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Magnetohydronamic Production of Relativistic Jets
Nick Cowan
UW Astronomy
December 2004
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Do these things really exist?
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M87 in the Radio
Guilty by Association
• Radio Galaxies and Quasars
• Microquasars• Supernovae• Gamma-ray bursts• T Tauri stars
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Good ol’ Crab
Synchrotron Radiation
• High brightness temperature
• Characteristic spectrum
• Highly polarized• Rapid variability (ns)
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Shocks
• Relativistic usually means supersonic
• The “hot spots” in many radio sources
• Nature’s own particle accelerator
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Superluminous Motion(How it looks)
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Superluminous Motion (How it works)
5 ly
3 ly
4 ly
V = 5/6 c
A
B
Light from A arrives here 2 yrs before light from B
Doppler Boosting & Focusing
D = sqrt(1 - 2) (1 - cos)
• Jet is D2 times brighter due to smaller solid angle
• Jet is D times brighter due to higher frequency of emission
Modelling MHD Jets: Some simplifying assumptions
Plasma behaves like a fluidPlasma is a perfect conductorFrozen-in field linesMagnetic pressureMagnetic tension
Limiting Cases
v2 < B2/8Plasma is flung out centrifugally along field lines
v2 > B2/8Field is bent back by rotating plasma
Characteristic Time Scales
d = R/cms
esc = (R3/2GM)1/2
d < esc Quasi-static outflow
d > esc Explosion or collapse
Recipe for a jet
Gravitating body+ Source of material+ Poloidal B-field+ Differential rotation Torsional Alfven
Wave Train (TAWT)
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Thick disks make powerful jets
• Jet is powered by the poloidal B-field
• Strength of poloidal field is proportional to (H/R)2
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R
H
Frame Drag
• Kerr black holes have angular momentum
• The space time near such a black hole is also rotating differentially
• This leads to more powerful jets
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