clusters (newly born stars)
Globular Clusters (old stars)

7. The Structure of the Milky Way
Distribution of dust
Distribution of stars and neutral hydrogen
Sun
Bar
Ring

8. The Problem of Identifying g-ray Sources
EGRET error contours
Pulsar
Black Hole X-Ray Binary
What’s the source of the g-ray emission?
Need more information (broadband spectrum; variability)

9. Unidentified g-Ray Sources (UIDs)
Out of 270 sources in the EGRET catalog (sources of > 100 MeV g-rays), 170 are unidentified!
Also, about two dozen TeV g-ray sources (detected by HESS, MAGIC) are unidentified.
Almost all within Galactic latitude |l| < 30o
=> Almost certainly of Galactic origin

10. Possible identifications:
The Nature of UIDs
Unidentified sources show a variety of different properties: They are certainly not one homogeneous source class.
Possible identifications:
Background AGN (→ Variability!)
Supernova remnants (→ Non-variable, extended)
Pulsars (→ Pulsed emission; hard spectrum)
Pulsar wind nebulae (→ non-variable, extended)
X-ray/g-ray binaries (→ periodic [orbital] variability)
O/B Associations (young, very massive stars with strong stellar winds) (→ non-variable, extended)

11. High-Latitude Sources
Comparison of spectral and variability properties
Complete EGRET catalog
High-Lat. UIDs
AGN
Pulsars
Number of Sources
Spectral Index
Variability Index

12. High-Latitude Sources
Similar flux – number diagrams as AGN
Number of Sources
Flux (> 100 MeV)
Most low-latitude sources are non-variable. Several previously unidentified TeV sources could be identified with pulsar wind nebula; most remain unidentified.

13. Examples: 1) The UID 3EG J1837-0423
Located only 1o off the Galactic plane
High peak g-ray flux for only 3.5 days
Never detected before or afterwards
Gamma-ray spectrum with photon index -2.1
Strong variability; hard photon spectrum → Background blazar?
Should be detectable in radio/infrared for normal blazar properties

14. 2) HESS J1303-631 TeV g-ray source 0.5o north of a known g-ray pulsar
Less variable than the pulsar.
HESS J
PSR B

15. Possible Counterparts
HESS J
Possible Counterparts
Radio and X-ray sources near HESS J
None of the nearby pulsars is powerful enough to power the TeV source
HESS J
Related to a powerful stellar wind (WR124)?