3: RADIATION PROCESSES

Question 1

What is cyclotron radiation?

Answer 1

For a non-relativistic electron, the radiation from the electron is not beamed and observer sees emission of radiation that varies sinusoidally. The spectrum of this radiation is Fourier transform of the time variation of this emission, giving a delta function at gyrofrequency.

Question 2

What does the synchrotron emission from a single electron look like?

Answer 2

A series of sharp pulses separated by the period corresponding to the reduced frequency

Question 3

What does the cyclotron emission from a single electron look like?

Answer 3

A smooth sine wave

Question 4

What is the energy spectrum determined by?

Answer 4

The width of the peak not the interval between peaks.

(The Fourier transform of the sharp peak gives an effectively continuous distribution, resulting in a continuum energy spectrum.)

Question 5

What is the width of the peak determined by?

Answer 5

Beaming and shortening of pulse

Question 6

What are the beaming and shortening effects?

Answer 6

1. Beaming effect means emission only seen between points A and B during which time electron has moved through angle 1/y
2. Pulse is shortened because electron travelling close to c so it almost keeps up with the emission from point A as it travels to point B

Question 7

Why is the synchrotron radiation many orders of magnitude higher than non-relativistic gyro-frequency?

Answer 7

Because of beaming and shortening

Question 8

What gives rise to the synchrotron energy spectrum?

Answer 8

The Fourier transform of the individual time series

Question 9

What are the observable signatures of synchrotron radiation that allows us to distinguish it from other emission mechanisms?

Answer 9

1. The power law spectrum that is produced at most energies and the value of its slope
2. Synchrotron self-absorption at low energies in compact sources
3. The polarisation of the radiation

Question 10

What is the electric vector of an accelerating charge?

Answer 10

It’s at a maximum anti-parallel to the direction of the acceleration, which is perpendicular to the direction of motion for a charged particle in a B field.

Question 11

Define high linear polarisation

Answer 11

Since radiation is strongly beamed along the direction of particle motion, most photons that are observed are from electrons with velocities directly toward observer so have closely aligned electric vectors

Question 12

What does the small beam width give?

Answer 12

A spread of frequency

Question 13

What does the intensity and polarisation depend on?

Answer 13

The viewing angle

Question 14

When is emission 100% linearly polarised?

Answer 14

When we look edge on

Question 15

When is a high degree of polarisation at the source not observed?

Answer 15

When the magnetic field at the source is tangled preventing any coherent polarisaiton and when the radiation passes through plasma on the way to the observer. If the plasma is non-uniform, Faraday rotation occurs which washes out polarisation

Question 16

Define brightness temperature of radiation

Answer 16

This is the temperature of a black body with the same intensity as the source in question at a particular frequency

Question 17

What increases at lower frequencies and what decreases towards lower frequencies?

Answer 17

1. Tb increases (brightness temp)
2. Te decreases (effective temp)

Question 18

Why can’t Tb be > than Te?

Answer 18

Because then electrons would be radiating more efficiently than a BB at their effective temperature, which is physically impossible.

Question 19

Define self-absorbed.

Answer 19

Electrons become opaque to their own synchrotron emission, absorbing the photons and obeying the condition Tb = Te.

Question 20

When does synchrotron radiation occur?

Answer 20

When energy losses are proportional to E^2

Question 21

What does τ represent?

Answer 21

Time for electrons to lose half its energy

Question 22

Define Bremsstrahlung emission

Answer 22

Emission due to charge acceleration from Coulomb interactions between particles (commonly electrons and ions) in a plasma. Free-free emission so gives continuum spectrum.

Question 23

Why are electrons not ions accelerated?

Answer 23

Due to their mass

Question 24

What is the Gaunt factor?

Answer 24

It corrects for QM effects and the effects of distant interactions.

Question 25

What can the exponential cut off at v = kT/h be used for?

Answer 25

Temperature diagnostic for clusters of galaxies

Question 26

Define inverse Compton scattering

Answer 26

Relativistic electrons lose energy by upscattering photons

Question 27

How does scattered radiation differ from synchrotron radiation?

Answer 27

Scattered is much less polarised

Question 28

What are examples of photon fields of known energy density?

Answer 28

The CMB radiation and electrons scattering their own synchrotron emission (synchrotron self-Compton)