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Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr 22-25 2008 A unified time-dependent view of relativistic jets G. Henri Laboratoire.

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Presentation on theme: "Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr 22-25 2008 A unified time-dependent view of relativistic jets G. Henri Laboratoire."— Presentation transcript:

1 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr A unified time-dependent view of relativistic jets G. Henri Laboratoire d ’Astrophysique de Grenoble, France

2 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Why is variability important ? Theoretical models of jets are often underconstrained : Steady-state models can fit instantaneous spectra with a large range of parameters and even basic assumptions (e.g. hadronic/leptonic models) Multi - , high sensitivity observations showing variability are much more constraining

3 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Good data available Highly sensitive instruments in gamma-ray (e.g. HESS) are crucial tools to get well resolved light curves.

4 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Simple models Assume leptonic models, basically synchrotron + SSC Simplest models = 1 zone « blob », homogeneously filled by B field, relativistic particles, moving relativistically with  b Must specify B, R,  b and particle distribution bb B R 

5 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Time dependent models 1-zone models can be transformed in time dependent models assuming some particle injection law (impulsive or continuous) Particle energy distribution : Power -law (1st order shock acceleration); (or broken power-law) Quasi-maxwellian or « pile-up » (2nd order diffusive acceleration, impulsive) (peaked around  0 )

6 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr A one-zone model with a pile-up One-zone injection of a pile-up distribution during a finite time Saugé & H. 2004

7 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Limits of the 1-zone model Does not reproduce the low energy part of the spectrum Evolution of geometrical parameters (Katarzynski 05..) ? Emission of « old » flares?

8 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr From the blob to the jet How to account for the long range emission? « Blob in jet » model (Katarzynski et al.) Successive flares -> succession of blobs Continuous emission -> time-dependent injection

9 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr The « Two flow » model  Two flow model : 2 distinct flows (Sol, Pelletier, Asséo ‘85, H. & Pelletier ‘91), introduced first for explaining radio observations (similar to later « spine in jet » model, Ghisellini et al.) MHD jet e - p+ mildly relativistic *carries most of the power *fuelled by accretion disk *large scale structures, hotspots Ultra relativistic e + -e - pair plasma * Generated in the « empty » funnel, no baryon load. * Produces high energy photons and relativistic motions * Energetically minor component

10 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr The « slow » MHD component Baryonic jet can be emitted from the accretion disk through MHD mechanism ( a la Blandford-Payne) (Ferreira et al., ‘97, ‘04) B field extract angular momentum and power from the JED (Jet Emitting Disk) Powerful, but only mildly relativistic (0,5 - 0,9 c)

11 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Formation of the relativistic pair plasma In situ generation of pair plasma in the inner MHD funnel (H.& Pelletier 91, Marcowith et al. ‘95) Produced through gamma-ray emission Injection of some relativistic particles X-ray and gamma-ray emission by IC and/or SSC  annihilation forms new pairs Continuous reacceleration by MHD turbulence necessary for a pair runaway to develop. Limited by the free energy available: saturation must occur at some point. Intermittent production possible and even probable !

12 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Recipe for a stratified, variable jet 1)A geometry R(z,t) 2)A B-field distribution B(z,t) 3)A Lorentz factor  b(z,t) 4)A Particle distribution n( ,z,t) Even in « thin » jet, 1-D approximation, requires full function of z and t : much more involved than 1-zone models To describe a continuous jet, one needs

13 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Parametrized by a « shifted » power-law Jet geometry Determined by MHD solutions (inner funnel) Assuming some interconversion process Conservation of poloidal flux Bp  R(z) -2 Conservation of current B   R(z) -1 ∝ R0R0 RiRi z0z0 Z=0

14 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Particle energy distribution In the spirit of 1-zone model, we adopt a pile-up distribution Apparent power-law can be reproduced by a spatial convolution of peaked functions (e.g. standard accretion « multicolor » disk model)

15 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Evolution of particle distribution along the jet Reacceleration necessary (short cooling time)  0 evolves following acceleration vs cooling Where acceleration is assumed to follow a power-law with a spatial cut-off Total particle flux evolves through pair production and annihilation

16 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Bulk Lorentz factor Light e+-e- very sensitive to the radiation field In an anisotropic photon field from an accretion disk, Compton force can be accelerating (radiation pressure) or decelerating (Compton drag) following bulk  b -> Bulk equilibrium Lorentz factor for which the aberrated net photon flux vanishes. Cold plasma Hot plasma Slowly accelerating Saturates to a asymptotic Lorentz factor (works only with external reheating (Compton rocket)-> 2-flow model only ! )

17 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Compton equilibrium velocity Compton rocket model does not seem to work for TeV blazars  b too low at small distances (imposed by variability) TeV blazars = BL Lacs = weak accretion disk !! (cf MHD accretion disks) Other acceleration (hydrodynamic?) mechanism ? Parametrized to vary from 1 to on a scale z 0

18 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Bulk Lorentz factor Bulk Lorentz factor constrained by  opacity Cospatial distribution of soft photons ->  ≥ 50 (Begelman et al 2008) Pair production necessary for the 2-flow model, needs  ~1 !! Choose the lowest value of  b compatible with pair production and variability. Stratified jet helps for lower  b..

19 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Limits on Lorentz factor Minimal constraint with hardest distribution (pile-up) H. & Saugé 2006

20 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Steady state solutions Instantaneous SED is a complicated convolution of the whole history of the jet, integrated all over the length. High energy data dominated by a single or a few flares Low energy data averaged over numerous flares (duty cycle f)

21 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Steady state solutions Construction of a « fake constant flaring state » by multiplying low energy points by f -1 (estimated from flaring duty cycle) Boutelier T., PhD thesis

22 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Time dependent solutions Procedure Construct a set of fake « constant » states (varying density and/or acceleration rate) from quiescent to « fake flaring » state Find a history of injections to fit light curves, taking into account light travel time.

23 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Variability constraints. i Z0=ctZ0=ct

24 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr PKS flare  b :15 cosi:1 R i : 1.1e+14 cm R 0 : 1.78e+14 cm Z 0 : 2e+15 cm Z max : 5e+19 m B: 5 G Q 0 : 6.5 N tot (z 0 ) : 40 cm-3 (quiesc.) 600 cm-3 (flare)  : 0.2 : 1.9  : 1.27

25 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Pair production flare With the chosen parameters, intense pair production occurs during a flare. Strongly non linear behavior

26 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Delayed variability Larger wavelengths variability is delayed and smoothed TeV injection optical X-ray

27 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr The video…. >200Gev flux

28 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Physical grounds for variability In the model, variability is reproduced by a small variation of initial density and/or acceleration rate only. Could be the result of non-linear feedback and hysteresis cycle, but very difficult to simulate (-> weather forecasts !) Pair production threshold sharply peaked-> strongly instable Onset of « Active » periods (yr time range) : changes in accretion rate, MHD structure Rapid flares (min to hr range) : bursts in pair production ? Leaves more room for complex variability pattern (possible long distance reacceleration sites in MHD jet, knots….

29 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Comments on bulk Lorentz factor Although in the lowest part of allowed range, bulk Lorentz factor still too high to be compatible with the « unification » model of radiogalaxie. Weak geometrical collimation ? Must go beyond the 1-D « thin jet » approximation. ( see Lenain et al… work ) FRI galaxy (unbeamed counterpart of BL lacs)  j > 1/  b 1/  b Lack of superluminal motion? Radial  b gradient?

30 Blazar Variability across the Electromagnetic Spectrum, Palaiseau, Apr Future TeV HESS 2, CTA GeV GLAST Hard X-rays : SIMBOL-X Will hopefully bring a complete coverage of high quality data…. High sensitivity, time and energy resolved observations are a key factor to test models


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