Transcript Document

High energy Astrophysics
Mat Page
Mullard Space Science Lab, UCL
6. Jets and
radio emission
Slide 2
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 3
First of all: what am I talking about?
• Collimated outflows
– Stuff being ejected in a straight line…
What can we learn from jets and why
are they important?
Slide 4
• 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 5
Big structures in the radio
3C296 optical (blue) and radio (red)
Slide 6
They really are outflows:
M87 observed by HST
Slide 7
Slide 8
Also seen in X-ray binaries
Radio
outflow
from
GRS1915,
probable
black hole
binary in
our own
galaxy.
Slide 9
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 10
3C311, a Fanaroff-Riley
class 1 radio galaxy.
Lower luminosity radio
galaxies
Diffuse lobes which
darken at the ends
Slide 11
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 12
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 13
Collimated outflows also seen in
young stellar objects
HST
image
of
HH47
Slide 14
So outflows common in accreting sources.
• Why might this be?
• Some possiblities:
– radiation pressure
– tangled magnetic fields
– angular momentum
Slide 15
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 16
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 17
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 18
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 = vsinq / (1-v cosq /c)
Slide 19
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 20
Synchrotron spectra
• Synchrotron spectra are typically power
laws Fu = ku-a where a 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 b of the electrons
 a=(b-1)/2
Slide 21
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 reacceleration, possibly in shocks.
Jets from radio-X-ray
Slide 22
PKS127 jet in X-rays (image) and radio (contours)
Slide 23
Multiwavelength emission
• In some cases synchrotron emission
extends all the way to X-ray or gammaray frequencies.
• However, inverse Compton losses can
also be important, especially in compact
hotspots.
Slide 24
X-ray Jet in Pictor A
Chandra image
Slide 25
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
Blazar spectra: synchrotron and inverse Compton
Slide 26
Slide 27
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 28
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 29
SS433
Artist’s impression
Emission lines also
observed in the optical
Spectra from Exosat
Slide 30
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 Xray emission may be synchrotron or inverse
Compton.