High-Energy Emission Processes in Active Galactic Nuclei
Summary
Active Galactic Nuclei (AGN) are powered by accretion onto supermassive black holes and can shine across the electromagnetic spectrum, with a substantial fraction of their luminosity emerging at X-ray and γ-ray energies. Relativistic jets launched from the central engine, organised and collimated by strong magnetic fields, accelerate particles to ultrarelativistic speeds. These charged particles radiate via synchrotron emission in the presence of magnetic fields, forming the low-energy hump of the spectral energy distribution (SED), and subsequently up-scatter low-energy photons to high energies via inverse Compton processes. In blazars, where the jet axis is closely aligned with our line of sight, emission is Doppler boosted, resulting in rapid variability, high polarisation and complex spectral features. Competing radiative mechanisms—synchrotron self-Compton, where electrons scatter their own synchrotron photons, and external Compton, which invokes photons from the accretion disc or broad-line region—yield distinct spectral shapes and polarisation signatures. Recent advances in polarimetric techniques and time-domain γ-ray monitoring have unveiled stratified emission zones, shock acceleration sites and magnetic reconnection events, deepening our understanding of jet composition, energy dissipation and the role of AGN as cosmic accelerators.
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High-Energy Emission Processes in Active Galactic Nuclei publication trend
The graph below shows the total number of articles in high-energy emission processes in active galactic nuclei across all publications each year (not limited to Nature Index journals).
Technical terms
Active Galactic Nucleus (AGN): A compact region at the centre of a galaxy where accretion onto a supermassive black hole releases vast amounts of energy across the electromagnetic spectrum.
Relativistic jet: A narrow, collimated outflow of plasma accelerated to speeds close to that of light, emanating from the vicinity of a black hole and transporting energy to large distances.
Synchrotron radiation: Emission produced by charged particles spiralling around magnetic field lines at relativistic velocities, typically forming the low-energy component of AGN spectra.
Inverse Compton scattering: A process by which relativistic electrons transfer energy to lower-energy photons, boosting them to X-ray or γ-ray energies.
Polarimetry: The measurement of the orientation and degree of polarisation of electromagnetic waves, used to infer magnetic field geometry and emission mechanisms in astrophysical sources.
References
- X-Ray Polarization of BL Lacertae in Outburst. The Astrophysical Journal Letters (2023).
- Magnetic field properties inside the jet of Mrk 421. Astronomy & Astrophysics (2024).
- X-Ray Polarization of the BL Lacertae Type Blazar 1ES 0229+200. The Astrophysical Journal (2023).
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