Active Galactic Nuclei (AGN)Galaxies with peculiar properties:

• Extremely bright nuclei

• Variability

• High-Energy (X-/-ray) emission

• Emission lines

• Polarization

• Relativistic outflows (jets)

Cosmic Jets and Radio LobesMany active galaxies show powerful radio jets

Radio image of Cygnus A

Material in the jets moves with almost the speed of light (“Relativistic jets”).

Hot spots:

Energy in the jets is released in interaction

with surrounding material

Radio Galaxy:

Powerful “radio lobes” at the end points of the jets, where power in the

jets is dissipated.

Cyg A (radio emission)

Types of radio-loud AGN and AGN Unification

Observing direction

Emission from the jet pointing towards us is enhanced

(“Doppler boosting”) compared to the jet moving in the other

direction (“counter jet”).

Types of radio-loud AGN and AGN Unification

Quasar or BL Lac object (properties very similar to

quasars, but no emission lines)

Observing direction

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

Blazar ModelsRelativistic jet outflow with ≈ 10

Injection, acceleration of ultrarelativistic

electrons

Qe (,

t)

Synchrotron emission

F

Compton emission

F

-q

Seed photons:

Synchrotron (SSC), Accr. Disk + BLR (EC)

Injection over finite length near the base of the jet.

Additional contribution from absorption along the jet

Leptonic Models

Blazar ModelsRelativistic jet outflow with ≈ 10

Injection, acceleration of ultrarelativistic electrons and

protons

Qe

,p (,

t)

Synchrotron emission of primary e-

F

Proton-induced radiation

mechanisms:

F

-q

• Proton synchrotron

Hadronic Models

• p → p0 0 → 2

• p → n+ ; + → +

→ e+e

→ secondary -, e-synchrotron

• Cascades …

Fitting the spectral variability of BL Lac in 2000

Böttcher & Reimer (2004)

• Electron acceleration up to ~ 25 GeV

(~ TeV in some blazars)

• Electron injection index q = 2.2

→ Relativistic parallel shocks?

• B = 1.4 G

Spectral modeling results along the Blazar Sequence

High-frequency peaked BL Lac (HBL):

No dense circumnuclear material → No strong external

photon field

SynchrotronSSC

Spectral modeling results along the Blazar Sequence

Radio Quasar (FSRQ)

Plenty of circumnuclear

material → Strong external

photon field

SynchrotronExternal Compton

Hartman & Boettcher (2001)