The observation of the Universe in the radio domain started in the 1930s, after the unexpected detection of radio waves from the Galactic Centre by Karl Jansky in 1931. Following that discovery, the first dedicated sky observations for astrophysical purposes revealed a radio emission dominated by non-thermal sources, i.e. astronomical objects emitting radiation from relativistic electrons (accelerated up to velocities very close to the speed of light). This early discovery showed that a lot of astronomical environments are able to accelerate particles up to relativistic velocities, opening up the field of high energy and cosmic-ray astrophysics.
Since then, many highly efficient radio observatories have been built throughout the world, and operate almost 24 hours a day. Several world-class radio observatories operate in interferometric mode, allowing one to combine the signal collected by tens of antennas.
One antenna of the Giant Metrewave Radio Telescope, India.
The investigation of non-thermal sources is based on the measurement of synchrotron radio emission. This radiation is produced by relativistic electrons in helical motion about magnetic field lines. This emission is thus used as a proxy for particle acceleration in astronomical sources. Galactic synchrotron radio emitters are typically supernova remnants, massive binaries at various stages of their evolution, or some young stellar objects. At the extragalactic scale, these emitters are mainly active galactic nuclei and galaxy clusters. Radio measurements constitute therefore a powerful tool to identify non-thermal sources, and thus particle accelerators in general.
Beside synchrotron emission, the radio study of astronomical objects allows one to investigate their potential thermal emission. This emission is produced by (non relativistic) free electrons undergoing an acceleration in the presence of protons or other nuclei in a plasma. This notably happens in thermal jets of young stellar objects or in stellar winds of massive stars. These objects can thus be composite sources, presenting both thermal and non-thermal radio emission. Radio investigations can thus provide a wealth of information about these objects.
Schematic view of a massive binary presenting both thermal and non-thermal radio emission.