Large-scale AGN JetsPhysical conditions and particle acceleration

Robert LaingOxford, July 3 2017

Extragalactic jets

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n Jet power ~1041 – 1047 ergs-1

n Relativistic bulk flow: pc-scale Lorentz factor Γ ~

Jets as particle accelerators

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Centaurus A radio + X-ray

Mkn 501 average (Abdo et al. 2011)

M87 TeV gamma rays

Radio Galaxy 3C31(RL et al. 2008)

FRI jets: Low-luminosityDeceleration

FRII: Powerful, fast, well-collimated jets(Swain, Bridle & Baum)

Why the difference ?

I will talk mostly about FRI jets (and one transition case). Markos will discuss FRII’s.

Hosts: old, red and undeadMassive elliptical galaxiesSlow rotators; core profiles Form early; grow by dry mergers

Very massive black holes

Old stars (mostly)

Hot plasma on galaxy andgroup scales

Molecular gas and dust – fuel?

Absorption line/LINER spectraBroad lines weak or absentNo evidence for obscuring torus

Energy output dominated by jets

Lacc/LEdd

•Flux density in observer's frame at frequency ν•S(θ,ν) = D2+α∫ ε'(θ,ν) dV/d2

•Doppler factor•Dj = [Γ(1- βcosθ)]-1 (approaching)•Dcj = [Γ(1+ βcosθ)]-1 (receding)

•sin θ' = D sinθ

•Approaching jet appears brighter and the two jets are observed at different angles to the line of sight in the flow rest frame θ'

•Polarization depends on θ' – different in the two jets

v ≈ c: Proper motions and aberration

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Meyer et al. (2015)βapp = 1.8 – 7.0

n What distributions of flow velocity, field geometry and rest-frame emissivity are consistent with observations?

n Observe:n Deep, high-resolution radio images; IQU, corrected for Faraday

rotationn Assume:

n Symmetrical, axisymmetric, stationary, relativistic flown Power-law energy distribution, optically-thin synchrotron

n Parametrised model of:n Geometryn Velocity field in 3Dn Emissivityn Magnetic-field component ratios

n Calculate I, Q, U; optimise

Jet Models

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Ten FRI radio galaxies (Laing & Bridle 2014) + one transition case

Consistency check: Faraday rotation

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Consistency check: core flux

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Core is the optically-thickbase of the jet

Assume intrinsic ratio ofcore/extended emission isconstant

Doppler beaming causesobserved ratio f to be anticorrelated with θ

Fit is simple single-speedmodel

NGC6251: a FRI/II transition jet

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Looking deeper

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Flying saucerNGC6252

I Model Fits (θ = 30o)

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I

Sidedness ratio→ transverse velocity gradient

Polarization model fits

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Colour: I

Vector length p

Vector angle χ0+90o

Q/I

Inferred velocity field

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Independent longitudinal velocity profiles for spine and layerNo transverse variation in either component

Simplest model:

Spine: constant β ≈ 0.89 (Γ ≈ 2.2)Layer: β ≈ 0.4 (Γ ≈ 1.1)

.... but cannot exclude faster spine close to the AGN wherejets are poorly resolved in width

β = v/c

This is very different from jets in FRI sources ...

FRI jet velocities: deceleration

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FRI jet physics: velocity

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Laing &Bridle (2014)

Deceleration and emissivity

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Deceleration startsin the high-emissivityregion and stops where the jetsrecollimate.

In close up

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Particle acceleration

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NGC315 spectral index Radio/X-ray (RL et al. 2006) (Worrall et al. 2007)

Distributed particle acceleration

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Cen A X-rayemission

(Goodger et al.2010)

M87 jet knot spectral energy distributions

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Consistent with asingle energy distributionfor radiating electrons.

Broken power law spectrumto a first approximation.

Typical for FRI jets.

Spectrum and speed

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Spectrum becomes flatter with increasing distance from AGNOpposite to effect of synchrotron lossesVelocity-dependent particle acceleration?

Laing & Bridle (2013)

FRI jets: relativistic particles

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Field components for NGC6251

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Longitudinal → Toroidal, just as in FRI sources ....

Field components in FRI jets

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Longitudinal Toroidal

FRI jets: field structure

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FRI jet dynamics: conservation law analysis

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Velocity fits from Laing & Bridle (2014)Conservation-lawanalysis followingLaing & Bridle (2002)

3C31

NGC315

B2 0326+39

3C 296

Tails and lobes

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Tails (Plumes) Lobes

Speed at recollimation

High Low

ρc2/w at recollimation(energy density cold:relativistic)

Low High

Entrainment Jets keeprestarts on going

NGC6251 (if pressure confined)

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Spine:- fast- (mildly) supersonic- very light- almost no entrainment- carries most of the energy

Kinetic power 3 x 1037 W

Layer:- mildy relativistic- transonic- carries more of the mass- but still very light

Kinetic power 6 x 1036 WLayerSpine

n FRI jetsn Widen rapidly, decelerate from relativistic to sub-relativistic speeds

and recollimaten Develop transverse velocity gradients as they decelerate, implying

external entrainmentn Have energy fluxes up to ~1037 W (1044 erg s-1)

n The jets in NGC6251 (a transition case)n Remain narrow and maintain roughly constant speedsn Show persistent transverse velocity structure, with a central

supersonic spine (β ≈ 0.9) and a slower (β ≈ 0.4) outer layern Entrain very littlen Have energy fluxes of ~3 x 1037 W

n I suspect that true FRII jets are faster ... but not fast enough to produce beamed iCCMB (see Markos’s talk)

n Observations of FRII counter-jets are very hard, but we are trying ….

Conclusions

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• In FRI jet bases:– Low frequency spectral index is significantly steeper than 0.5 (energy index > 2)– Correlation with flow speed: faster flow → (slightly) steeper spectrum– Particle acceleration up to electron γ ≈ 107 for bulk Γ ≈ 2– Characteristic “broken power-law” spectrum– Acceleration not restricted to a few discrete sites (distributed/multiple small sites)

• Shocks?– Complex, non-axisymmetric brightness structure at start of deceleration – No evidence for shocks where jets recollimate– Conservation-law analysis (no dynamically important field) → transonic flow– Weak relativistic shocks might explain velocity - spectral index relation (Summerlin

& Baring)• Shear?

– Evidence for velocity gradients– One example of association with spectral index

Particle acceleration questions

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Are any of the proposed mechanisms efficient enough?