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

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Presentation on theme: "Model Constraints from Multiwavelength Variability of Blazars Markus Böttcher North-West University Potchefstroom South Africa."— Presentation transcript:

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 (B ӧ ttcher et al. 2007)

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

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

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) Non-thermal spectra with two broad bumps: Low-energy (probably synchrotron): radio-IR-optical(-UV-X-rays) High-energy (X-ray –  -rays) 3C66A Collmar et al. (2010)

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 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 → P. Mimica's Talk

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 R s 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 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 … (Mannheim & Biermann 1992; Aharonian 2000; Mücke et al. 2000; Mücke et al. 2003)

13 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

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

15 Lepto-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 X-ray/  -ray Polarization

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

17 Correlated Multiwavelength Variability in Leptonic One-Zone Models Example: Variability from short-term increase in 2 nd - order-Fermi acceleration efficiency X-rays anti-correlated with radio, optical,  -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  b >>  a ) bb aa ff rr Shock acceleration → Injection of particles with Q(  ) = Q 0  -q for     Sokolov et al. (2004), Mimica et al. (2004), Sokolov & Marscher (2005), Graff et al. (2008), B ӧ ttcher & Dermer (2010), Joshi & B ӧ ttcher (2011), Chen et al. (2011, 2012) Time-dependent, inhomogeneous radiation transfer Synchrotron SSC (→ Light travel time effects!) External Compton (Chen et al. 2012) → X. Chen's Talk

21 Internal Shock Model Parameters / SED characteristics typical of FSRQs or LBLs (B ӧ ttcher & Dermer 2010)

22 Internal Shock Model Discrete Correlation Functions X-rays lag behind HE  -rays by ~ 1.5 hr Optical leads HE  - rays by ~ 1 hr Optical leads X-rays by ~ 2 hr (B ӧ ttcher & Dermer 2010)

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

24 Polarization Angle Swings Optical +  -ray variability of LSP blazars often correlated Sometimes O/  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 obs. Frame:  obs ~ 1/  Viewing direction in comoving frame:  obs ~  /2 B

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

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 PKS Synchrotron + Accretion Disk SEDs Frequency-dependent Degree of Polarization  Degree of Polarization  vs. time Polarization angle vs. time (Zhang et al. 2014)

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

32 Summary 1.Both leptonic and hadronic models can generally fit blazar SEDs well. 2.Distinguishing diagnostics: Variability, Polarization, Neutrinos? 3.Time-dependent hadronic models are able to predict uncorrelated synchrotron vs. gamma-ray variability 4.Synchrotron polarization swings (correlated with  -ray flares) do not require non-axisymmetric jet features!

33

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

35 Requirements for lepto-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) => Synchrotron cooling time: t sy (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): No dense circum- nuclear material → No strong external photon field Synchrotron SSC Low magnetic fields (~ 0.1 G); High electron energies (up to TeV); Large bulk Lorentz factors (  > 10) The “classical” picture (Acciari et al. 2010)

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

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

39 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

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) (Dotson et al. 2012) If EC(BLR) dominates: Blazar zone should be inside BLR →  absorption on BLR photons → GeV spectral breaks → No VHE  -rays expected! →VHE  -rays from FSRQs must be from outside the BLR (e.g., Barnacka et al. 2013) (Poutanen & Stern 2010)

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


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