PlasmaAstro-TOK-Seminar 2025

Abstract: How magnetized turbulent plasmas can accelerate charged particles to high energies forms a long-standing question with far-reaching implications for multi-messenger astrophysics. It indeed goes back to the seminal works of Enrico Fermi in the late 1940s and nowadays, it is commonly invoked to model the generation of non-thermal particle spectra in a broad variety of astrophysical sites, including extreme, relativistic sources such as black hole accretion disks, pulsar wind nebulae, or relativistic jets from active galactic nuclei. In a first part, this seminar will summarize recent progress in this area and propose a modern theoretical picture of the physics at play, which can be regarded as a generalization of the original Fermi scenario. In a second part, these results will be placed in the context of magnetized turbulent coronae of active galactic nuclei to understand if and how this process can explain the origin of high-energy neutrinos recently detected by the Ice Cube experiment in the direction of Seyfert galaxies. [mehr]

Abstract: Blazars are promising candidates for astrophysical neutrino sources. Multi-messenger lepto-hadronic models based on proton--photon (p-gamma) interactions predict spectra that peak at high energies, whereas statistical searches often assume a power-law shape, emphasising lower energies. We investigate how these spectral assumptions impact neutrino--blazar associations by incorporating physically motivated spectra into our Bayesian point-source framework. Using predictions from Rodrigues et al. (2024), we analyse 10 years of IceCube data and identify five candidate sources. Our results show that $p\gamma$ spectra suppress low-energy associations but may enhance high-energy ones. Strong associations then imply that energetic neutrino events likely have much higher true energies than those inferred under a power-law assumption. This is particularly relevant in light of the recent KM3NeT detection of the highest-energy neutrino, reinforcing the need for theory-driven models to interpret multi-messenger signals. [mehr]

Abstract: The origin and propagation of cosmic rays (CRs) is a multi-disciplinary problem in which plasma (astro)physics plays a major role. Plasma processes are indeed determining how these charged particles are injected and accelerated at their astrophysical sources, as well as the type of turbulence that CRs encounter as they propagate through different Galactic environments; cosmic-ray propagation itself is defined by plasma processes underlying their non-linear interaction with turbulence. More than a century after their discovery, our understanding of the processes yielding cosmic rays and defining their journey through the Galaxy cannot be considered satisfactory.In this talk, I will discuss how the most-recent direct and indirect CR measurements challenge the naive picture of cosmic-ray transport that is usually adopted by the astroparticle community, delving in what are the issues with a standard description of CR scattering based on Alfvénic fluctuations. I will also present recent results based on different approaches that may solve some of the typical issues, but that simultaneously lead to new open questions. [mehr]

Abstract: Cosmic rays (CRs) with energies below the 'knee' at about 3 PeV are believed to be of Galactic origin, but it is still not fully understood in which astrophysical objects they are produced. The classical paradigm states that Supernova remnants (SNRs) are the main sources of Galactic CRs mainly on the account of their energy budget that is sufficient to explain the energy density of low-energy CRs. But whether SNRs are responsible for all the Galactic CRs or not remains an open question as many pieces of the puzzle are still missing. To answer this question it is necessary to understand whether SNRs can accelerate particles beyond PeV energies and whether the total yield of CRs from SNRs is compatible with observations in terms of the spectral shaper and elemental composition. In this talk I will critically assess the capabilities of SNRs as CR accelerators by the means of multiwavelength observations and numerical simulations. [mehr]

Abstract: The detection of a flux of TeV neutrinos from the nearby Seyfert galaxy NGC1068 is particularly intriguing. The neutrino flux measured by the IceCube neutrino observatory is at least two orders of magnitude higher than its gamma-ray counterpart, in contrast with the hypothesis of correlation between high-energy neutrinos and gamma-rays in AGN. This finding motivated further searches for neutrino emission in sources similar to NGC 1068. This source is among the brightest X-ray Seyfert galaxies, supporting models that suggest that high-energy neutrinos are produced in the vicinity of the supermassive black hole, most likely in the AGN corona. I will present the most recent IceCube results on searches for neutrino sources, including both a follow-up of already published results and findings on a newly compiled list of X-ray bright non-blazars AGN. In particular, we confirm the measurement of a neutrino flux from NGC1068, and we report the evidence of neutrino emission from a subset of 11 X-ray bright AGN at a 3.3σ level. [mehr]

Abstract: Shocks are promising sites of particle acceleration in extragalactic jets. In electron-ion shocks, electrons can be heated up to large Lorentz factors, making them an attractive explanation for the high minimum electron Lorentz factors regularly needed to model blazar SEDs. Still, the thermal electron component is commonly neglected when modelling observations. In this talk, we present a multi-wavelength modelling of the blazar Mrk421 employing a particle distribution whose shape is directly motivated by predictions of plasma PIC simulations. We demonstrate that fluxes in the optical/UV and MeV-GeV bands efficiently restrict the emission from the thermal (relativistic) Maxwellian electrons, allowing us to obtain constraints on the shock properties. Notably, we find that at least ~10% of the shock energy must be transferred to the nonthermal electrons in order to stay compatible with the fluxes in the optical/UV and MeV-GeV bands. These results are almost insensitive to the shock velocity, although radio observations suggest a shock Lorentz factor above ~5. In the second part of the talk, we present preliminary results of dedicated PIC simulations to verify under which physical conditions of electron-ion shocks these findings can be matched. [mehr]