Frank Rieger

Context . Very high-energy (VHE) γ -ray emission detected from the central molecular zone (CMZ) hints at diffusive propagation of cosmic rays (CRs) injected continuously near the Galactic center (GC). Aims . Using H.E.S.S. VHE γ -ray observations, we aim to construct a multicomponent description of the region in order to derive the spatial and spectral distributions of CRs and discuss their potential origin. Methods . We rely on a spectro-morphological analysis, in which spectral and spatial parameters of different components are fit simultaneously, and on a three-dimensional description of the CMZ and CR distributions. Results . The GC ridge emission is well reproduced with the assumed gas distribution model weighted by a 1 /r ^α CR density profile with α = 1.10 ± 0.05 _stat ± 0.10 _syst . We detect no significant spectral variations across the CMZ, which supports a continuous injection from the GC. We report a significant (>3 σ ) curvature in the GC ridge γ -ray spectrum, implying a parent proton break (or cutoff energy) of E _b ∼ 15–50 TeV ( E _cut ∼ 60–160 TeV). We also derive the best-fit spectral parameters of the other sources in the region and report a significant cutoff at E ∼ 5 TeV in the spectrum of the pulsar wind nebula G0.9+0.1. Finally we constrain the position of the injection site to be close to the GC (ruling out the Arches and Quintuplet clusters as major contributors to the emission). We discuss possible origins for the GC ridge emission, potential mechanisms for causing the observed curvature, and a possible contribution of the gas content to the emission of HESS J1745–290, and derive qualitative constraints on the line-of-sight positions of the clouds shaping the CMZ.

The propagation of very-high-energy (VHE; E _γ ≥ 100 GeV) gamma rays from extragalactic sources is affected by interactions with photons of the extragalactic background light (EBL), resulting in pair production that attenuates the intrinsic gamma-ray flux. This interaction renders the Universe increasingly opaque to VHE photons at high energies and redshifts. New physics scenarios involving axion-like particles (ALPs) could modify this expected optical depth. In particular, ALPs with masses m _a ∼ 10 eV can decay into two photons over cosmological timescales, thereby contributing to the diffuse EBL. If such ALPs constitute a significant fraction of the dark matter density, their decay would enhance the EBL intensity and consequently increase the gamma-ray optical depth. In this study, we investigate this scenario using a large sample of gamma-ray spectra observed with the High Energy Stereoscopic System (H.E.S.S.) and the Fermi Large Area Telescope. We model the contribution of decaying ALPs to the EBL and assess their impact on the spectra of blazars across redshifts. By comparing these observations with standard EBL models, we place constraints on the properties of heavy ALPs — specifically their mass and photon coupling — and evaluate their viability as a dark matter candidate capable of modifying the gamma-ray transparency of the Universe. From the combined analysis, and under the assumption that ALPs constitute the entire dark matter density, we derive 95% confidence exclusion limits on the photon-ALP coupling down to m _aγ ∼ 7×10 ^-12 GeV ^-1 for masses m _a ∼ 15 eV. These constraints are competitive with existing astrophysical bounds and provide complementary sensitivity to other techniques, closing a previously unconstrained region of parameter space in the m _a ∼ 2.5–20 eV range.

Stochastic particle acceleration in magnetised turbulent plasmas has emerged as a key mechanism to explain multi-messenger signals from compact astrophysical environments. Self-consistent modelling remains challenging because it requires one to treat several non-linear kinetic processes simultaneously, especially turbulence-driven acceleration and its feedback on the turbulent cascade, as well as the radiative and hadronic losses, including the reprocessing of electromagnetic radiation in radiatively dense environments. The present paper introduces the hybrid numerical code , which is designed for this purpose. This hybrid numerical code couples the state-of-the-art time-dependent lepto-hadronic radiative solver with a stochastic acceleration module that incorporates recent theoretical advances in turbulent acceleration and accounts for the dynamical damping of turbulence by accelerated particles. In the second part of the paper, we use this code to provide self-consistent time-dependent models of proton acceleration in the turbulent black hole corona of NGC 1068. We find that the IceCube neutrino signal is well reproduced for a standard set of physical parameters describing the black hole corona. The same template model accounts for in a satisfactory way for IceCube observations of other active galactic nuclei. Furthermore, our exploration of parameter space allowed us to predict detailed template spectral shapes for the TeV neutrino spectrum, which in turn help clarify how future neutrino observations can constrain the properties of turbulent AGN coronae and the underlying acceleration mechanism. This framework provides a powerful tool to model multi-messenger emission in a broad variety of compact astrophysical environments. Turb-AM3 AM3 Turb-AM3

Context. XMM-Newton has been monitoring Sgr A*, the supermassive black hole located at the dynamical centre of the Milky Way for more than two decades. A total of 91 observations were conducted from 1999 to 2023. Aims. In this paper we focus on a time-binned analysis of 2019 observations with a time coverage of ≈460 ks to investigate the flaring activities of Sgr A*. Methods. We proceeded background-subtracted light curves of the 2019 datasets for various on- and off-extraction regions to analyse the signal-to-noise ratio of the detected flares of Sgr A* and enhance the statistical significance of its variability in the X-ray band. Results. Our results reveal that Sgr A* underwent six very bright flaring events during the 2019 XMM-Newton observations, from March to the end of September. We identified the five brightest flares of Sgr A*, observed over more than two decades (1999–2023), with two of these events occurring in 2019. Remarkably, Sgr A* exhibited its brightest flaring event reported so far on August 31, 2019 with a peak brightness and duration exceeding the previous four brightest flares by more than a factor of 2. This exceptionally bright flare, with a duration of 6.6 ks, featured an asymmetric energy-independent double-peak morphology. Spectral analysis of this double-peaked flare suggests spectral evolution during the event and highlights substructures on timescales as short as 100 s. We discuss the implications of the observed flaring activity for synchrotron and hotspot emission models.

At the center of our Galaxy lies the bright γ -ray point-like source HESS J1745-290, which is compatible in position with Sgr A ^⋆ , although an association between the two remains uncertain. Using data obtained between 2004 and 2019 with the High Energy Stereoscopic System (H.E.S.S.) on the Galactic center region, we studied the variability of HESS J1745-290 over 353 hours of observations collected over 16 years, representing the largest dataset gathered yet on this region at TeV energies. We performed a 3D maximum-likelihood analysis of the central source and the diffuse γ -ray emission in the Galactic center region. This analysis allowed us to extract the spectral and morphological intrinsic behavior of the two components. By performing this analysis on an annual basis, we derived the light curve of HESS J1745-290 and the diffuse emission over the past 16 years. The 3D maximum-likelihood analysis method allowed us to separate the central source from the overlapping diffuse emission, enabling a recalibration of the former by the latter and alleviating some of the systematic effects. We find no long-term or yearly variability. We also provide an estimate of the sensitivity of H.E.S.S. to variation of this specific source over 16 years. We rule out any yearly γ -ray flux variation of this source larger than 30%, as well as any linear flux variation exceeding 30% over this time period.

Shear acceleration is a promising candidate mechanism for particle acceleration in extragalactic relativistic jets. For this study, we explored the application of the shear acceleration model to 17 X-ray-bright jet regions in the large-scale jets of Fanaroff–Riley (FR) type I radio galaxies. We studied the jet properties by fitting the multiwavelength spectral energy distributions (SEDs) in a leptonic framework including synchrotron radiation and inverse-Compton scattering off the cosmic microwave background photons. In order to improve spectral modeling, we analyzed Fermi -LAT data for four sources and reanalyzed archival data of Chandra on three X-ray-bright jet regions. We show that synchrotron radiation from a second, shear-accelerated electron population reaching multi-TeV energies satisfactorily models the X-ray SEDs. We explored three different velocity profiles, including linearly decreasing, power-law, and Gaussian profiles and we find that the inferred jet spine velocities are significantly dependent on the choice of velocity profile. The derived magnetic field strengths range from a few to several tens of μG, and the required power in nonthermal particles is well below the Eddington constraint. For M 87, we find that the summed emission from all jet knots is comparable to H.E.S.S. low-state flux, suggesting that the large-scale jet may play a dominant role for the persistent very high energy emission. A comparison with previous results for FR II jets shows that FR I jets tend to have stronger magnetic fields but lower total electron energy. The larger shear viscosity (slower flows and softer shear electron spectral indices) of FR I jets may imply enhanced entrainment and stronger jet-environment interactions compared to FR II jets. Finally, we discuss the potential contribution of FR I jets to ultra-high-energy cosmic rays.

Dwarf spheroidal galaxies (dSphs) are excellent targets for indirect dark matter (DM) searches using gamma-ray telescopes because they are thought to have high DM content and a low astrophysical background. The sensitivity of these searches is improved by combining the observations of dSphs made by different gamma-ray telescopes. We present the results of a combined search by the most sensitive currently operating gamma-ray telescopes, namely: the satellite-borne Fermi -LAT telescope; the ground-based imaging atmospheric Cherenkov telescope arrays H.E.S.S., MAGIC, and VERITAS; and the HAWC water Cherenkov detector. Individual datasets were analyzed using a common statistical approach. Results were subsequently combined via a global joint likelihood analysis. We obtain constraints on the velocity-weighted cross section 〈 σv 〉 for DM self-annihilation as a function of the DM particle mass. This five-instrument combination allows the derivation of up to 2-3 times more constraining upper limits on 〈 σv 〉 than the individual results over a wide mass range spanning from 5 GeV to 100 TeV. Depending on the DM content modeling, the 95% confidence level observed limits reach 1.5×10 ^-24 cm ³ s ^-1 and 3.2×10 ^-25 cm ³ s ^-1 , respectively, in the τ ⁺ τ ^- annihilation channel for a DM mass of 2 TeV.

HESS J0852-463 is a TeV γ-ray source located in the Galactic plane. The region consists of a supernova remnant (SNR, RX J0852.0-4622) with a shell-like morphology, commonly referred to as Vela Junior, and a pulsar denoted PSR J0855-4644. Pulsars are among the most efficient leptonic accelerators in our Galaxy, making this region particularly interesting to study. We utilise the most recent data taken by the High Energy Stereoscopic System (H.E.S.S.), to investigate any γ-ray emission associated with the pulsar in this region, PSR J0855-4644. We applied a full forward folding method on the H.E.S.S. data. Utilising 3D modelling techniques, we evaluated the TeV γ-ray emission towards the various components of this complex system. The distinct energy-dependent morphology observed in our data motivates further investigation of this source. We resolved the emission in the Vela Junior region into various components, several of which correspond to the SNR itself. In particular, we find a new extended component which is coincident with the position of PSR J0855-4644. The spectrum follows a power-law distribution with a best-fit index of ΓE = 1.81 \pm 0.07stat which differs from the properties of the surrounding γ-ray emission of the Vela Junior SNR. A one-zone leptonic joint fit between the X-rays (from XMM-Newton) and γ-rays (from H.E.S.S.) leads to a lower limit on the magnetic field of 1.6μG and a spectral index of α = 1.88 \pm 0.01, in line with expectations of pulsar wind nebulae (PWNe). In this paper, we report the first detection of the PWN of PSR J0855-4644 at TeV energies with the H.E.S.S. experiment at a significance of 12.2σ. This is attributed to the advanced techniques of the 3D analysis. Based on the pulsar's characteristics, its PWN is consistent with the known TeV PWNe population in the Galaxy.

Aims. Plasma shock waves stand out as one of the most promising sites of efficient particle acceleration in extragalactic jets. In electron-ion plasma shocks, electrons can be heated up to large Lorentz factors, making them an attractive scenario to explain the high minimum electron Lorentz factors regularly needed to describe the emission of BL Lac type objects. Still, the (relativistic) thermal electron component is commonly neglected when modelling the observations, although it holds key information on the shock properties. Methods. Considering a shock acceleration scenario, we modelled the broadband emission of the archetypal high synchrotron peaked blazar Markarian 421; we employed particle distributions that included a thermal (relativistic) Maxwellian component at low energies followed by a non-thermal power law, as motivated by particle-in-cell simulations. The observations, in particular in the optical/UV and MeV-GeV bands, efficiently restricted the non-thermal emission from the Maxwellian electrons, which we used to derive constraints on the basic properties, such as the fraction ϵ _e of the total shock energy stored in the non-thermal electrons. Results. The best-fit model yields a non-thermal electron power law with an index of ∼2.4, close to predictions from shock acceleration. Successful fits are obtained when the ratio between the Lorentz factor at which the non-thermal distribution begins ( γ _nth ) and the dimensionless electron temperature ( θ ) satisfies γ _nth / θ ≲ 8. Since γ _nth / θ controls ϵ _e , the latter limit implies that at least ϵ _e ≈ 10% of the shock energy is transferred to the non-thermal electrons. These results are almost insensitive to the shock velocity γ _sh , but radio observations indicate γ _sh ≳ 5 since for lower shock velocities the fluxes in the millimetre band are overproduced by the Maxwellian electrons. Therefore, if shocks drive the particle energisation, our findings indicate that they operate in the mildly to fully relativistic regime with efficient electron acceleration. This paper lays the ground for future works, in which we will use plasma simulations to investigate if, and under which conditions, the findings presented here can be reproduced.