6. Jets and radio emission
• This lecture:• Some observations showing collimated
outflows• Some reasons we might expect them• Their properties: speeds, what they are
made of, how they emit• Radio galaxies• Jets from stars in the Galaxy, SS433
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• Collimated outflows– Stuff being ejected in a straight line…
First of all: what am I talking about?
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What can we learn from jets and why are they important?
• Are a historical record of nuclear activity– Give us a minimum lifetime for the AGN
• Important source of mechanical heating – affect the energy balance of the intergalactic medium
• Probe of the surrounding medium• Kinetic luminosity of the central engine• Important source of high energy electrons
– perhaps cosmic rays
• Tell us the geometry, orientation of the system• Ultimately tell us about conditions close to an
accreting black hole
Slide 4
Also seen in X-ray binariesRadio outflow from GRS1915, probable black hole binary in our own galaxy.
Slide 8
Types of radio galaxies
• Radio galaxies split into Fanaroff-Riley class 1 and 2 by the jet/lobe properties.
• Quite a lot of lobe morphologies seen, eg head-tail sources.
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3C311, a Fanaroff-Riley class 1 radio galaxy.
Lower luminosity radio galaxies
Diffuse lobes which darken at the ends
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Cygnus A, a Fanaroff-Riley class II radio galaxy. These are more powerful than FR-1s, and the ends of the lobes are brightened with hot spots.
Slide 11
3C83: a head-tail radio galaxy (radio in red, optical in blue).
The bending of the jets is caused by interaction with the hot gas in a cluster of galaxies.
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So outflows common in accreting sources.
• Why might this be?
• Some possiblities:– radiation pressure– tangled magnetic fields– angular momentum
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Why collimated?
• Magnetic fields– Already seen collimated accretion flows in
magnetic white dwarfs
• Geometry– Something about the geometric layout of the
system has a preferred direction for outflow– Axis of a rotating body is a ‘special direction’– Magnetic fields generated in rotating systems
Slide 15
How fast is the material moving?
• In some quasars and micro-quasars we can observe individual blobs of material in the jets.
• In some cases the apparent velocity of motion is > the speed of light!– termed superluminal motion– How can this work?
Slide 16
Clue to superluminal motion:
• Sources which show superluminal motion tend to be one-sided, whereas most radio sources are relatively symmetrical.– Effect of beaming– The plasma is moving towards us!– So the material must be moving close to
the speed of light
Slide 17
Superluminal motion
Worried astronomers at first!
Projection effect. If blob of plasma has a high velocity in your direction, the motion of the material in the plane of the sky has the appearance of moving faster than light.
vapparent = vsin / (1-v cos /c)
Slide 18
Emission mechanism
• Radio emission from radio jets is often strongly polarized
• Preferred direction for electric and magnetic wave vectors
• -> magnetic fields• We already know v->c• -> synchrotron radiation
Slide 19
Synchrotron spectra
• Synchrotron spectra are typically power laws F = k- where is the called the spectral index and is typically ~0.8
• Featureless over a wide energy range
• The power law of the synchrotron radiation is related to the power law energy spectral index of the electrons
=(-1)/2
Slide 20
Energy losses
• The rate of energy loss of a synchrotron electron is proportional to E2
– The highest energy electrons lose their energy the fastest
– High energy cutoff in the synchrotron spectrum unless energy is continuously injected.
– High frequency synchrotron far from the source of electrons must imply re-acceleration, possibly in shocks.
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Multiwavelength emission
• In some cases synchrotron emission extends all the way to X-ray or gamma-ray frequencies.
• However, inverse Compton losses can also be important, especially in compact hotspots.
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What if you look straight down the jet?
• The beamed emission from the jet should dominate over everything else: continuum spectra.
• We know of some of these objects, they are called BL Lac objects, named after the variable ‘star’ BL Lac. Also known as ‘blazars’ (actually blazars are a slightly broader class including emission line sources)
• Highly variable because of the compressed timescales
Slide 25
What are the jets made of?
• Electrons
• Protons or positrons? We don’t know!
• If positrons, then a large amount of electromagnetic energy must also be coming down the jet.
• In one Galactic source called SS433 we do know.
Slide 27
SS433
• Source 433 from the 1977 catalogue of emission line objects of Stephenson and Sanduleak.
• Neutron star or black hole binary.• Jets which precess on a 164 day
period.• Unusual jet: thermal emission• Doppler shifted emission lines (80000
km/s) seen from the jet: the jet contains normal matter!
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Some key points:
• Collimated outflows (‘jets’) are found in many accretion powered systems.
• The jets from active galaxies and accreting binaries consist of electrons (+ protons or positrons) moving at relativistic speeds.
• Jets interact with their surroundings. They could be important sources of mechanical energy and cosmic rays.
• They give us a lower limit to the lifetimes of accreting systems.
• The radio emission is synchrotron; optical and X-ray emission may be synchrotron or inverse Compton.
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