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 Galaxies Many 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). Unification of 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 ≤ 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 > 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, Hz < sy ≤ 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  ≈ 10 Injection, acceleration of ultrarelativistic electrons Q e ( ,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) Q e ( ,t)     -(q+1) bb  -q or  -2  bb 11  b :  cool (  b ) =  esc

Hadronic Blazar Model Relativistic jet outflow with  ≈ 10 Injection, acceleration of ultrarelativistic electrons and protons Q e,p ( ,t)  Synchrotron emission of primary e - F Proton- induced radiation mechanisms: F    -q Proton synchrotron p  → p  0  0 → 2  p  → n  + ;  + →  +     → e + e  → secondary  -, e-synchrotron Cascades … Radiative + adiabatic cooling Q e ( ,t)   -(q+1)  -q

Requirements for hadronic models To exceed p-  pion production threshold on interactions with synchrotron (optical) photons: E p > 7x10 16 E -1 ph,eV eV For proton synchrotron emission at multi-GeV energies: E p up to ~ eV (=> UHECR) Require Larmor radius r L ~ 3x10 16 E 19 /B G cm ≤ a few x 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  ~ 1/3 e -     ӧ ttcher, 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) (B ӧ ttcher, 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) (B ӧ ttcher, 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 & B ӧ ttcher, 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 & B ӧ ttcher, 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 & B ӧ ttcher, 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 & B ӧ ttcher, 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 & B ӧ ttcher, 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 % 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)