1. Active Galactic Nuclei (AGN)
Galaxies with peculiar properties:
Extremely bright nuclei
Variability
High-Energy (X-/g-ray) emission
Emission lines
Polarization
Relativistic outflows (jets)

2. Cosmic Jets and Radio Lobes
Many active galaxies show powerful radio jets
Hot spots:
Energy in the jets is released in interaction with surrounding material
Radio image of Cygnus A
Material in the jets moves with almost the speed of light (“Relativistic jets”).

3. Types of radio-loud AGN and AGN Unification
Observing direction
Cyg A (radio emission)
Radio Galaxy:
Powerful “radio lobes” at the end points of the jets, where power in the jets is dissipated.

4. Types of radio-loud AGN and AGN Unification
Quasar or BL Lac object (properties very similar to quasars, but no emission lines)
Emission from the jet pointing towards us is enhanced (“Doppler boosting”) compared to the jet moving in the other direction (“counter jet”).
Observing direction

5. Blazars
Class of AGN consisting of BL Lac objects and gamma-ray bright quasars
Rapidly (often intra-day) variable
Strong gamma-ray sources
Radio jets, often with superluminal motion
Radio and optical polarization

6. Blazar Models Leptonic Models g-q g1 g2 Synchrotron emission
Synchrotron emission
Relativistic jet outflow with G ≈ 10
Injection, acceleration of ultrarelativistic electrons
nFn n
Compton emission
g-q
Qe (g,t) g1 g2 g
Leptonic Models
nFn
Injection over finite length near the base of the jet. n
Seed photons:
Synchrotron (SSC), Accr. Disk + BLR (EC)
Additional contribution from gg absorption along the jet

7. Blazar Models Hadronic Models g-q g1 g2
Proton-induced radiation mechanisms:
Injection, acceleration of ultrarelativistic electrons and protons
Relativistic jet outflow with G ≈ 10
nFn
g-q
Qe,p (g,t) n
Proton synchrotron g1 g2 g
pg → pp0 p0 → 2g
pg → np+ ; p+ → m+nm
m+ → e+nenm
Synchrotron emission of primary e-
→ secondary m-, e-synchrotron
nFn
Hadronic Models
Cascades … n

8. Fitting the spectral variability of BL Lac in 2000
Linj = 3*1040 erg/s
g1 = 1100
g2 = 4*104 → 5*104
q = 2.6 → 2.2
D = 17
eB = 1
B = 1.4 G
RB = 2.5*1015 cm
Electron acceleration up to ~ 25 GeV
(~ TeV in some blazars)
Electron injection index q = 2.2
→ Relativistic parallel shocks?
B = 1.4 G
Böttcher & Reimer (2004)

9. Spectral modeling results along the Blazar Sequence
High-frequency peaked BL Lac (HBL):
Synchrotron
No dense circumnuclear material → No strong external photon field
SSC

10. Spectral modeling results along the Blazar Sequence
Radio Quasar (FSRQ)
External Compton
Plenty of circumnuclear material → Strong external photon field
Synchrotron
Hartman & Boettcher (2001)