1. Lecture 4
The TeV Sky
Cherenkov light
Sources of Cherenkov radiation

2. Causes of attenuation

3. Gamma-rays interacting in the atmosphere create
Energy > 30 GeV
Gamma-rays interacting
in the atmosphere create
what is called an air shower.
Electron-positron pairs
Travel in the atmosphere
With a velocity LARGER than
The velocity of light IN THE
ATMOSPHERE, and produce
Cherenkov light. This is
Optical (blue) light, that is
Detected by TeV telescopes.
Current generation: HESS,
MAGIC, VERITAS, Cangaroo
(future: Cherenkov Telescope
Array)

4. Huygens’ construction of wave fronts

5. Observation of Crab Nebula
HEGRA
Observation of Crab Nebula
Aharonian et al. 2004, ApJ, 614, 897
1 arcmin

6. HESS spectrum of Crab Nebula

7. Cherenkov Telescopes: MAGIC Major Atmospheric Gamma-ray Imaging Cherenkov Telescope
Camera: 3.5°FoV
17 m
17 m

8. High Energy Stereoscopic System (H.E.S.S.)

9. Cherenkov Telescope Array (www.cta-observatory.org)
2 sites, 1 in each Hemisphere: Canary Islands and Paranal
Sensitivity: ~0.1% Crab (factor of 10 improvement over HESS, MAGIC….)
Angular Resolution: ~1 arcmin (factor of 5-10 improvement)
Fielf of View: virtually whole accessible sky (as opposed to 5 x 5 deg)

11. Comparison of optical images before and after cosmic ray
correction

12. (25% still unidentified)
TeV Sky interactive
map
(M82, NGC253)
~180 sources
(25% still unidentified)

13. HESS observations of SNR RX J1713.7-3946 (1 kpc)
Aharonian et al. 2007

14. Extragalactic TeV sources:
Blazars (Lorentz factors of ~10):
Up to z = 0.6, and probably beyond
Nearby radiogalaxies with blazar-like nuclei and jets:
Cen A (4 Mpc)
M87 (Virgo, 20 Mpc)
IC 310 (Perseus, 80 Mpc)
NGC1275 (Perseus, 80 Mpc)
Very nearby starbursts:
M82 (3 Mpc)
NGC253 (3 Mpc)

15. AGN scheme with various photon fields and geometrical structures
10% of all AGNs are
radio-loud, and a
small fraction of
these are blazars
(Lorentz factors of ~10-20)
UNIFIED SCHEME OF AGN
(Urry & Padovani 1995)

16. AGN scheme

17. Extragalactic Jets: radiogalaxies and blazars Lorentz factors ~ 10-20
Virgo A (M87), distance: 16 Mpc
Cen A
(4 Mpc)
5’ ~ 6 kpc
0.5’~ 30 kpc
2 pc
(z = 0.056)

18. M87 (20 Mpc): TeV emitting region is few dozens of Schwarzschild radii
Acciari et al. 2009