3: RADIATION PROCESSES Flashcards
What is cyclotron radiation?
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.
What does the synchrotron emission from a single electron look like?
A series of sharp pulses separated by the period corresponding to the reduced frequency
What does the cyclotron emission from a single electron look like?
A smooth sine wave
What is the energy spectrum determined by?
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.)
What is the width of the peak determined by?
Beaming and shortening of pulse
What are the beaming and shortening effects?
- Beaming effect means emission only seen between points A and B during which time electron has moved through angle 1/y
- 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
Why is the synchrotron radiation many orders of magnitude higher than non-relativistic gyro-frequency?
Because of beaming and shortening
What gives rise to the synchrotron energy spectrum?
The Fourier transform of the individual time series
What are the observable signatures of synchrotron radiation that allows us to distinguish it from other emission mechanisms?
- The power law spectrum that is produced at most energies and the value of its slope
- Synchrotron self-absorption at low energies in compact sources
- The polarisation of the radiation
What is the electric vector of an accelerating charge?
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.
Define high linear polarisation
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
What does the small beam width give?
A spread of frequency
What does the intensity and polarisation depend on?
The viewing angle
When is emission 100% linearly polarised?
When we look edge on
When is a high degree of polarisation at the source not observed?
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
Define brightness temperature of radiation
This is the temperature of a black body with the same intensity as the source in question at a particular frequency
What increases at lower frequencies and what decreases towards lower frequencies?
- Tb increases (brightness temp)
- Te decreases (effective temp)
Why can’t Tb be > than Te?
Because then electrons would be radiating more efficiently than a BB at their effective temperature, which is physically impossible.
Define self-absorbed.
Electrons become opaque to their own synchrotron emission, absorbing the photons and obeying the condition Tb = Te.
When does synchrotron radiation occur?
When energy losses are proportional to E^2
What does τ represent?
Time for electrons to lose half its energy
Define Bremsstrahlung emission
Emission due to charge acceleration from Coulomb interactions between particles (commonly electrons and ions) in a plasma. Free-free emission so gives continuum spectrum.
Why are electrons not ions accelerated?
Due to their mass
What is the Gaunt factor?
It corrects for QM effects and the effects of distant interactions.
What can the exponential cut off at v = kT/h be used for?
Temperature diagnostic for clusters of galaxies
Define inverse Compton scattering
Relativistic electrons lose energy by upscattering photons
How does scattered radiation differ from synchrotron radiation?
Scattered is much less polarised
What are examples of photon fields of known energy density?
The CMB radiation and electrons scattering their own synchrotron emission (synchrotron self-Compton)
