The signal strength from natural radio sources may be thousands of times smaller than those from radio and TV stations, which are usually around 100uV/m. Radio astronomers use power flux density as the unit of signal strength and this is in Watts per square metre per Hz (W m-2 Hz-1). In honour of the first radio astronomer Karl Jansky, the very small value of 10-26W m-2 Hz-1 is called 1 Jansky - or 1 Jy.
The relationship between typical power densities of some radio sources
discussed in this paper is given in Figure 2.1. This illustrates the wide
range of source signal strengths. Each source has a different signal strength
as a function of frequency and in Figure 2.1 this is covered by representing
the range of signal strengths as a coloured block.
Figure 2.1 Power density of various radio sources
There are nearly eight orders of magnitude between the strongest and weakest sources that an amateur might detect. Very strong solar bursts can have strengths up to 109 Jy and an external radio galaxy like Virgo A may have a strength of only 100 Jy. The level of equipment sophistication required to detect these sources will therefore vary considerably - from a standard communications receiver to detect solar bursts, to special temperature controlled receivers and preamplifiers to detect external galaxies. Almost all amateurs begin by detecting the Sun. This is easy to do, especially in the middle of the Sun spot cycle where there are many solar storms and bursts. The next goal might be to detect Jupiter or a quiet Sun, followed by a supernova remnant such as Cassiopeia. With this level of sensitivity it is then possible to make maps of radio noise in the Milky Way and to detect and plot the distribution of the neutral Hydrogen emission line at 1420.4MHz.
Spectra of radio sources
There are three main types of astronomical radio emission mechanism: " Thermal noise " Non-thermal e.g. synchrotron generation " Line emissions from atoms and molecules Each of these mechanisms produces a different spectrum of radiation and in a real measurement they will all be present to some extent. The task of the radio astronomer is to separate out the different spectra that make up the observed spectrum in order to determine the physical generation mechanism - or mechanisms - that are taking place in the region of space being observed. This is how knowledge of some of the physics behind the observed Universe is uncovered.
Solid objects at some temperature T, radiate a continuum of electromagnetic radiation due to the vibration of the constituents of atoms and molecules.
There are very few free electrons, and displacements are small, resulting in low levels of emission. For objects at a few hundred degrees Kelvin most of the radiation is at infra red wavelengths. The hotter the object becomes the wavelength of the peak emission of the radiation decreases toward the optical - the object becomes 'red or yellow hot'. The emission spectrum of a perfect hot 'black body' was derived theoretically in 1901 by the famous physicist Max Plank and is shown in Figure 2.2.
Figure 2.2 Black body spectrum
It can be seen that as the temperature increases, the wavelength of the peak emission decreases. For wavelengths much longer than λ max the power of the emissions fall according to the Rayleigh-Jeans law P= kT/λ2 .
A gas heated to a high temperature can emit more radiation as the particles
move with greater speed. Ultimately, the gas atoms break up into charged
ions and free electrons - a so called plasma. See Figure 2.3. Under these
conditions the gas can radiate a significant amount of electromagnetic
energy across a broad spectrum. In radio astronomy we do detect thermal
emissions from cool solid objects such as the moon, but more often from
regions of ionised gas such as in nebulae or around stars.
Figure 2.3 Emission from an accelerating electron
In this figure the electron is deflected (accelerated|) by the electric field of the ion and this causes it to emit radiation. The radiation may however be absorbed by other electrons and an energy balance between radiation and particles is achieved. With trillions of such encounters in a body of gas, all with random accelerations, a broad thermal noise spectrum is produced.
The radio emission spectrum of a dense excited gas (where self absorption occurs) can be described by the Rayleigh-Jeans law, but if the gas is tenuous - i.e. it is semi-transparent to the radio emissions - the equation must be modified to include a constant ε which depends on the self absorption of the gas. Thus we have:
For a dense opaque gas ε = 1 but in a semi-transparent gas (which is often observed in space) ε is proportional to λ2 , so the wavelength dependency in equation 2.1 falls out and the radiated power is constant.
This is shown in Figure 2.4 where the semi-transparent region is shown from x to y and the opaque region from y to z.
We know that the emission mechanism is thermal if this sort of spectrum
is measured. It is also possible to determine something about the density
of the gas cloud from its emission spectrum.
Figure 2.4 Thermal radio emission spectrum from a gas
Non-thermal radio emission
There are mechanisms that produce strong radio emissions that are not due to random thermal motions of electrons. The processes are more ‘organised’ and the agent is often a magnetic field. The emission spectrum produced has the opposite frequency dependence to thermal emissions. The radiated power increases with wavelength as shown in Figure 2.5.
Figure 2.5 Typical spectrum of a non thermal source
Fast moving charged particles from very hot plasma or cosmic rays will interact with a magnetic field that may be present in the medium in a certain way. They will rotate around the field lines as shown in Figure 2.6 and as their direction is constantly changing, they are being accelerated – and thus they radiate electromagnetic energy.
Figure 2.6 Charged particle in a magnetic field
When a charged particle moving with a velocity v encounters a magnetic
Figure 2.7 Relativistic electron – Synchrotron process
The inclusion of relativistic effects results in the radiation being beamed in a narrow cone away from the electron as it rotates about the field lines. This magnifies the power density of the radiation ‘beamed’ toward the observer.
Figure 2.8 Synchrotron and thermal spectra
The spectra of many astronomical sources are non-thermal as can be seen in Figure 2.9.
Figure 2.9 Non-thermal spectra of sources
Spectral line emissions
These narrow bandwidth emissions are created when the quantum state of
E = quantum energy
Consider the case shown in Figure 2.10 where a neutral hydrogen atom is shown in two quantum states. On the left the particle ‘spins’ are aligned – on the right they are opposed. There is a difference in energy between these two states and this manifests itself by the emission of radiation with a specific frequency vT (the transition frequency).
This is the frequency emitted by the ‘spin transition’ in the ‘lowest energy state of the hydrogen atom - known as the ‘ground state’ - and occurs at 1420.4MHz , or ~ 21cm wavelength.
Figure 2.10 Spin transition in Hydrogen ground state
Many radio line emissions can be generated by atoms and
molecules. The most common is Hydrogen, due to its abundance in the Universe,
but OH, CO, H2O and many other have been detected and mapped by modern
professional radio telescopes. The frequencies of some of these emissions
are given in Table 2.1.
We have seen the key mechanisms by which radio sources emit signals. Each mechanism has its own spectral characteristics and will be found in different radio objects in space. By measuring the total emissions from a source and picking out the various spectral components much can learned about the physics of the region being studied. We move now to a description of some individual radio sources and discuss how easy or difficult they are for detection and measurement by amateur radio astronomers.
© Dr David Morgan 2011