Large-scale AGN Jetsgroup scales Molecular gas and dust –fuel? Absorption line/LINER spectra Broad...
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Large-scale AGN JetsPhysical conditions and particle acceleration
Robert LaingOxford, July 3 2017
n Jet power ~1041 – 1047 ergs-1
n Relativistic bulk flow: pc-scale Lorentz factor Γ ~
Jets as particle accelerators
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
•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
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
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
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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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I Model Fits (θ = 30o)
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Sidedness ratio→ transverse velocity gradient
Polarization model fits
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Vector length p
Vector angle χ0+90o
Inferred velocity field
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Independent longitudinal velocity profiles for spine and layerNo transverse variation in either component
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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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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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)
Tails and lobes
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Tails (Plumes) Lobes
Speed at recollimation
ρc2/w at recollimation(energy density cold:relativistic)
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 ….
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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?