Chapter 2 Flashcards
(38 cards)
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
