arXiv subjects

Constantin Steppa

Publications and source records attributed to Constantin Steppa.

15 recordsLinked to original sources

arXiv2026

Estimatingthe Contribution of Galactic Neutrino Sources

The Milky Way hosts astrophysical accelerators capable of producing high-energy cosmic rays. These cosmic rays can interact with the interstellar medium (ISM) across the Galaxy to produce neutrinos and gamma rays (propagation component), while their interactions with ambient material at their acceleration sites, such as supernova remnants, can give rise to the source component of the gamma-ray and neutrino flux. In this paper, we estimate the source component of the Galactic neutrino flux using simulated populations of Galactic gamma-ray sources. We compare our results with observations from neutrino experiments in the energy range of 1-30 TeV. Using simulated populations of Galactic TeV gamma-ray sources, we exploit the correlation between gamma rays and neutrinos and introduce a bracketing approach to constrain the range for the source contribution of the Galactic neutrino flux. For the upper limit, we used a simulation describing the entity of Galactic gamma-ray sources, whereas the lower limit was estimated using the hadronic component of the Galactic supernova remnant population. Our results show that the difference between this maximum and minimum is less than an order of magnitude and the flux range is comparable to the Galactic neutrino flux from the cosmic-ray interaction with the ISM. The results agree with the observed signals from IceCube and ANTARES and suggest that the propagation component, combined with the minimum source contribution predicted by the supernova-remnant model, approaches the observed neutrino flux, leaving little room for significant enhancements of the emission originating from propagating cosmic rays.

astro-ph.HE↗

arXiv2025

On the Transients Handler for the Cherenkov Telescope Array Observatory

The Cherenkov Telescope Array Observatory (CTAO) is a next-generation gamma-ray observatory in both the Southern (Paranal, Chile) and Northern Hemisphere (La Palma, Spain) and will consist of up to 100 imaging atmospheric Cherenkov telescopes. With sensitivity far exceeding current facilities, CTAO will provide detailed measurements of gamma rays from GeV up to a few 100s of TeV. CTAO has a nominal field of view of 10{\deg} and will rely on external alerts from observatories such as IceCube (neutrinos), Fermi-LAT (GeV gamma rays), LIGO (gravitational waves (GWs)) or the upcoming Vera C. Rubin Observatory (optical) to trigger observations of targets of opportunity (ToOs). The Transients Handler (TH) is a subsystem of the Array Control and Data Acquisition that will provide the means of handling alerts and schedule follow-up observations. The TH is responsible for (i) filtering of thousands of internal and external alerts per night, (ii) processing of alerts according to the corresponding observation proposal to determine the optimal observation schedule with minimum interruptions to observations and (iii) broadcasting of events detected by CTAO to external facilities. In this work, we will discuss the architecture of the Transients Handler, its latest improvements and future updates

astro-ph.IM↗

arXiv2025

Runaway stars and the Galactic supernova remnant landscape: non-thermal emission and observational evidence

Context. A significant fraction (~30%) of massive stars in our Galaxy are moving supersonically through the interstellar medium, which strongly governs their location at the time they end their lives, e.g. die as a supernova and give birth to a supernova remnant (SNR). These dead stellar environments accelerate particles, emitting by non-thermal mechanisms up to the TeV range, and they are considered as a major contributor to the very-high-energy band of the local cosmic-ray spectrum. Aims. This study investigates the effect of the runaway motion of supernova progenitors on the spatial distribution of SNRs in the Milky Way and how this influences the deduced properties of the population. Methods. We construct Galactic populations of SNRs by Monte Carlo simulation, taking into account the bulk motion and the evolution history of their progenitor stars once ejected from their parent clusters. The gamma-ray domain emission of each population is then calculated, to be compared with the High Energy Stereoscopic System (H.E.S.S.) Galactic Plane Survey. Results. We find that including the runaway motion of supernova progenitors strongly modifies the detectability of the simulated emission of their remnants in the very-high-energy band. Particularly, our best fit model using a Reid Milky Way model for core-collapse supernova progenitors requires 33% of massive runaway stars, which is close to the known fraction of runaway high-mass stars, to be in accordance with the H.E.S.S. Galactic Plane Survey data. Conclusions. Our results show that the runaway nature of supernova progenitors must be taken into account in the study of the Galactic population of SNRs within the H.E.S.S. Galactic Plane Survey and the forthcoming Galactic Plane Survey of the Cherenkov Telescope Array Observatory, as it is a governing factor of the detectability of non-thermal emission of their subsequent SNRs.

astro-ph.HE↗

arXiv2024

The population of Galactic supernova remnants in the TeV range

Supernova remnants (SNRs) are likely to be significant sources of cosmic rays up to the knee of the local cosmic-ray (CR) spectrum. They produce gamma-rays in the very-high-energy (VHE) ($E>0.1$ TeV) range via: hadronic interactions with the interstellar medium and leptonic interactions with soft photons. Current observations have lead to the detection of about a dozen of VHE SNRs and future instruments should increase this number. The details of particle acceleration at SNRs, and of the mechanisms producing VHE gamma-rays at SNRs are poorly understood. We aim to study the population of SNRs detected in the TeV range and its properties, and to address fundamental questions of particle acceleration at SNR shocks: What is the spectrum of accelerated particles? What is the efficiency of acceleration? Is the VHE emission dominated by hadronic or leptonic interactions? By means of Monte Carlo methods, we simulate the population of SNRs in the VHE domain and confront our simulations to H.E.S.S. Galactic Plane Survey (HGPS). We explore the parameter space: the slope of accelerated particles $\alpha$, the electron-to-proton ratio $K_{\rm ep}$, and the efficiency of particle acceleration $\xi $. We found sets of parameters for which $\gtrsim 90$\% of realisations are found in agreements with the HGPS data. These parameters are found $ 4.2 \gtrsim \alpha \gtrsim 4.1 $, $10^{-5} \lesssim K_{\rm ep} \lesssim 10^{-4.5}$, and $0.03 \lesssim \xi\lesssim 0.1 $ . We were able to strongly argue against some regions of the parameter space: $\alpha \lesssim 4.05$, $\alpha \gtrsim 4.35$, or $K_{\rm ep} \gtrsim 10^{-3}$. Our model is so far able to explain the SNR population of the HGPS. Our approach, confronted to the results of future systematic surveys, will help remove degeneracy in the solutions, and to better understand particle acceleration at SNR shocks.

astro-ph.HE↗

arXiv2023

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.

astro-ph.HE↗

arXiv2023

Probing Galaxy structure with VHE $\gamma$ 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.

astro-ph.HE↗

arXiv2023

The Galactic TeV sky: sources or diffuse emission?

Gamma-ray observations have recently shifted the focus to higher and higher energies, with capable ground-based instruments enabling measurements in the TeV to PeV domain. While a clear prevalence of diffuse emission is observed in the GeV sky, energy-dependent cosmic-ray transport suggests a reversal of this hierarchy at higher energies. Measurements, however, are at strife regarding this question. While imaging atmospheric Cherenkov telescopes (IACTs) see a source-dominated Galactic plane, air-shower particle detectors (ASPDs) report a dominance of diffuse emission. Reconciling these claims might require a closer look at the involved instrument limitations: IACTs have a small field of view, resulting in poorer performance for large-scale emission due to the applied background subtraction technique. ASPDs have reduced resolution capabilities, resulting in unresolved sources contributing to the measurable diffuse emission signal. Here we contribute to this controversy by investigating the amount of unresolved sources in current TeV measurements in a population synthesis approach and discuss the unique capabilities for high-resolution diffuse-emission measurements with IACTs and their possibilities for overcoming their background limitations.

astro-ph.HE↗

arXiv2022

The Transients Handler System for the Cherenkov Telescope Array Observatory

The Cherenkov Telescope Array Observatory (CTAO) will be the largest and most advanced ground-based facility for gamma-ray astronomy. Several dozens of telescopes will be operated at both the Northern and Southern Hemisphere. With the advent of multi-messenger astronomy, many new large science infrastructures will start science operations and target-of-opportunity observations will play an important role in the operation of the CTAO. The Array Control and Data Acquisition (ACADA) system deployed on each CTAO site will feature a dedicated sub-system to manage external and internal scientific alerts: the Transients Handler. It will receive, validate, and process science alerts in order to determine if target-of-opportunity observations can be triggered or need to be updated. Various tasks defined by proposal-based configurations are processed by the Transients Handler. These tasks include, among others, the evaluation of observability of targets and their correlation with known sources or objects. This contribution will discuss the concepts and design of the Transients Handler and its integration in the ACADA system.

astro-ph.IM↗

arXiv2022

Galactic diffuse gamma rays meet the PeV frontier

The Tibet AS$\gamma$ and LHAASO collaborations recently reported the observation of a $\gamma$-ray diffuse emission with energy up to the PeV from the Galactic plane. We discuss the relevance of non-uniform cosmic-ray transport scenarios and the implications of these results for cosmic-ray physics. We use the {\tt DRAGON} and {\tt HERMES} codes to build high-resolution maps and spectral distributions of that emission for several representative models under the condition that they reproduce a wide set of local cosmic-ray data up to 100 PeV. We show that the energy spectra measured by Tibet AS$\gamma$, LHAASO, ARGO-YBJ and Fermi-LAT in several regions of interest in the sky can all be consistently described in terms of the emission arising by the Galactic cosmic-ray "sea". We also show that all our models are compatible with IceTop $\gamma$-ray upper limits. Our results favor transport models characterized by spatial-dependent diffusion although some degeneracy remains between the choice of the transport scenario and that of the cosmic-ray spectral shape above 10 TeV. We discuss the role of forthcoming measurements in resolving that ambiguity.

astro-ph.HE↗

arXiv2021

Exploring the population of Galactic very-high-energy $\gamma$-ray sources

At very high energies (VHE), the emission of $\gamma$ rays is dominated by discrete sources. Due to the limited resolution and sensitivity of current-generation instruments, only a small fraction of the total Galactic population of VHE $\gamma$-ray sources has been detected significantly. The larger part of the population can be expected to contribute as a diffuse signal alongside emission originating from propagating cosmic rays. Without quantifying the source population, it is not possible to disentangle these two components. Based on the H.E.S.S. Galactic plane survey, a numerical approach has been taken to develop a model of the population of Galactic VHE $\gamma$-ray sources, which is shown to account accurately for the observational bias. We present estimates of the absolute number of sources in the Galactic Plane and their contribution to the total VHE $\gamma$-ray emission for five different spatial source distributions. Prospects for CTA and its ability to constrain the model are discussed. Finally, first results of an extension of our modelling approach using machine learning to extract more information from the available data set are presented.

astro-ph.HE↗

arXiv2020

Modelling the Galactic very-high-energy $γ$-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 $γ$-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 $γ$-ray sources comprise only a small and biased subsample of the overall population. Still, numerical modelling allows us to study the VHE $γ$-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.

astro-ph.HE↗

arXiv2020

A NECTAr-based upgrade for the Cherenkov cameras of the H.E.S.S. 12-meter telescopes

The High Energy Stereoscopic System (H.E.S.S.) is one of the three arrays of imaging atmospheric Cherenkov telescopes (IACTs) currently in operation. It is composed of four 12-meter telescopes and a 28-meter one, and is sensitive to gamma rays in the energy range ~30 GeV - 100 TeV. The cameras of the 12-m telescopes recently underwent a substantial upgrade, with the goal of improving their performance and robustness. The upgrade involved replacing all camera components except for the photomultiplier tubes (PMTs). This meant developing new hardware for the trigger, readout, power, cooling and mechanical systems, and new software for camera control and data acquisition. Several novel technologies were employed in the cameras: the readout is built around the new NECTAr digitizer chip, developed for the next generation of IACTs; the camera electronics is fully controlled and read out via Ethernet using a combination of FPGA and embedded ARM computers; the software uses modern libraries such as Apache Thrift, ZMQ and Protocol buffers. This work describes in detail the design and the performance of the upgraded cameras.

astro-ph.IM↗

arXiv2019

HexagDLy - Processing hexagonally sampled data with CNNs in PyTorch

HexagDLy is a Python-library extending the PyTorch deep learning framework with convolution and pooling operations on hexagonal grids. It aims to ease the access to convolutional neural networks for applications that rely on hexagonally sampled data as, for example, commonly found in ground-based astroparticle physics experiments.

cs.CV↗

arXiv2018

Application of Deep Learning methods to analysis of Imaging Atmospheric Cherenkov Telescopes data

Ground based gamma-ray observations with Imaging Atmospheric Cherenkov Telescopes (IACTs) play a significant role in the discovery of very high energy (E > 100 GeV) gamma-ray emitters. The analysis of IACT data demands a highly efficient background rejection technique, as well as methods to accurately determine the energy of the recorded gamma-ray and the position of its source in the sky. We present results for background rejection and signal direction reconstruction from first studies of a novel data analysis scheme for IACT measurements. The new analysis is based on a set of Convolutional Neural Networks (CNNs) applied to images from the four H.E.S.S. phase-I telescopes. As the H.E.S.S. cameras pixels are arranged in a hexagonal array, we demonstrate two ways to use such image data to train CNNs: by resampling the images to a square grid and by applying modified convolution kernels that conserve the hexagonal grid properties. The networks were trained on sets of Monte-Carlo simulated events and tested on both simulations and measured data from the H.E.S.S. array. A comparison between the CNN analysis to current state-of-the-art algorithms reveals a clear improvement in background rejection performance. When applied to H.E.S.S. observation data, the CNN direction reconstruction performs at a similar level as traditional methods. These results serve as a proof-of-concept for the application of CNNs to the analysis of events recorded by IACTs.

astro-ph.IM↗

arXiv2017

The International Cosmic Day - An Outreach Event for Astroparticle Physics

The International Cosmic Day (ICD) is an astroparticle physics outreach event for high-school students and brings together students and different physics outreach projects from all over the world. Groups of scientists, teachers, and students meet for one day to learn about cosmic rays and perform an experiment with atmospheric muons. All participating groups investigate an identical question. The students are enabled to work together like in an international collaboration, discussing their results in joint video conferences. Analyzing data, comparing and discussing with other "young scientists" gives the students a glimpse of how professional scientific research works. Scientists join the video conferences and give lectures to provide an insight in current astroparticle physics research. Several participating research experiments analyze big science data tailored to the questions addressed by the students and present their results on equal terms with the students. To create a lasting event, proceedings with measurement results of all participating groups are published. Every participant receives a personal e-mail with his certificate and the proceedings booklet. Organized by DESY in cooperation with Netzwerk Teilchenwelt, IPPOG, QuarkNet, Fermilab, and national partners like INFN, the ICD is a growing event with more and more popularity. We present the organization of the event and the experience from five years of ICD.

astro-ph.IM↗