Chapter 2 Flashcards
What is Bremsstrahlung radiation?
Braking radiation
Produced in any hot ionised gas or “astrophysical plasma” which contains electrons and ions accelerating as they encounter each other’s electrostatic fields
As electrons are so much lighter than ions it is mainly they which are accelerated
What does the acceleration of the electron which emit Bremsstrahlung radiation depend on?
The nuclear charge (+Ze)
how close it gets (given by collision parameter b)
What is power proportional to?
Acceleration and thus electrostatic force which is proportional to 1/d^2
What is the Bremsstrahlung spectrum dominated by?
The perpendicular component of a (I1)
What happens at high frequencies of the Bremsstrahlung spectrum?
There is an exponential cut-off in the spectrum
What happens to faster electrons?
They have shorter duration of collision and a higher frequency cut off
What happens to the spectrum at low frequencies?
The duration of the collision is much less than the period of the emitted radiation (flat spectrum)
What do cosmic ray electrons contribute to?
The diffuse galactic gamma ray emission
When is the highest photon energy expected?
it is that corresponding to the maximum kinetic energy which the electron can lose in one go
What does the observed bremsstrahlung spectrum have the same slope as?
As the electron spectrum
What must happen to produce high frequency photons?
We require fast electrons (they cannot radiate more than their excess kinetic energy
What is thermal bremsstrahlung radiation?
A Maxwell-Boltzmann distibution of electrons typical of a hot plasma
What is relativistic bremsstrahlung radiation?
A relativistic electron population with a typical cosmic ray like power law spectrum
What does emissivity depend on?
The electron and ion number densities and the plasma temperature
What is the LTE?
Radiation field in any small region is a blackbody with temperature T
What happens to Bremsstrahlung emission when in thermodynamic equilibrium?
The bremsstrahlung emission is exactly balanced by absorption by the inverse process and the electrons in the plasma can absorb photons and gain energy and momentum.
This results in a black body spectrum
What suggests that radio emission from young SNR is predominantly synchrotron?
Polarisation and a power law spectrum
What is the total energy responsible for radio emission?
A combination of energy density of electron and of magnetic field over volume V
What is the minimum energy condition required to produce an observed power?
It is close to a condition of equipartition of energy between magnetic fields and relativistic particles
What does a supernova shock front provide?
A very efficient manner for particle acceleration from which we may obtain a power law energy spectrum
What can happen in strong shocks?
Particle acceleration
In SN shock what does the compression of the ionised gas cause?
Compression of the magnetic fields associated with it and creates turbulence and a high density of scattering centres
What causes particles to gain energy at a shock front?
When the ISM is overtaken by the shock and a particle enters it, it is scattered across and ahead of the shock only to be overtaken again (repeated crossings)
What happens when material is compressed?
The magnetic field is compressed and it becomes stronger causing particles to bounce off it as it acts like a wall
What can be used to estimate magnetic field strengths of SNR shock fronts?
Thin filaments of X-ray synchrotron emitting gas
What happens to particles across a supernova shock?
The particles accelerate and then they loose energy through synchrotron emission
What happens when particles cross the shock?
Some are lost because the relativistic particles on this side (inside) will move away from the shock front with 1/4 of shock velocity of the shock
When the spectrum of a nebula goes from radio to optical or there is a turn down/ “break” in the spectrum what does this imply?
The balance shifts between the particles’ acceleration and their energy losses via synchrotron
What happens during pair production?
2 photons collide and give a positron and electron pair as a result
In pair productions what kind of photons can be involved?
Target photons can be real (infrared or optical background photons)
Target photons can be virtual (photons from the electrostatic field of a nucleus)
Why can’t pair production be detected on Earth?
The gamma rays would never reach the ground. They interact with atmosphere and create theses secondary particles which are detected as cosmic rays
What is Inverse Compton Scattering?
When a photon of low energy scatters off an relativistic electron to become a high energy photon
What does the observed emission spectrum depend on?
Urad not just the electron spectrum
What does a typical nebulas’s spectrum consist of?
A combination of synchrotron emission and a second component of gamma rays due to Inverse Scattering.
(more likely to meet a synchrotron photon than a stellar photon)
When do electrons loose the most energy?
The higher the energy of the incoming radiation field
The more they start off with, the more they loose
How much more likely is loosing energy via Inverse Compton Scattering than Synchrotron emission?
3 times more likely
An electron in interstellar space will loose its energy quicker due to inverse Compton scattering than to synchrotron
What are faster electrons more likely to encounter?
Synchrotron photons
infrared photons from warm dust
2.7 K microwave background photons
What is Synchrotron Self Compton mechanism?
Inverse Compton radiation produced when synchrotron emitting particles up scatter their own synchrotron radiation
