Particle composition in relativistic jets

Relativistic jets launched by supermassive black holes are the most extreme and energetic persistent phenomena in the Universe.

They are capable of accelerating particles to energies inaccessible by particle accelerators on Earth, and deposit enormous amount of energy in their surroundings during their lifetime. They are believed to have a significant impact on galaxy evolution via feedback mechanisms.

Yet, despite decades of studies, one of the most fundamental properties, the composition of these jets is still unknown.

The jets can consist of either electron-positron pairs, electron-proton plasma where also protons are present and accelerated, or an admixture of the two.

The composition affects the power of the jet and the presence or lack of hadrons is a crucial ingredient in modelling the multi-wavelength and multi-messenger emission of these jets.

If the jets are hadronic, they are likely the origin of the highest energy cosmic rays.Circular polarization is the only direct way to determine the particle composition of jets, and we have designed a project that will overcome limitations of previous programs to finally resolve the particle composition of relativistic jets.

Previously, this has only been attempted on individual objects at low radio frequencies, or using single-frequency data lacking the required discriminating power.

PARTICLES will deliver the first multifrequency-radio high-cadence observations of relativistic jets in total intensity, linear, and circular polarization for a sample of objects.

In parallel, we will conduct very long baseline interferometry observations to connect our light curves with the spatially resolved structures in the jets.

This unprecedented dataset will be combined with radiative transfer simulations, crucial for modelling the origin of circular polarization.

These advances will enable us to answer one of the most central questions related to relativistic jets, what is their particle composition?