Radio Astronomy Emission Mechanisms

Ronald J. Maddalena

What is a SPECTRUM

Thermal Continuum Radiation

Non-Thermal Continuum Radiation

Spectral-Line Radiation

References

Advanced undergraduate or lower graduate level:

M. Harwit, "Astrophysical Concepts" (1st or 2nd editions).

G. Verschuur and K. Kellerman (eds.), "Galactic and Extra-Galactic Radio Astronomy" (1st and 2nd editions; 1st is at a lower level than 2nd).

G. Verschuur, "Invisible Universe Revealed".

Kraus, "Radio Astronomy" (latest edition).

 

Thermal Continuum Radiation

Object's characteristics

• Opaque "black" body
• Isothermal
• In equilibrium
  
  

What is Happening:

1. iAtoms, molecules, etc. constantly emit and absorb radiation -- radiation bath.
2. Photons act like a gas at a temperature
3. Photons diffuse outward and eventually escape -- "Random Walk" process.
4. Very similar to 'Brownian' motion -- an 'ideal' gas at a certain temperature.
   
   

Intensity of a "black body" at a certain temperature at a certain frequency?

Planck's Law:

h = Plank's constant;

k = Boltzman's constant

T in K, in Hz

INTENSITY is the energy emitted by an object per unit surface area at a certain frequency. It is INTRINSIC to the source -- all observers, no matter where they are in the universe would see the same I(). Units: ergs/cm²/sec/Hz

In radio (approximately):

Non-Thermal Continuum Radiation

Anything that doesn't follow the "black-body" intensity curve.

I'll only consider two non-thermal methods:

• Free-Free

• Synchrotron

Whenever a charge particle is accelerated it gives off radiation.

Free-Free Continuum Radiation

• Electron accelerated by an encounter by with a proton but isn't 'captured' by the proton.
• Mostly IONIZED gases (also called a PLASMA).
• Radiation at LOW FREQUENCIES cannot directly escape but goes through multiple absorptions and re-emission. The radiation "random" walks its way out.
• It LOOKS like a THERMAL source at LOW frequencies.
• At high frequencies, radiation can escape directly and I = constant (i.e., independent of frequency).

Synchrotron Continuum Radiation

Electrons moving at relativistic velocities across a magnetic field will spiral around the lines of magnetic force.

Particle experiences an acceleration. In most cases:

where alpha usually has a value between -0.2 and -1.2 at high frequencies.

Very different from thermal or free-free.

• For thermal radiation, = 2 at ALL radio wavelengths.
• For free-free radiation, = 2 at low frequencies but = 0 at high.
  

Mechanisms that alter radiation spectra

Absorption

Any mechanism that can produce radiation can also absorb it.

Dispersion

Space does not have an index of refraction equal to one -- electrons in space.

Speed at which light travels = c times the index of refraction.

Index of refraction is different at different frequencies.

Different frequencies will travel at different speeds.

Some frequencies will arrive earlier or later than others.

 

Doppler effect

If an object is moving toward or away from the observer, the radiation spectrum is shifted by approximately:

V = velocity of the object toward or away from us. V is < 0 if moving toward us, > 0 if away.

Spectra are shifted (and distorted) if object and observer move toward or away from one another.

Makes no difference whether it is the source or observer who is moving.

Spectral-Line Emission Mechanisms

The ENERGY of atoms and molecules is quantized.

They lose energy in quantized amounts.

They can lose energy by emitting light.

E = h x frequency so the frequency of the light emitted can only be at certain frequencies.

Note: anything that emits light can also absorb it so atoms and molecules can absorb light but only if the light have the frequencies that correspond to the quantized energy.

Atomic Spectral Lines

Atoms can emit or absorb radiation as electrons move from one orbit to another. For hydrogen:

n and m are the 'number' of the electron orbits

If n and m are small numbers, the radiation occurs in or near the visible.

If n and m are large (e.g., m = 176 and n = 177), the radiation will occur at radio frequencies.

Hyper-fine Transitions

Electrons and protons have "spin".

Spin is quantized.

In a H atom, the proton's "spin" may be aligned or anti-aligned with the electron's spin.

A H atom with the spins aligned has more energy than one that is anti-aligned.

An 'aligned' H atom in 11 million years will flip the spin of the electron and emit light at 1420 MHz.

10⁶⁷ H atoms in the Milky Way ==> about 3x10⁵² H atoms per second are emitting at 1420 MHz.

Molecular Transitions

Molecular rotations and vibrations are also quantized.

When a molecule slows its vibration, it might emit at infra-red frequencies.

When a molecule slows its rotation, it might emit at radio frequencies.

Molecules can also produce "Maser" emission -- equivalent to "laser" emission except at radio frequencies and is a natural phenomenon.