Gamma-Ray Astrophysics and Relativistic Cosmic Outflows
Final Report Abstract
During this project we developed a generalised steady gap model to explore the non-thermal processes in the vicinity of supermassive black holes. By incorporating realistic ambient soft photon fields we were able to show that the absorption of gamma-rays, produced in black-hole magnetospheric gaps via inverse Compton processes, can drive efficient pair cascades and eventually provide the plasma source that is required for force-free jet formation in AGN. Using this model, we then determined self-consistent solutions for the archetypal radio galaxy M87, demonstrating that its observed gamma-ray variability on black-hole horizon crossing times is compatible with a magnetospheric origin. Time-dependent considerations will be of value for future gap studies. In general, since gap size and gap power are linked to each other, rapid magnetospheric processes are experimentally only testable in nearby sources. Extending our analysis, we showed that the black hole in the centre of our Galaxy (Sgr A*) is a potential gammaray emitter and capable of accelerating cosmic-ray protons to PeV energies. As such it could be the source powering the diffuse gamma-ray emission seen in the Galactic Centre region. Further application to active stages in the past promises insights into the contribution of Sgr A* to the origin of Galactic cosmic rays. AGN typically exhibit variability on a variety of timescales. As we have shown, advanced timing analysis implies that the observed VHE emission in gamma-ray blazars during different states is indeed shaped by different radiative and/or seed injection processes. In general, the preference for a log-normal distribution of fluxes rules out additive processes for the origin of variability and instead favours multiplicative cascade-like models. In addition, the increased interest in year-type, gamma-ray QPOs, as possible signature of a (sub-parsec) binary black hole stage, must be handled with caution, as the reported QPOs are not yet (globally) significant. The next few years are likely to bring further clarification on this issue. On larger spatial scales, evidence is mounting that electron synchrotron emission is the favoured mechanisms for the origin of the X-ray emission in the large-scale jets of nearby AGN. Drawing on the unique statistics available for M87, we were able to show that its radio to X-ray spectra rules out a Compton origin, yet is compatible with a (broken power-law) synchrotron model. A special breakthrough in this context has been achieved with the detection of extended VHE emission along the large-scale jet of Centaurus A, directly proving the presence of ultra-relativistic electrons on kilo-parsec scales. As we have shown, this requires a distributed (in situ) acceleration mechanism to sustain them. Fermi-type particle acceleration in shearing flows currently represents one of the most promising mechanism for distributed acceleration. In particular, shear acceleration can in principle generate power-law like particle distributions with indices depending on the turbulence spectrum. We have developed a simple physical model showing that, since acceleration is tied to the particle transport across the velocity shear, gradual shear acceleration generally requires relativistic flow speeds to be efficient and to counter-balance diffusive particle escape. Extending the model to incorporate second-order Fermi as well as radiative processes, we verified that stochastic-shear particle acceleration can provide a self-consistent framework for continued particle energisation, and account for the production of multi-component particle distributions. In particular, in relativistic AGN jets, efficient electron acceleration up to several PeV (synchrotron-limited), and proton acceleration up to ultra-high energies (several EeV; confinementlimited) is feasible. The results we have obtained here significantly advance the understanding of cosmic particle acceleration and make it now possible to explore the physics of individual sources.
Publications
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Shear acceleration in expanding flows. Astrophysical Journal, Vol. 833. 2016, Number 1, 34.
Rieger F.M. & Duffy P.
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Characterizing the γ-ray long-term variability of PKS 2155-304 with H.E.S.S. and Fermi-LAT. Astronomy and Astrophysics, Vol. 598. 2017, A39.
Abdalla H., …Rieger, F. …(H.E.S.S. Collaboration)
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Particle Acceleration in Mildly Relativistic Shearing Flows: The Interplay of Systematic and Stochastic Effects, and the Origin of the Extended High-energy Emission in AGN Jets. Astrophysical Journal, Vol. 842. 2017, Number 1, 39.
Liu R-Y., Rieger F.M. and Aharonian, F.A.
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Magnetospheric Gamma-Ray Emission in Active Galactic Nuclei.
Astrophysical Journal, Vol. 852. 2018, Number 2, 112.
Katsoulakos G. & Rieger F.M.
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Particle Acceleration in Shearing Flows: Efficiencies and Limits.
Astrophysical Journal Letters, Vol. 886. 2019, Number 2, L26.
Rieger F.M. & Duffy P.
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Evaluating Quasi-Periodic Variations in the gamma-ray Lightcurves of Fermi-LAT Blazars. Astronomy and Astrophysics, Vol. 634. 2020, A120.
Ait Benkhali F., Hofmann W., Rieger F.M., Chakraborty N.
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Gap-type particle acceleration in the magnetospheres of
rotating supermassive black holes. Astrophysical Journal, Vol. 895. 2020, Number 2, 99.
Katsoulakos G. & Rieger F.M.
