Christopher Eckner

The Galactic Center is potentially hosting the largest indirect signal from particle dark matter (DM), which in many well-motivated models would produce gamma rays as their final states. However, this region has often been dismissed for DM studies because of the evidence for an unexpected gamma-ray component over astrophysical backgrounds at GeV energies, firstly discovered in the data of the Large Area Telescope (LAT), the so-called Galactic Center excess (GCE). While this was initially considered to hint at GeV thermal relics, recent work supports a GCE interpretation in terms of a stellar population of millisecond pulsarlike sources in the Galactic bulge. Building on this preference, we reevaluate the Galactic Center as a powerful target for indirect DM searches via gamma rays. This is achieved by combining adaptive template fitting and photon-count statistical methods to assess the role of subthreshold point sources in the observed -LAT gamma-ray counts while minimizing the mismodeling of Galactic diffuse-emission backgrounds. In a fully self-consistent way, the gamma-ray data are fitted with a mixed model comprising a DM signal and a stellar bulge, both potentially contributing to the GCE. The space left for signals from weak-scale DM particle annihilations is quantified by extracting 95% confidence level upper limits on the annihilation cross section, which, depending on the DM density profile, result in stringent limits for masses ≲ 300 GeV . The robustness of our results is supported by tests on simulated data.

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.

Numerous observations confirm the existence of dark matter (DM) at astrophysical and cosmological scales, yet the fundamental nature of this elusive component of our Universe remains unknown. Theory and simulations of Galaxy formation predict that DM should cluster on small scales in bound structures called subhalos or DM clumps. While the most massive DM subhalos host baryonic matter and are observed as dwarf galaxies of the Milky Way (MW), less massive, unpopulated subhalos could be abundant in the Galaxy as well and yield high-energy gamma rays as final products of DM annihilation. Recently, it has been highlighted that the brightest halos should also have a sizeable extension in the sky. In this study, we examine the prospects offered by the Cherenkov Telescope Array Observatory (CTAO), a next-generation gamma-ray instrument, for detecting and characterizing such objects. Previous studies have primarily focused on high-latitude observations; here, we assess the potential impact of the CTAO’s Galactic Plane Survey, which will provide unprecedentedly deep survey data for the inner five degrees of the Galactic plane. Our modeling accounts for tidal effects on the subhalo population, examining the conditions under which DM subhalos can be detected and distinguished from conventional astrophysical sources. We find that regions a few degrees above or below the Galactic plane offer the highest likelihood for DM subhalo detection. For an individual subhalo—the brightest from among various realizations of the MW subhalo population—we find that detection at the 5 σ level is achievable for an annihilation cross section of ⟨ σ v ⟩ ∼ 3 × 10 − 25 cm 3 / s for TeV-scale DM annihilating into b b ¯ . For a full population study, depending on the distribution and luminosity model of Galactic subhalos, still unconstrained cross sections in the range ⟨ σ v ⟩ ∼ 10 − 23 – 10 − 22 cm 3 / s for TeV DM candidates are necessary for the brightest subhalos to be detected.

We present a numerical linear programming bootstrap to construct dual model scattering amplitudes. Dual models describe tree-level exchanges of higher spin resonances in theories like string theory and large N gauge theories. Despite being very simple objects, their numerical bootstrap has proven challenging due to slow convergence of the infinite sums over resonances. Our bootstrap succeeds thanks to an efficient parametrization of the amplitude in terms of Mandelstam-Regge poles and the use of combined regions that make crossing symmetry constraining. Along the way, we discover and conjecture a property of “superunitarity” of the Veneziano amplitude, which we use to keep a linear problem. As results, we present first the study of a class of stringlike amplitudes with linear trajectories, for which we observe that the Veneziano amplitude lies at a preferred location, at the bottom of a pit, which minimizes crossing. Then, we introduce a toy-model deformation to nonlinear trajectories, mimicking some features of QCD, for which our algorithm also detects a clear pit. This gives compelling evidence that our bootstrap is able to produce amplitudes that can exhibit nontrivial phenomenological features.

These lecture notes provide an overview of high-energy astrophysical processes involving axions, axion-like particles (ALPs), and other weakly interacting slim particles (WISPs) focusing on their potential observational signatures in astrophysical environments. After introducing key concepts in high-energy astrophysics, we present the fundamental properties of WISPs, emphasising their phenomenological implications. Particular attention is given to ALP-photon conversion in strong magnetic fields and the possible decay signatures of ALPs in sources such as active galactic nuclei, galaxy clusters, and cosmic-ray accelerators. These effects can lead to distinctive modifications in astrophysical spectra, spatial distributions, and polarisation patterns, providing unique probes of physics beyond the Standard Model. We discuss their role in dark matter scenarios and their potential impact on high-energy observations. The lecture series is supplemented by hands-on tutorials, including exercises on axion electrodynamics and an analysis of gamma-ray data from NGC 1275 to search for ALP-photon conversion signatures.

These lecture notes provide an overview of high-energy astrophysical processes involving axions, axion-like particles (ALPs), and other weakly interacting slim particles (WISPs) focusing on their potential observational signatures in astrophysical environments. After introducing key concepts in high-energy astrophysics, we present the fundamental properties of WISPs, emphasizing their phenomenological implications. Particular attention is given to ALP-photon conversion in strong magnetic fields and the possible decay signatures of ALPs in sources such as active galactic nuclei, galaxy clusters, and cosmic-ray accelerators. These effects can lead to distinctive modifications in astrophysical spectra, spatial distributions, and polarization patterns, providing unique probes of physics beyond the Standard Model. We discuss their role in dark matter scenarios and their potential impact on high-energy observations. The lecture series is supplemented by hands-on tutorials, including exercises on axion electrodynamics and an analysis of gamma-ray data from NGC 1275 to search for ALP-photon conversion signatures.

Regge poles connect the analytic structure of scattering amplitudes, analytically continued in angular momentum, to their high-energy limit in momentum space. Dual models are expected to have only Regge poles as singularities in angular momentum space, and string theory suggests there should be an infinite number of them. In this study, we investigate the number of Regge trajectories these models may have. We prove, based solely on crossing symmetry and unitarity, that meromorphic amplitudes, with or without subtractions, cannot produce a reggeizing amplitude if they contain any finite number of Regge trajectories, and show that this excludes the existence of such amplitudes altogether. Additionally, we develop and apply a linear programming dual bootstrap method to exclude these amplitudes directly in momentum space.