Leptonic and Hadronic Models for the Spectral Energy Distributions and High-

Energy Polarization of Blazars

Markus Böttcher

North-West University

Potchefstroom

South Africa

Jets of Active GalaxiesMany active galaxies show powerful radio jets

Radio image of Cygnus A

Material in the jets moves at

relativistic speed

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)

Unification of Extragalactic Jet Sources

Observing direction

Emission from the jet pointing towards us is enhanced

(Doppler boosting) compared to the jet moving in the opposite

direction (counter jet).

Unificationof Extragalactic Jet Sources

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 Spectral Energy Distributions (SEDs)

Non-thermal spectra with two broad bumps:

• Low-energy (probably synchrotron):

radio-IR-optical(-UV-X-rays)

• High-energy (X-ray – -rays)

3C66A

Blazar Classification

Flat Spectrum Radio Quasars (FSRQs):

Low-frequency component from radio to

optical/UV,

sy ≤ 1014 Hz

High-frequency component from X-rays

to -rays, often dominating total power

(Hartman et al. 2000)

High-frequency peaked BL Lacs (HBLs):

Low-frequency component from radio to UV/X-rays,

sy > 1015 Hz

often dominating the total power

High-frequency component from hard X-rays to high-

energy gamma-rays

Low-frequency peaked / Intermediate BL Lacs

(LBLs/IBLs):

Peak frequencies at IR/Optical and GeV gamma-

rays,

1014 Hz < sy ≤ 1015 Hz

Intermediate overall luminosity

Sometimes -ray dominated

(Abdo et al. 2011)

3C66A

(Acciari et al. 2009)

LSP = Low-Synchrotron

Peaked Blazars

HSP = High-Synchrotron

Peaked Blazars

ISP = Intermediate-Synchrotron

Peaked Blazars

Leptonic Blazar Model

Relativistic jet outflow with ≈ 10Injection, acceleration of ultrarelativistic

electrons

Qe (,

t)

Synchrotron emission

F

Compton emission

F

-q

Radiative cooling ↔ escape =>

Seed photons:

Synchrotron (within same region [SSC] or slower/faster earlier/later emission regions

[decel. jet]), Accr. Disk, BLR, dust torus (EC)

Qe (,

t)

-(q+1)

b

-q or -2

b 1

b:

cool(b) = esc

Hadronic Blazar ModelRelativistic jet outflow with ≈ 10Injection, acceleration

of ultrarelativistic electrons and protons

Qe

,p (,

t)

Synchrotron emission of primary e-

F

Proton-induced radiation

mechanisms:

F

-q

• Proton synchrotron

• p → p0 0 → 2

• p → n+ ; + → +

→ e+e

→ secondary -, e-synchrotron

• Cascades …

Radiative + adiabatic cooling

Qe (,

t)

-(q+1)

-q

Requirements for hadronic models

• To exceed p- pion production threshold on interactions with synchrotron (optical) photons: Ep > 7x1016 E-1

ph,eV eV

• For proton synchrotron emission at multi-GeV energies: Ep up to ~ 1019 eV (=> UHECR)

• Require Larmor radius

rL ~ 3x1016 E19/BG cm ≤ a few x 1015 cm => B ≥ 10 G

(Also: to suppress leptonic SSC component below synchrotron)

Semi-analytical hadronic model• Primary e- synchrotron + SSC as for leptonic model• Power-law injection spectrum of ultrarelativistic protons• Proton spectrum: Quasi-equilibrium - injection; sy., p, adiabatic cooling;• Production rates of final decay products (electrons, positrons, neutrinos,

0-decay photons) from Kelner & Aharonian (2008) templates• Semi-analytical representation

of cascades:

- pair production through

delta-function approximation

- synchrotron emissivity

from single electron

through

j ~ 1/3e-

ӧttcher, Reimer, et al. 2013)

Leptonic and Hadronic Model Fits along the Blazar Sequence

Red = Leptonic

Green = Hadronic

Synchrotron

Synchrotron self-Compton (SSC)

Accretion Disk

External Compton of direct accretion disk

photons (ECD)

External Compton of emission from BLR

clouds (ECC)

(Bӧttcher, Reimer et al. 2013)

Electron synchrotron

Accretion Disk

Proton synchrotron

Leptonic and Hadronic Model Fits along the Blazar Sequence

Red = Leptonic

Green = Hadronic

Synchrotron

Synchrotron self-Compton (SSC)

External Compton of emission from BLR

clouds (ECC)

(Bӧttcher, Reimer et al. 2013)

Electron synchrotron

Proton synchrotron

Electron SSC

Proton synchrotron + Cascade synchrotron

Hadronic models can more easily produce VHE emission through cascade synchrotron

Leptonic and Hadronic Model Fits Along the Blazar Sequence

Red = leptonic

Green = lepto-hadronic

3C66A (IBL)

Leptonic and Hadronic Model Fits Along the Blazar Sequence

Red = leptonic

Green = lepto-hadronic

(HBL)

In many cases, leptonic and hadronic models can produce equally

good fits to the SEDs.

Possible Diagnostics to

distinguish:

• Neutrinos

• Variability

• Polarization(?)

X-Ray and Gamma-Ray Polarization

• Synchrotron polarization:

Standard Rybicki & Lightman description

• SSC Polarization:

Bonometto & Saggion (1974) for Compton scattering in Thomson regime

For both leptonic and hadronic models:

Upper limits on high-energy polarization, assuming perfectly ordered magnetic field perpendicular to the line of sight

(Zhang & Bӧttcher, 2013, ApJ, in press)

Integral (X-rays / soft -rays)

GEMS(X-rays )

Fermi(polarimetry at E < 200 MeV)

X-Ray and Gamma-Ray Polarization: FSRQs

Hadronic model: Synchrotron dominated

=> High generally increasing with energy

(SSC contrib. in X-rays).

Leptonic model:

X-rays SSC dominated: ~ 20 – 40 %;

-rays EC dominated

=> Negligible .

(Zhang & Bӧttcher, 2013)

X-Ray and Gamma-Ray Polarization: LBLs

Hadronic model:

Mostly synchrotron dominated => High

except for X-rays, where SSC may

dominate.

Leptonic model:

X-rays = transition from sy. to SSC:

rapidly decreasing with energy;

-rays EC dominated

=> Negligible .

(Zhang & Bӧttcher, 2013)

X-Ray and Gamma-Ray Polarization: IBLs

Hadronic model:

Synchrotron dominated => High throughout

X-rays and -rays

Leptonic model:

X-rays sy. Dominated => High rapidly

decreasing with energy;

-rays SSC/EC dominated

=> Small .

(Zhang & Bӧttcher, 2013)

X-Ray and Gamma-Ray Polarization: HBLs

Hadronic model:

Synchrotron dominated => High

Leptonic model:

X-rays sy. Dominated => High rapidly

decreasing with energy;

-rays SSC/EC dominated

=> Small .

(Zhang & Bӧttcher, 2013)

Observational Strategy• Results shown here are upper limits (perfectly ordered

magnetic field perpendicular to line of sight)

• Scale results to actual B-field configuration from known synchrotron polarization (e.g., optical for FSRQs/LBLs) => Expect 10 - 20 % X-ray and -ray polarization in hadronic models!

• X-ray and -ray polarization values substantially below synchrotron polarization will favor leptonic models, measurable -ray polarization clearly favors hadronic models!

Summary

1. Both leptonic and hadronic models can generally fit blazar SEDs well.

2. Distinguishing diagnostics: Variability, Polarization, Neutrinos

3. X-Ray / Gamma-Ray polarimetry as potential diagnostic? Hadronic Models predict high degree of X/gamma polarization

Partially Dis-ordered Magnetic Fields

Conical standing shock

Mach disk

Looking at the jet from the side

• Many turbulent cells across jet cross-section;

• Each cell has random B direction;

• Explain rapid variability on > pc scales by

only a small fraction of cells being active

(A. Marscher)

→ Turbulent Extreme Multi-zone (TEMZ) Model (Marscher 2012)