Multi-Messenger Searches in Astrophysics

Multi-messenger astronomy has experienced an explosive development in the past few years. While not being a particularly young field, it has recently attracted a lot of attention by several major discoveries and unprecedented observation campaigns covering the entity of the electromagnetic spectrum as well as observations of cosmic rays, neutrinos, and gravitational waves. The exploration of synergies is in full steam and requires close cooperation between different instruments. Here I give an overview over the subject of multi-messenger astronomy and its virtues compared to classical "single messenger" observations, present the recent break throughs of the field, and discuss some of its organisational and technical challenges.Comment: 10 pages, 3 figure

Gamma-Hadron Separation in Very-High-Energy gamma-ray astronomy using a multivariate analysis method

In recent years, Imaging Atmospheric Cherenkov Telescopes (IACTs) have discovered a rich diversity of very high energy (VHE, > 100 GeV) gamma-ray emitters in the sky. These instruments image Cherenkov light emitted by gamma-ray induced particle cascades in the atmosphere. Background from the much more numerous cosmic-ray cascades is efficiently reduced by considering the shape of the shower images, and the capability to reduce this background is one of the key aspects that determine the sensitivity of a IACT. In this work we apply a tree classification method to data from the High Energy Stereoscopic System (H.E.S.S.). We show the stability of the method and its capabilities to yield an improved background reduction compared to the H.E.S.S. Standard Analysis.Comment: 10 pages, 9 figures, accepted for publication in Astroparticle Physic

Probing Galaxy structure with VHE γ\gamma rays

As an observer from within the Milky Way, it is difficult to determine its global structure. Despite extensive observational data from surveys at different wavelengths, we have no conclusive description of the structure of our own Galaxy. For very-high-energy (VHE) $\gamma$ rays, the most comprehensive catalogue of Galactic sources resulting from the H.E.S.S. Galactic Plane Survey (HGPS) shows a striking asymmetry in the distribution of the sources in the latitudinal direction. This could be the result of a local feature in the spatial distribution of the sources or it could be due to the position of the Sun above the Galactic plane. In this contribution, we estimate the position of the Sun based on the latitudinal flux profile of VHE $\gamma$-ray sources, assuming three mirror-symmetric models for the spatial distribution of the sources in three-dimensional space and taking into account the observational bias of the HGPS. We verify our method using simulations and find values for $z_{\odot}$ between $-6\,\mathrm{pc}$ and $94\,\mathrm{pc}$ depending on the considered model. Our results show that the position of the Sun has a significant impact on the observed source distribution and must therefore be taken into account when modelling the population of Galactic VHE $\gamma$ sources. However, it is not conclusive whether the Sun's offset from the Galactic plane is the only factor leading to the asymmetry in the latitudinal profile.Comment: 6 pages, 3 figures, Proceedings of the 7th Heidelberg International Symposium on High-Energy Gamma-Ray Astronomy ($\gamma$-2022), Barcelona, Spai

Modelling the Galactic very-high-energy γ\gamma-ray source population

The High Energy Stereoscopic System (H.E.S.S.) Galactic plane survey (HGPS) is to date the most comprehensive census of Galactic $\gamma$-ray sources at very high energies (VHE; $100\,\mathrm{GeV}\leq E\leq 100\,\mathrm{TeV}$). As a consequence of the limited sensitivity of this survey, the 78 detected $\gamma$-ray sources comprise only a small and biased subsample of the overall population. Still, numerical modelling allows us to study the VHE $\gamma$-ray source population in the Milky Way based on this sample. In this work, several azimuth-symmetric and spiral-arm models are compared for spatial source distribution. The luminosity and radius function of the population are derived from the source properties of the HGPS data set and are corrected for the sensitivity bias of the HGPS. Based on these models, VHE source populations are simulated and the subsets of sources detectable according to the HGPS are compared with HGPS sources. The power-law indices of luminosity and radius functions are determined to range between $-1.6$ and $-1.9$ for luminosity and $-1.1$ and $-1.6$ for radius. Azimuth-symmetric distributions and a distribution following a four-arm spiral structure without Galactic bar describe the HGPS data reasonably well. The total number of Galactic VHE sources is predicted to be in the range from 800 to 7000 with a total luminosity and flux of $(1.6-6.3) \cdot 10^{36}$~ph~s$^{-1}$ and $(3-15) \cdot 10^{-10}$~ph~cm$^{-2}$~s$^{-1}$, respectively. Depending on the model, the HGPS sample accounts for $(68-87)\%$ of the emission of the population in the scanned region. This suggests that unresolved sources represent a critical component of the diffuse emission measurable in the HGPS. With the foreseen jump in sensitivity of the Cherenkov Telescope Array, the number of detectable sources is predicted to increase by a factor between 5 - 9.Comment: 11 pages, 11 figures, accepted for publication in Astronomy & Astrophysic

The Energy Spectrum of Cosmic-Ray Electrons Measured with H.E.S.S.

The spectrum of cosmic-ray electrons has so far been measured using balloon and satellite based instruments. At TeV energies, however, the sensitivity of such instruments is very limited due to the low flux of electrons at very high energies and small detection areas of balloon/satellite based experiments. The very large collection area of ground-based imaging atmospheric Cherenkov telescopes gives them a substantial advantage over balloon/ satellite based instruments when detecting very-high-energy electrons (> 300 GeV). By analysing data taken by the High Energy Stereoscopic System (H.E.S.S.), this work extends the known electron spectrum up to 4 TeV - a range that is not accessible to direct measurements. However, in contrast to direct measurements, imaging atmospheric Cherenkov telescopes such as H.E.S.S. detect air showers that comic-ray electrons initiate in the atmosphere rather than the primary particle. Thus, the main challenge is to differentiate between air showers initiated by electrons and those initiated by the hadronic background. A new analysis technique was developed that determines the background with the support of the machine-learning algorithm Random Forest. It is shown that this analysis technique can also be applied in other areas such as the analysis of diffuse gamma rays from the Galactic plane

The population of Galactic supernova remnants in the TeV range

SNRs are likely to be significant sources of Galactic cosmic rays up to the knee. They produce gamma rays in the very-high-energy (E>100 GeV) range mainly via two mechanisms: hadronic interactions of accelerated protons with the interstellar medium and leptonic interactions of accelerated electrons with soft photons. Observations with current instruments have lead to the detection of about a dozen SNRs in VHE gamma rays and future instruments will help significantly increase this number. Yet, the details of particle acceleration at SNRs, and of the mechanisms producing VHE gamma-ray at SNRs remain poorly understood: What is the spectrum of accelerated particles? What is the efficiency of particle acceleration? Is the gamma-ray emission dominated by hadronic or leptonic origin? To address these questions, we simulate the population of SNRs in the gamma-ray domain, and confront it to the current population of TeV SNRs. This method allows us to investigate several crucial aspects of particle acceleration at SNRs, such as the level of magnetic field around SNR shocks or scanning the parameter space of the accelerated particles (spectral index, electron to proton ratio and the acceleration efficiency of the shock) with the possibility to constrain some of the parameters.Comment: 8 pages, 5 figures, Proceedings for the 38th International Cosmic Ray Conference (ICRC2023

Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign

Abstract: In 2017, the Event Horizon Telescope (EHT) Collaboration succeeded in capturing the first direct image of the center of the M87 galaxy. The asymmetric ring morphology and size are consistent with theoretical expectations for a weakly accreting supermassive black hole of mass ∼6.5 × 109 M ⊙. The EHTC also partnered with several international facilities in space and on the ground, to arrange an extensive, quasi-simultaneous multi-wavelength campaign. This Letter presents the results and analysis of this campaign, as well as the multi-wavelength data as a legacy data repository. We captured M87 in a historically low state, and the core flux dominates over HST-1 at high energies, making it possible to combine core flux constraints with the more spatially precise very long baseline interferometry data. We present the most complete simultaneous multi-wavelength spectrum of the active nucleus to date, and discuss the complexity and caveats of combining data from different spatial scales into one broadband spectrum. We apply two heuristic, isotropic leptonic single-zone models to provide insight into the basic source properties, but conclude that a structured jet is necessary to explain M87’s spectrum. We can exclude that the simultaneous γ-ray emission is produced via inverse Compton emission in the same region producing the EHT mm-band emission, and further conclude that the γ-rays can only be produced in the inner jets (inward of HST-1) if there are strongly particle-dominated regions. Direct synchrotron emission from accelerated protons and secondaries cannot yet be excluded

Mixing of materials in magnetised core-collapse supernova remnants

Core-collapse supernova remnants are structures of the interstellar medium (ISM) left behind the explosive death of most massive stars (smaller or equal to 40 Mo). Since they result in the expansion of the supernova shock wave into the gaseous environment shaped by the star wind history, their morphology constitutes an insight into the past evolution of their progenitor star. Particularly, fast-moving massive stars can produce asymmetric core-collapse supernova remnants. We investigate the mixing of materials in core-collapse supernova remnants generated by a moving massive 35 Mo star, in a magnetised ISM. Stellar rotation and the wind magnetic field are time-dependently included into the models which follow the entire evolution of the stellar surroundings from the zero age main sequence to 80 kyr after the supernova explosion. It is found that very little main sequence material is present in remnants from moving stars, that the Wolf-Rayet wind mixes very efficiently within the 10 kyr after the explosion, while the red supergiant material is still unmixed by 30 per cent within 50 kyr after the supernova. Our results indicate that the faster the stellar motion, the more complex the internal organisation of the supernova remnant and the more effective the mixing of ejecta therein. In contrast, the mixing of stellar wind material is only weakly affected by progenitor motion, if at all