Monochromatic gamma-ray signals constitute a potential smoking gun signature for annihilating or decaying dark matter particles that could relatively easily be distinguished from astrophysical or instrumental backgrounds. We provide an updated assessment of the sensitivity of the Cherenkov Telescope Array (CTA) to such signals, based on observations of the Galactic centre region as well as of selected dwarf spheroidal galaxies. We find that current limits and detection prospects for dark matter masses above 300 GeV will be significantly improved, by up to an order of magnitude in the multi-TeV range. This demonstrates that CTA will set a new standard for gamma-ray astronomy also in this respect, as the world's largest and most sensitive high-energy gamma-ray observatory, in particular due to its exquisite energy resolution at TeV energies and the adopted observational strategy focussing on regions with large dark matter densities. Throughout our analysis, we use up-to-date instrument response functions, and we thoroughly model the effect of instrumental systematic uncertainties in our statistical treatment. We further present results for other potential signatures with sharp spectral features, e.g. box-shaped spectra, that would likewise very clearly point to a particle dark matter origin.

Context. Microquasars have emerged as promising candidates to explain the cosmic-ray flux at petaelectronvolt energies. LHAASO observations revealed V4641 Sgr as the most extreme example so far. Its gamma-ray spectrum extends up to 800 TeV, which requires particles with multi-PeV energy. The TeV emission is highly extended, which challenges expectations given the reported low-inclination angle of the V4641 Sgr jets. Aims. We spatially and spectrally resolved the gamma-ray emission from V4641 Sgr and investigated the particle acceleration in the system. Methods. Using ≈100 h of H.E.S.S. data, we performed a spectro-morphological study of the gamma-ray emission around V4641 Sgr. We employed HI and dedicated CO observations of the region to infer the target material for cosmic-ray interactions. Results. We detected multi-TeV emission around V4641 Sgr with a high significance. The emission region is elongated, and its major and minor axes are 0.34° ±0.01 _syst ^{±0.04 _stat} and 0.06° ±0.01 _syst ^{±0.01 _stat} , respectively. We found a power-law spectrum with an index ≈1.8, and together with results from other gamma-ray instruments, this reveals a spectral energy distribution (SED) that peaks at energies of ≈100 TeV for the first time. We found indications (3 σ ) of a two-component morphology, with indistinguishable spectral properties. The position of V4641 Sgr is inconsistent with the best-fit position of the single-component model and with the dip between the two components. We found no significant evidence of an energy-dependent morphology. No dense gas was found at any distance towards V4641 Sgr, which places an upper limit of n _gas ≲ 0.2 cm ⁻³ within the gamma-ray emission region. Conclusions. The peak of the SED at ≈100 TeV identifies V4641 Sgr as a candidate cosmic-ray accelerator beyond the so-called knee. The absence of dense target gas places stringent energetic constraints on hadronic interpretations, however. The H.E.S.S. measurement requires an unusually hard (≈1.5) spectral index for the protons. A leptonic scenario faces fewer obstacles if the particle transport is fast enough to avoid losses and to reproduce the observed energy-independent morphology. The absence of bright X-ray emission across the gamma-ray emission region requires a magnetic field strength ≲3 μG, however. Our findings favour a leptonic origin of the gamma-ray emission. This conclusion does not exclude hadron acceleration in the V4641 Sgr system.

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.

FlashCam is a high-performance, fully digital camera system developed for use in imaging atmospheric Cherenkov telescopes, particularly for the Medium-Sized Telescopes of the upcoming Cherenkov Telescope Array Observatory (CTAO). The camera features a fully digital trigger and readout architecture, enabling high event rates and precise control over the trigger system. A novel front-end electronics design provides a wide dynamic range of over 3,000 photoelectrons per pixel using a single readout channel, simplifying hardware without compromising performance. This contribution presents an overview of the development and performance of FlashCam, focusing on time reconstruction, charge reconstruction and temperature stability.

FlashCam is an advanced camera developed for Imaging Atmospheric Cherenkov Telescopes (IACTs), aiming to significantly enhance ground-based astronomical observations. FlashCam has delivered exceptional performance within the High Energy Stereoscopic System (H.E.S.S.) for over 5 years. Its design has been thoroughly studied using laboratory data as well as accurate simulations, all of which show good agreement between simulated and observed results. Minor adjustments are being made to the camera design in preparation for its integration with the Cherenkov Telescope Array Observatory (CTAO). Calibration is a critical aspect of any detector, and muons provide an effective means to calibrate the telescope’s full optical throughput. For FlashCam, a specialized muon detection method achieves over 95% efciency in identifying muons, with a low background acceptance.