High energy Astrophysics

Mat Page

Mullard Space Science Lab, UCL

6. Jets and radio emission

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

Slide 2

• Collimated outflows– Stuff being ejected in a straight line…

First of all: what am I talking about?

Slide 3

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

Big structures in the radio

3C296 optical (blue) and radio (red)

Slide 5

They really are outflows: M87 observed by HST

Slide 6

Slide 7

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.

Slide 9

3C311, a Fanaroff-Riley class 1 radio galaxy.

Lower luminosity radio galaxies

Diffuse lobes which darken at the ends

Slide 10

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.

Slide 12

Collimated outflows also seen in young stellar objects

HST image of HH47

Slide 13

So outflows common in accreting sources.

• Why might this be?

• Some possiblities:– radiation pressure– tangled magnetic fields– angular momentum

Slide 14

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.

Slide 21

Jets from radio-X-ray

PKS127 jet in X-rays (image) and radio (contours)

Slide 22

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.

Slide 23

X-ray Jet in Pictor A

Chandra image

Slide 24

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

Blazar spectra: synchrotron and inverse Compton Slide 26

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!

Slide 28

SS433

Artist’s impression

Spectra from ExosatEmission lines also observed in the optical

Slide 29

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.

Slide 30