Acceleration in Active Galactic Nuclei I

Overview of AGN

Roger BlandfordKIPAC

Stanford

Brief History

Fath 1918- M87 JetReber 1939 - Cygnus ASeyfert 1943 – Active Galactic NucleiJennison, Das Gupta 1953 – Double Radio SourcesBaade, Minkowski 1954 - Elliptical GalaxiesSchmidt 1963 – QuasarsKerr 1963 – Black Hole MetricVLBI 1969 – Superluminal ExpansionRees 1971 – JetsEGRET 1991 – Gamma rays

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For Particle PhysicistsAGN (and galaxies) are more like people than particles• Properties historical not quantum numbersAGN astrophysics is mostly inductive not deductive• Hard to calculate implications of fundamental modelAGN studies have been led/limited by observations• GLAST should make and stimulate many discoveries

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Anatomy

DiskWindHoleJet

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AGN

10-8 < L/L* < 104

AGN

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Physiology

Black HolesAccretion ModesOutflows MechanismsJet formationSuperluminal MotionJet CollimationEnvironmental Impact

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(T “Standard Model of AGN”Million to Billion solar mass spinning black holes• In most normal galaxies

Mass supplied through accretion disk• Angular momentum ~ r1/2

Unification Models• Orientation very important

- Beaming – radio and γ-ray jets- Obscuration – Seyferts and keV X-rays

Grand Unification/Central Dogma• Major physical (not orientation) properties of AGN depend upon:

- M’S/M’E; (M’E =4πGMmp/σTc); “Passive” black hole- α = Ωm; (0 < α < 0.5); “Active” black hole

• Mass M provides a scale for size for length, time, power (temperature)- Stellar black holes

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Intermediate Mass Supply

Thin, cold, steady, slow, radiative diskSpecific energy e = -Ωl/2

G − ′ M l = const ≈ 0dLdr

=d(ΩG − ′ M e)

dr≈ 3 ′ M de

drG

Energy radiated is 3 x the local energy loss

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Low, High Mass Supply

M’<<M’E, tenuous flow cannot heat electrons and cannot coolM’>>M’E, dense flow traps photons and cannot cool Thick, hot, steady, slow, adiabatic diskBernoulli function: b =e+h

G − ′ M l ≈ 0ΩG − ′ M b ≈ 0⇒ b ≈ −2e > 0

G

Energy transported by torque unbinds gas => outflow

Galaxy black hole (Sgr A*), BALQ

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“Active” Black HolesSpin acts passively to increase efficiency of thin disk accretion• 6-42 percent efficiency

- Very sensitive to pressure, radiation, magnetic stress

Rotational energy associated with spinning Kerr spacetimeSpin energy can be extractedelectromagnetically from spacetime and relativistic disk B

MΩ

EMF ~ ΩΦ ~ ZVPower ~ EMF2/Z0 ~1039W

UHECR!

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Angular MomentumFluid torques• Hole spun up by accretion• Spinning holes spun up less• Retrograde holes spun up moreMagnetic torques• Magnetorotational instability• Disks can support MHD wind• Gas flung out if q > 60o• Remove energy, ang. mom., mass• Decelerate hole

Superluminal Motion

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βob =β sinθ

1− β cosθ

D =1

γ(1− β cosθ)Sν ∝ IνΩ ∝ nν '3 ∝ D3+α

•Typically, γ ~ βob~θ-1 ~ 10•Doppler Beaming

•Powerful amplifier

•Observe oncoming sources•Approaching/Receding~(2γ)7 ~109!

Radio Jets

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•Relativistically beamed•Dominated by approaching jets

•Core – shock structure•Core is stationary, self-absorbedsynchrotron emission

•Radio photosphere•ν ∼ γ2B MHz

•r ~ λ ∼ 1018−20 cm, B ∼ 0.1G,T~1012K•Moving features is shock or wave

•Moving superluminally•Jets may be magnetic pinches

•Relativistic cores

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Pinched JetsToroidal magnetic field B in jet frameCurrent I in rest frame• I=2πrΓB/μ0

Static equilibriumMech. ~ 3-10 EM power

dPdr

+μ0

8π 2r2ddr

IΓ

⎛ ⎝ ⎜

⎞ ⎠ ⎟

2

= 0

LEM =Z0

2πdr'r'0

r∫ (1− Γ−2)1/ 2 I2

LMech = 8πc dr'r'Γ(Γ2

0

r∫ −1)1/ 2 P

X Bφ

I[r]

r

Biochemistry

Disk emissionJet emission Radiative TransferParticle Acceleration

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Disk emissionOptical/UV emission• Black body T ~ r-3/4

• Scattering=>GreyX-ray emission• Comptonisation by hot electrons in active corona• Second order Fermi acceleration of photons• <Δν/ν> = 4kTe/mec2

- Limited by Compton recoil around MeV

• Spectral index α(α+3)= (kTeτ(1+τ)/mec2)-1

• Slope controlled by energetics- spectrum flattens with heating

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Corona – X-rays

Disk - OUV

γ-ray JetsInverse Compton scattering• r ~ 1015-17cm? ν ∼ γ2νs

Soft photons• X-rays can be S or C-1; ν ∼ γ2B MHz• Internal synchrotron (lo L?)• External disk (hi L?)

Particle acceleration• >10 TeV electrons!• One zone

- standing shock?

• TeV emission inside GeV- Particle acceleration faster at small radius

• TeV emission outside GeV- Opacity domintes: TeV + eV -> e+ + e-

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TeV

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Some Big Questions on JetsLocation

• TeV, GeV, Radio, X-ray…

Emission Mechanisms• Leptonic vs Hadronic, Synchrotron vs Coherent

Composition• EM, pairs, ionic plasma

Velocity Field• Monolithic, vs laminar vs turbulent

Collimation• Pinch vs inertial vs pressure

Particle acceleration• Shocks vs reconnection vs stochastic…

Transport• Power, thrust, discharge, current

Origin• Black hole, relativistic accretion disk

Some References

Krolik – Active Galactic NucleiFrank, King, Raine – Accretion Power in AstrophysicsBlandford, Netzer, Woltjer – Active Galactic NucleiRector, De Young (ed) – Extragalactic Jets

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