1. Model Constraints from Multiwavelength Variability of Blazars
Markus Böttcher
North-West University
Potchefstroom
South Africa

2. Blazars
Class of AGN consisting of BL Lac objects and gamma-ray bright quasars
Rapidly (often intra-day) variable

3. Blazar Variability: Example: The Quasar 3C279
X-rays
Optical
Radio
(Bӧttcher et al. 2007)

4. Blazar Variability: Variability of PKS 2155-304
VHE g-rays
VHE g-rays
Optical
X-rays
(Aharonian et al. 2007)
VHE g-ray variability on time scales as short as a few minutes!
(Costamante et al. 2008)
VHE g-ray and X-ray variability often closely correlated
→ See D. Dorner's and S. Ciprini's, and V. Karamanavis' Talks

5. Polarization Angle Swings
Optical + g-ray variability of LSP blazars often correlated
Sometimes O/g flares correlated with increase in optical polarization and multiple rotations of the polarization angle (PA)
g-rays (Fermi)
Optical
PKS (Marscher et al. 2010)

6. Blazars
Class of AGN consisting of BL Lac objects and gamma-ray bright quasars
Rapidly (often intra-day) variable
Strong gamma-ray sources

7. Blazar Spectral Energy Distributions (SEDs)
3C66A
Collmar et al. (2010)
Non-thermal spectra with two broad bumps:
Low-energy (probably synchrotron):
radio-IR-optical(-UV-X-rays)
High-energy (X-ray – g-rays)

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

9. Superluminal Motion
(The MOJAVE Collaboration)

10. Leptonic Blazar Model g-q → P. Mimica's Talk g1 g2 g-q or g-2 g-(q+1)
Synchrotron emission
Injection, acceleration of ultrarelativistic electrons
Relativistic jet outflow with G ≈ 10
nFn
g-q
Qe (g,t) n
→ P. Mimica's Talk g1 g2 g
Compton emission
Radiative cooling ↔ escape =>
nFn
g-q or g-2 n
Qe (g,t)
g-(q+1)
Seed photons:
Synchrotron (within same region [SSC] or slower/faster earlier/later emission regions [decel. jet]), Accr. Disk, BLR, dust torus (EC) g1 gb g2 g
gb:
tcool(gb) = tesc gb g1 g2

11. Sources of External Photons (↔ Location of the Blazar Zone)
Direct accretion disk emission (Dermer et al. 1992, Dermer & Schlickeiser 1994)
→ d < few 100 – 1000 Rs
Optical-UV Emission from the BLR (Sikora et al. 1994)
→ d < ~ pc
Infrared Radiation from the Obscuring Torus (Blazejowski et al. 2000)
→ d ~ 1 – 10s of pc
Synchrotron emission from slower/faster regions of the jet (Georganopoulos & Kazanas 2003)
→ d ~ pc - kpc
Spine – Sheath Interaction (Ghisellini & Tavecchio 2008)
→ d ~ pc - kpc
→ M. Georganopoulos' talk

12. Hadronic Blazar Models
Injection, acceleration of ultrarelativistic electrons and protons
Proton-induced radiation mechanisms:
Relativistic jet outflow with G ≈ 10
nFn n
g-q
Qe,p (g,t)
Proton synchrotron g1 g2 g
pg → pp0 p0 → 2g
Synchrotron emission of primary e-
pg → np+ ; p+ → m+nm
m+ → e+nenm
nFn
→ secondary m-, e-synchrotron
(Mannheim & Biermann 1992; Aharonian 2000; Mücke et al. 2000; Mücke et al. 2003)
Cascades … n

13. Leptonic and Hadronic Model Fits along the Blazar Sequence
Red = Leptonic
Green = Hadronic
Accretion Disk
Accretion Disk
External Compton of direct accretion disk photons (ECD)
Synchrotron self-Compton (SSC)
External Compton of emission from BLR clouds (ECC)
Electron synchrotron
Synchrotron
Proton synchrotron
(Bӧttcher, Reimer et al. 2013)

14. Leptonic and Hadronic Model Fits Along the Blazar Sequence
3C66A (IBL)
Red = leptonic
Green = lepto-hadronic

15. Lepto-Hadronic Model Fits Along the Blazar Sequence
(HBL)
Red = leptonic
Green = lepto-hadronic
In many cases, leptonic and hadronic models can produce equally good fits to the SEDs.
Possible Diagnostics to distinguish:
Neutrinos
Variability
X-ray/g-ray
Polarization

16. Distinguishing Diagnostic: Variability
Time-dependent leptonic one-zone models produce correlated synchrotron + gamma-ray variability (Mastichiadis & Kirk 1997, Li & Kusunose 2000, Bӧttcher & Chiang 2002, Moderski et al. 2003, Diltz & Böttcher 2014)
→ See also M. Zacharias' Talk
→ Time Lags
→ Energy-Dependent Cooling Times
→ Magnetic-Field Estimate!
Time-dependent leptonic one-zone model for Mrk 421

17. Correlated Multiwavelength Variability in Leptonic One-Zone Models
Example: Variability from short-term increase in 2nd-order-Fermi acceleration efficiency
X-rays anti-correlated with radio, optical, g-rays;
delayed by ~ few hours.
(Diltz & Böttcher, 2014, JHEAp)

18. Distinguishing Diagnostic: Variability
Time-dependent hadronic models can produce uncorrelated variability / orphan flares (Dimitrakoudis et al. 2012, Mastichiadis et al. 2013, Weidinger & Spanier 2013)
(M. Weidinger)

19. Inhomogeneous Jet Models
Internal Shocks (see next slides)
Radially stratified jets (spine-sheath model, Ghisellini et al. 2005, Ghisellini & Tavecchio 2008)
Decelerating Jet Model (Georganopoulos & Kazanas 2003)
Mini-jets-in-jet (magnetic reconnection → D. Giannios' Talk)

20. The Internal Shock Model
Central engine ejects two plasmoids (a,b) into the jet with different, relativistic speeds (Lorentz factors Gb >> Ga) Gb Ga Gr Gf
Shock acceleration → Injection of particles with
Q(g) = Q0 g-q for g1 < g < g2
Time-dependent, inhomogeneous radiation transfer
Synchrotron
SSC (→ Light travel time effects!)
External Compton
(Chen et al. 2012)
Sokolov et al. (2004), Mimica et al. (2004), Sokolov & Marscher (2005), Graff et al. (2008), Bӧttcher & Dermer (2010), Joshi & Bӧttcher (2011), Chen et al. (2011, 2012)
→ X. Chen's Talk

21. Parameters / SED characteristics typical of FSRQs or LBLs
Internal Shock Model
Parameters / SED characteristics typical of FSRQs or LBLs
(Bӧttcher & Dermer 2010)

22. Internal Shock Model Discrete Correlation Functions
X-rays lag behind HE g-rays by ~ 1.5 hr
Optical leads HE g-rays by ~ 1 hr
Optical leads X-rays by ~ 2 hr
(Bӧttcher & Dermer 2010)

23. Varying the External Radiation Energy Density
Parameter Study
Varying the External Radiation Energy Density
DCFs / Time Lags
Reversal of time lags!
(Bӧttcher & Dermer 2010)

24. Polarization Angle Swings
Optical + g-ray variability of LSP blazars often correlated
Sometimes O/g flares correlated with increase in optical polarization and multiple rotations of the polarization angle (PA)
PKS (Marscher et al. 2010)

25. Polarization Swings
3C279 (Abdo et al. 2009)

26. Previously Proposed Interpretations:
Helical magnetic fields in a bent jet
Helical streamlines, guided by a helical magnetic field
Turbulent Extreme Multi-Zone Model (Marscher 2014)
Mach disk
Looking at the jet from the side

27. Tracing Synchrotron Polarization in the Internal Shock Model
Viewing direction in comoving frame:
qobs ~ p/2 B
Viewing direction in obs. Frame:
qobs ~ 1/G

28. Light Travel Time EffectsB B
Shock propagation B
(Zhang et al. 2014)
Shock positions at equal photon-arrival times at the observer

29. Flaring Scenario: Magnetic-Field Compression perpendicular to shock normal
Baseline parameters based on SED and light curve fit to PKS (Chen et al. 2012)

30. Flaring Scenario: Magnetic-Field Compression perpendicular to shock normal
Synchrotron + Accretion Disk SEDs
Degree of Polarization P vs. time
Frequency-dependent Degree of Polarization P
Polarization angle vs. time
PKS
(Zhang et al. 2014)

31. Flaring Scenario: Magnetic-Field Compression perpendicular to shock normal
Synchrotron + Accretion Disk SEDs
Degree of Polarization P vs. time
Frequency-dependent Degree of Polarization P
Polarization angle vs. time
Mrk 421
(Zhang et al. 2014)

32. Summary
Both leptonic and hadronic models can generally fit blazar SEDs well.
Distinguishing diagnostics: Variability, Polarization, Neutrinos?
Time-dependent hadronic models are able to predict uncorrelated synchrotron vs. gamma-ray variability
Synchrotron polarization swings (correlated with g-ray flares) do not require non-axisymmetric jet features!

34. Superluminal Motion
Apparent motion at up to ~ 40 times the speed of light!

35. Requirements for lepto-hadronic models
To exceed p-g pion production threshold on interactions with synchrotron (optical) photons: Ep > 7x1016 E-1ph,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)
=> Synchrotron cooling time: tsy (p) ~ several days
=> Difficult to explain intra-day (sub-hour) variability!
→ Geometrical effects?

36. Spectral modeling results along the Blazar Sequence: Leptonic Models
High-frequency peaked BL Lac (HBL):
Low magnetic fields (~ 0.1 G);
High electron energies (up to TeV);
Large bulk Lorentz factors (G > 10)
The “classical” picture
Synchrotron
No dense circum-nuclear material → No strong external photon field
SSC
(Acciari et al. 2010)

37. Spectral modeling results along the Blazar Sequence: Leptonic Models
High magnetic fields (~ a few G);
Lower electron energies (up to GeV);
Lower bulk Lorentz factors (G ~ 10)
FSRQ
External Compton
Plenty of circum-nuclear material → Strong external photon field
Synchrotron

38. Intermediate BL Lac Objects
3C66A October 2008
(Abdo et al. 2011)
(Acciari et al. 2009)
Spectral modeling with pure SSC would require extreme parameters
(far sub-equipartition B-field)
Including External-Compton on an IR radiation field allows for
more natural parameters and near-equipartition B-fields
→ g-ray production on > pc scales?

39. Leptonic and Hadronic Model Fits along the Blazar Sequence
Hadronic models can more easily produce VHE emission through cascade synchrotron
Red = Leptonic
Green = Hadronic
Synchrotron self-Compton (SSC)
Proton synchrotron + Cascade synchrotron
Proton synchrotron
External Compton of emission from BLR clouds (ECC)
Electron synchrotron
Synchrotron
Electron SSC
(Bӧttcher, Reimer et al. 2013)

40. Diagnosing the Location of the Blazar Zone
Energy dependence of cooling times: Distinguish between EC on IR (torus → Thomson) and optical/UV lines (BLR → Klein-Nishina)
If EC(BLR) dominates: Blazar zone should be inside BLR → gg absorption on BLR photons → GeV spectral breaks
(Poutanen & Stern 2010)
→ No VHE g-rays expected! →VHE g-rays from FSRQs must be from outside the BLR (e.g., Barnacka et al. 2013)
(Dotson et al. 2012)

41. Time-dependent SED and light curve fits to PKS 1510-089
Internal Shock Model
Time-dependent SED and light curve fits to PKS
(SSC + EC[BLR])
(Chen et al. 2012)
→ X. Chen's Talk