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Model Constraints from Multiwavelength Variability of Blazars

0. Model Constraints from Multiwavelength Variability of Blazars. Markus B öttcher North-West University Potchefstroom South Africa. 0. Blazars. Class of AGN consisting of BL Lac objects and gamma-ray bright qu asars Rapidly (often intra-day) variable.

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Model Constraints from Multiwavelength Variability of Blazars

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  1. 0 Model Constraints from Multiwavelength Variability of Blazars Markus Böttcher North-West University Potchefstroom South Africa

  2. 0 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 0 X-rays Optical Radio (Bӧttcher et al. 2007)

  4. Blazar Variability: Variability of PKS 2155-304 0 VHE g-rays VHE g-rays Optical (Aharonian et al. 2007) X-rays 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 1510-089 (Marscher et al. 2010)

  6. 0 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. 0 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. 0 LeptonicBlazarModel Synchrotron emission Injection, acceleration of ultrarelativistic electrons Relativistic jet outflow with G≈ 10 nFn g-q Qe (g,t) n → P. Mimica's Talk Compton emission g1 g2 g 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. 0 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. 0 HadronicBlazarModels Proton-induced radiation mechanisms: Injection, acceleration of ultrarelativistic electrons and protons Relativistic jet outflow with G≈ 10 nFn n g-q Qe,p (g,t) • Proton synchrotron g1 g2 g • pg→ pp0p0 →2g Synchrotron emission of primary e- • pg→ np+ ; p+ → m+nm • m+→ e+nenm → secondary m-, e-synchrotron nFn (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) Synchrotron Electron 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) Gr Gf Gb Ga Shock acceleration → Injection of particles with Q(g) = Q0g-qfor 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. 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. 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 1510-089 (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 Effects B 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 1510-089 (Chen et al. 2012)

  30. Flaring Scenario: Magnetic-Field Compression perpendicular to shock normal Degree of Polarization P vs. time Synchrotron + Accretion Disk SEDs Frequency-dependent Degree of Polarization P Polarization angle vs. time PKS 1510-089 (Zhang et al. 2014)

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

  32. 0 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!

  33. 0 Superluminal Motion Apparent motion at up to ~ 40 times the speed of light!

  34. Requirements for lepto-hadronic models • To exceed p-gpion 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 ~ 1019eV (=> 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?

  35. 0 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)

  36. 0 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

  37. 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?

  38. 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) Synchrotron Electron synchrotron Electron SSC (Bӧttcher, Reimer et al. 2013)

  39. 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 → gg absorption on BLR photons → GeV spectral breaks → No VHE g-rays expected! →VHE g-rays from FSRQs must be from outside the BLR (e.g., Barnacka et al. 2013) (Poutanen & Stern 2010)

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

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