During 2021–2022 the Chair of Astronomy and Astrophysics at the University of Würzburg carried out a funded project (grant 50OR2107 from the German Aerospace Center) to investigate hadronic signatures in the X‑ray spectrum of the blazar Mrk 421. The study combined multi‑wavelength observations, theoretical modelling and numerical simulations to disentangle leptonic and hadronic emission processes in relativistic jets.
The core of the work was a multi‑wavelength campaign that spanned almost twenty orders of magnitude in photon energy. Data were collected with the INTEGRAL satellite (SPI, IBIS/ISGRI, JEMX, OMC), ground‑based Cherenkov telescopes (FACT, MAGIC, H.E.S.S.), and radio interferometers (VLBI). The campaign was coordinated with the International collaborations FACT, MAGIC, LST and CTA, and supplemented by archival monitoring from the DFG research group “Relativistic Jets in Active Galaxies”. The INTEGRAL observations targeted a flare of Mrk 421, allowing the team to construct a contemporaneous spectral energy distribution (SED) that could be compared with theoretical models.
On the theoretical side, the team solved kinetic equations for electrons and protons using stochastic differential equations, and coupled the results to relativistic magnetohydrodynamic (MHD) simulations of jet dynamics. The models predict that non‑thermal radiation originates from particle acceleration in magnetic reconnection sites and shock fronts. Electrons produce synchrotron and inverse‑Compton emission, while protons lose energy mainly through pion production, leading to high‑energy γ‑rays and neutrinos. The calculated SEDs show that both leptonic and hadronic scenarios can reproduce the observed X‑ray and γ‑ray fluxes of Mrk 421. However, the two models diverge most strongly in the MeV band, where the leptonic inverse‑Compton component and the hadronic pion‑decay spectrum differ by more than an order of magnitude. The current INTEGRAL SPI sensitivity is insufficient to resolve this difference, and the project concluded that future missions with higher MeV sensitivity, such as the proposed COSI SMEX, would be required to discriminate between the two scenarios. The study also highlighted that neutrino production in hadronic jets would be detectable only with next‑generation detectors like IceCube‑Gen2 or KM3NeT, as present instruments lack the necessary sensitivity.
The project’s collaborative framework involved both internal and external partners. Internally, the team comprised Bernd Schleicher, Dr. Daniela Dorner, Laura Eisenberger, Dr. Christoph Wendel, Patrick Günther, Sarah Wagner, Felix Pfeil, Marcel Vorbrugg, Marc Berger, Dr. Thomas Siegert, Prof. Dr. Karl Mannheim, and Secretary Peer Meißner. External collaborators included David Paneque (MAGIC), Michael Zacharias (H.E.S.S.), Albert Domingo Garau (OMC), Jérôme Chenevez (JEMX), Elisabeth Jourdain (SPI), Guillaume Belanger (IBIS/ISGRI), and Ignacio de la Call. The project was led by Prof. Dr. Karl Mannheim and was part of the DLR “Extraterrestrial” consortium, with funding from the German Aerospace Center. The research period ran from 1 July 2021 to 31 December 2022, during which the team produced a comprehensive analysis of the non‑thermal emission processes in Mrk 421 and identified key observational requirements for future high‑energy astrophysics missions.