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Galactic continuum emissions

7.1 Constituents of continuum emissions

Very broadband continuous radio emissions are referred to as 'continuum' emissions and are composed of two main types:

  • Thermal emissions
  • Cyclotron / synchrotron emissions

The generation mechanisms for both types were discussed in section 2. In this section we look at continuum emissions that can be observed by amateur radio astronomers. Because of the different spectra, emissions from either type will tend to dominate in certain frequency bands. Thermal emission is more easily detected at high frequencies above a few GHz, whereas synchrotron emission increases with decreasing frequency as is more easily detected at low frequencies in the HF to UHF bands. It follows that observers should construct equipment that is capable of operation in either frequency regime depending on what type of continuum emission they seek to measure.

7.2 Synchrotron emissions

We will give an example of thermal emission from nebulae later in this paper but here we will examine how the amateur radio astronomer can plot the distribution of synchrotron emission in the Milky Way. If equipment has been designed to detect the neutral Hydrogen line at 1420MHz it is convenient to use frequencies close to this to observe synchrotron emissions. Because the line emission of Hydrogen is narrow (<1MHz) we can tune the receiver to a frequency close by and detect only the broad band synchrotron signal. In the example that follows, the chosen frequency was 1453MHz - which was far enough away from the Hydrogen line, but still within the bandwidth of the antenna and receiving equipment.

Figure 7.1 3m dish with 1400-1500MHz feed

By setting the antenna at a series of declinations over a period of several days, a succession of transit scans through the galaxy can be made resulting in a set of plots as shown in Figure 7.2.

Figure 7.2 Transit scan data (synchrotron emissions)

The data can be assembled as a 'false colour' map using suitable software(9) as shown in Figure 7.3. In this diagram we see the radio map at 1453.5MHz converted to galactic coordinates(12) and superimposed on an optical picture of the Milky Way. There are several interesting things to note about this map:

  • The low signal level of the emission at galactic longitude of 60° corresponds to the gap seen between spiral arms in Figure 6.2. The antenna is pointed in a direction where there is low concentration of matter and plasma and the integrated signal along the line of sight is low. This also shows up on the optical image where the region is largely dark.
  • Closer to the centre of the galaxy at a longitude of ~35° there is an excursion on the radio map known as the North Galactic Spur, thought to be part of the giant arc of an ancient supernova remnant. There is no optical counterpart to this feature.
  • The radio and optical emissions peak again at a galactic longitude of 80° where we are looking along the Cygnus spiral arm and the integrated radio and optical emissions are bright.

As has been shown, detecting the galactic synchrotron emission is a credible project for the amateur radio astronomer

Figure 7.3 Map of galactic radio synchrotron emission

© Dr David Morgan 2011