The Radio Sky

The sky looks different at different wavebands. We find some characteristics and features of the source at one waveband and in addition to that if we observe the same source at a different waveband we may find some other characteristics and features of the sources. By doing the multi-wavelength observations of the source we can have a full picture of the source.

In the optical band, the dominant emitters are stars, luminous clouds of gas, and galaxies. All of these sources are thermal sources with temperatures in the range $10^{3}$-$10^{4}$ K. At these temperatures the emitted spectrum peaks in the optical band. Sources with temperatures, which are outside this range and emitters of non-thermal radiation are relatively weak emitters of radiation in the optical band. Nevertheless, such sources can be strong emitters in other wave bands.

Since the universe contains all the thermal and non-thermal objects, one needs to make multi-band studies in order to fully understand the nature of the sources. For a thermal source with temperature $\sim$ 100 K, the flux density in the radio band can be well approximated by the Rayleigh-Jeans law,

$$\rm {S = (2kT/\lambda^2) d\Omega}.$$ (2..1)

Where $S$ is the the flux density, $T$ is the temperature of the source and $\lambda$ is the wavelength of observation.

By this law the predicted flux densities at radio wavelengths are miniscule and then one might hence imagine that the radio sky should be dark and empty. However, radio observations reveal a variety of radio sources all of which have flux densities much greater than that given by the Rayleigh-Jeans Law. Or in other words, the radio emission that they emit is not thermal in nature. Today it is well known that the bulk of the radio emission is produced via the synchrotron emission mechanism. Energetic electrons spiraling in strong magnetic fields emit synchrotron radiation. Unlike thermal emission where the flux density increases with frequency, for synchrotron emitters, the flux density decreases with frequency. Synchrotron emitting sources are hence, best studied at low radio frequencies. (Ref : Low Frequency Radio Astronomy, Course Notes from a School, NCRA, 1999. eds by Jayaram N. Chengalur, Yashawant Gupta, K.S. Dwarakanath, May 2003.)