Definitions Flashcards
Synchrotron radiation
Electron performs helical orbits in the presence of an external magnetic field.
Larmor formula
Gives the energy emitted by an accelerated electron.
Bremsstrahlung radiation
Electrons in plasma are accelerated by the electrostatic fields due to ions - braking radiation.
Fermi’s golden rule
Equation for calculating transition rates between quantum states.
Dipole Approximation
Ignore any variation of the EM field over the atom.
Synchrotron self-absorption
There is more energy in the radiation than the electrons, and the electrons absorb that energy, causing the spectrum to turn over.
Eddington limit
Limit to the maximum luminosity of an accreting system which occurs when outflowing radiation exerts a pressure which exceeds the gravitational force inwards.
Shock wave
Discontinuous change in the fluid flow variables which occurs when there is motion faster than the sound speed of gas.
Equivalent width
The area between the spectrum and the extrapolated continuum (divided by continuum value) - it is a block that has the same depth as the line and therefore the same effective width.
Cooling time
The time the cluster would take to radiate away all its energy at the current rate
Cooling flows
If the gas in the central regions
cools then there is a drop in pressure and, to maintain hydrostatic equilibrium, matter must flow
inwards from outside the cooling region, leading to a ‘cooling flow’. We expect highly peaked central surface brightness profiles.
Downstream
Post-shock gas.
Upstream
Pre-shock gas.
Fermi acceleration
The acceleration of charged particles after repeatedly crossing a shock, every time they cross, they gain a small fraction of energy leading to acceleration.
Lorentz gauge
∇⋅A+(1/c^2)∂φ/∂t=0. Partial gauge fixing of the electromagnetic vector potential, requiring ∂μAμ=0.
Maxwell I
del dot E = rho/epsilon_0
Maxwell II
del dot B = 0
Maxwell III
del cross E = -partial B dot
Maxwell IV
del cross B = mu_0*j + 1/c^2 *E dot
Maxwell I Physical significance
Conservation of electric field within a closed surface not enclosing any electric charge.
Maxwell II Physical significance
Magnetic field is conserved everywhere.
Maxwell III Physical significance
An electric charge moving in a certain direction will induce a magnetic field in a direction orthogonal to the charge.
Maxwell IV Physical significance
Magnetic field is produced due to the combined effect of current density and displacement current density.
Distant zone
The distance is significantly larger than the size of the source, so we can assume the distance is constant.
Dipole selection rules
(1) JY = JX 1, JX, JX +1 (J is total angular momentum quantum number)
(2) Parity change
(3) In the case of LS coupling, also
(3a) no spin change
(3b) same rules for orbital AM L as for total AM J.
Maser emission
Opacity is negative and specific intensity grows - upper state is more populated than the lower state.
Optical depth
tau = opacity x length
Source function
A measure of how photons in a light beam are removed and replaced by new photons by the material it passes through.
Thermal equilibrium
Means intensity doesn’t vary with depth: dI/dL = 0. Ratio of states is equal to the Boltzmann distribution.
n > > n_crit
Boltzmann – collision dominated regime
n < < n_crit
up collisions = down radiative spontaneous emissions
