Radio emission from galactic sources
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, several 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, therefore substantially improving the angular resolution as compared to single dish antennas operated at the same wavelength.
One of the antennas of the Giant Metrewave Radio Telescope, India.
The radio emission from galactic sources can be either thermal or non-thermal. 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 binary stellar systems at various stages of their evolution, or some young stellar objects. 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, telling us about various aspects of their physics.
Schematic view of a massive binary system presenting both thermal and non-thermal radio emission.
Typical observation strategies involve measurements at several frequencies to determine spectral indices that constitute precious diagnostic tools to infer the nature of the radio source (thermal vs non-thermal). For sources expected to be variable such as Particle-Accelerating Colliding-Wind Binaries, measurements at several epochs are justified for mainly two reasons. First, the synchrotron emission may be revealed only in a fraction of the orbit, because of the strong free-free absorption due to the stellar wind material that is surround the colliding-wind region. Second, the variation of the physical conditions in the particle acceleration and synchrotron emission region requires to monitor the orbit in order to better characterize the underlying physics. Data collected for this purpose by members of this group are mainly obtained using interferometers such as the GMRT (India), the JVLA (USA) and ATCA (Australia), along with very long baseline interferometers such as the European VLBI Network or the Long Baseline Array. In the latter case, the enhanced angular resolution allows us to produce images of the distribution of the radio emission from specific sources, therefore telling us about its spatial distribution.
Radio image of the PACWB Apep obtained with the LBA. The synchrotron emission region appears as an elongated source coincident with the wind-wind interaction region between the two WR stars. Figure taken from Marcote et al. 2021, MNRAS, 501, 2478.
.
