Alicja Wierzcholska

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.

The detection of gravitational waves (GWs) from a binary neutron star (BNS) merger by Advanced LIGO and Advanced Virgo (GW170817), together with its electromagnetic counterpart, the short gamma-ray burst GRB 170817A, heralded the birth of multimessenger astronomy. The detection of TeV emission from GRBs motivates follow-up observations with the Cherenkov Telescope Array Observatory (CTAO), which is ideal for detecting such signals due to its unprecedented sensitivity, rapid response, and wide-field survey capabilities. The aim of this work is to evaluate GeV–TeV GW follow-up strategies for CTAO using a multistep simulation pipeline and to estimate the expected rate of joint GW–GRB detections during observing run O5. Using a simulated sample of BNS systems with corresponding GW detections, gamma-ray emission is simulated through phenomenological prescriptions based on the observed population of short GRBs, including off-axis jet scenarios. CTAO observations are simulated to account for instrument response, sky tiling strategies, integration times, and varying observing conditions. Strategies with variable and constant integration times are investigated. We find that, via an optimized follow-up strategy, about 5% of simulated GW-associated short GRBs produce GeV–TeV radiation detectable by CTAO. Detectability is strongly influenced by the jet opening angle and viewing angle, suggesting that even rough estimates of the viewing angle in GW alerts could enhance targeting. This framework motivates future follow-ups of GW-detectable events, including neutron star–black hole mergers, and further supports the development of advanced strategies incorporating galaxy distributions and synergies with future detectors such as the Einstein Telescope.

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.

Context: BL Lacertae is a blazar known for its high flux variability and occasional broadband flares, the origins of which remain unknown. BL Lacertae was found to be in an extended flaring state in July 2020 which continued until the end of 2021. Aims: The long-term flaring activity makes it an ideal candidate to study its spectral and temporal properties during different flux states. This study explores the X-ray temporal and spectral variability of BL Lacertae. Methods: We analysed five observations of BL Lacertae with the XMM-Newton EPIC instrument taken up to the end of 2021. Temporal properties were investigated using the fractional variability method, minimum variability timescale, and the discrete correlation function. Detailed spectral modeling was performed on the two most variable observations, including an investigation of correlations between the soft (0.3 - 2.0 keV) and hard (2.0 - 10.0 keV) energy bands. Results: Out of five observations, two observations were found to be highly variable with F _var =19.16±0.32 and 6.27±0.43. The observation taken in 2021 corresponds to the highest flux state. The shortest variability timescale in the 0.3-10 keV band is estimated as 1.24 ks. Assuming the X-ray emission is dominated by the synchrotron process, this variability timescale constrains the size of the emission region. Under the assumption of equipartition between the magnetic field and radiating particles, this implies a magnetic field strength of B ≈ 0.4G. The spectral analysis reveals a softer-when-brighter trend, which is commonly seen in blazars. We modeled the X-ray spectra with single power-law, log-parabola, and broken power-law models. In most cases, a broken power-law provided the best fit based on corrected Akaike Information Criterion (AICc) statistics, and a strong correlation was observed between the break energy and the source flux. When a thermal blackbody component was added to the model, its temperature also showed a positive correlation with flux in some observations. Conclusions: Our work indicates the complex spectral evolution of BL Lacertae during this flare. The spectral break, interpreted as the cooling break within the synchrotron component, shifts to higher energies with increasing flux. The source consistently displayed softer-when-brighter behavior. In only one observation were the soft and hard bands found to be significantly correlated. The data suggest a scenario where the peak of the synchrotron emission moves into or across the X-ray band as the source brightens.

Context. Recent observational efforts using imaging atmospheric Cherenkov telescopes (IACTs) have led to firm detections of very-high-energy (VHE) signals from bright gamma-ray bursts (GRBs), often at moderate redshifts. Aims. This work presents 15 years of H.E.S.S. GRB observations and examines their implications through population comparisons and selected modelling cases. Methods. GRBs are a key science target of the High Energy Stereoscopic System (H.E.S.S.). With a low-energy threshold (≲100 GeV) and rapid repointing capabilities, H.E.S.S. can begin follow-up observations within tens of seconds after a GRB trigger, covering the late prompt or early afterglow phases. Results. We report GRB follow-up observations with H.E.S.S. from 2004 to 2019, which resulted in no significant VHE signals (aside from the detections of GRB 180720B and GRB 190829A). The resulting upper limits comprise the largest set available for GRBs at VHE. Conclusions. A subset of bursts with favourable conditions were selected for X-ray analysis and emission modelling. Population studies were performed to compare detected and non-detected GRBs. The results indicate that VHE-detected GRBs are not a distinct population, but tend to feature luminous X-ray emission and favourable redshift and observing conditions. This highlights the potential of next-generation IACTs such as the Cherenkov Telescope Array Observatory (CTAO), whose lower energy threshold will enhance the detection of fainter and more distant GRBs.