Gammapy: high level data analysis for extragalactic science cases with the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) is the future large observatory in the very high energy (VHE) domain. Operating from 20 GeV to 300 TeV, it will be composed of tens of Imaging Air Cherenkov Telescopes (IACTs) displaced in a large area of a few square kilometers in both the southern and northern hemispheres. The CTA/DATA On-Site Analysis (OSA) is the system devoted to the development of dedicated pipelines and algorithms to be used at the CTA site for the reconstruction, data quality monitoring, science monitoring and realtime science alerting during observations. The OSA integral sensitivity is computed here for the most studied source at Gamma-rays, the Crab Nebula, for a set of exposures ranging from 1000 seconds to 50 hours, using the full CTA Southern array. The reason for the Crab Nebula selection as the first example of OSA integral sensitivity is twofold: (i) this source is characterized by a broad spectrum covering the entire CTA energy range; (ii) it represents, at the time of writing, the standard candle in VHE and it is often used as unit for the IACTs sensitivity. The effect of different Crab Nebula emission models on the CTA integral sensitivity is evaluated, to emphasize the need for representative spectra of the CTA science targets in the evaluation of the OSA use cases. Using the most complete model as input to the OSA integral sensitivity, we obtain a significant detection of the Crab nebula (about 10% of flux) even for a 1000 second exposure, for an energy threshold less than 10 TeV.

With very high energy (VHE) gamma-ray astronomy becoming a mature branch of observational astrophysics, and the multi-messenger sky being opened up by neutrino observatories and gravitational wave detectors, we here propose to set up a ring of imaging air Cherenkov telescopes – the Cherenkov Telescope Ring (CTR). The aim of this proposed project would be to facilitate continuous monitoring of sources on the VHE- and multi-messenger sky with minimal time delays and with high sensitivity. Development time and construction cost could be kept comparatively low by both including existing facilities into the monitoring effort, and by relying on substantial previous expertise gained in the community on the road towards the Cherenkov Telescope Array (CTA). This way, the Ring could prove to be a highly efficient facility greatly enhancing the science prospects for future ground-based high-energy astrophysics.

The Review of [formula omitted]-ray Astronomical Observing Techniques

We provide an updated assessment of the power of the Cherenkov Telescope Array (CTA) to search for thermally produced dark matter at the TeV scale, via the associated gamma-ray signal from pair-annihilating dark matter particles in the region around the Galactic centre. We find that CTA will open a new window of discovery potential, significantly extending the range of robustly testable models given a standard cuspy profile of the dark matter density distribution. Importantly, even for a cored profile, the projected sensitivity of CTA will be sufficient to probe various well-motivated models of thermally produced dark matter at the TeV scale. This is due to CTA's unprecedented sensitivity, angular and energy resolutions, and the planned observational strategy. The survey of the inner Galaxy will cover a much larger region than corresponding previous observational campaigns with imaging atmospheric Cherenkov telescopes. CTA will map with unprecedented precision the large-scale diffuse emission in high-energy gamma rays, constituting a background for dark matter searches for which we adopt state-of-the-art models based on current data. Throughout our analysis, we use up-to-date event reconstruction Monte Carlo tools developed by the CTA consortium, and pay special attention to quantifying the level of instrumental systematic uncertainties, as well as background template systematic errors, required to probe thermally produced dark matter at these energies.

Ground-based observatories allow for the detection of the most energetic forms of cosmic radiation in a multi-messenger context, including cosmic rays with energies above 10 15 eV and gamma rays with energies above 10 10 eV. Current experiments, such as the Pierre Auger Observatory, and forthcoming ones, such as the Cherenkov Telescope Array (CTA), have to deal with interpreting extensive air showers to reconstruct the fluxes of particles arriving at Earth. In this scenario, the general purpose of this thesis is to contribute to the improvement of the techniques used to reconstruct the properties of the primary particle in measurements of extensive air showers. In particular, four individual studies based on the simulation of air showers are presented. First, a study of the fluctuations in the atmospheric depth at which a shower reaches its maximum (X max ) is performed. The best functional form to describe the simulated X max distributions is selected and, along with two alternative functions, parametrized. This study aims to improve the reconstruction of the relative abundances of primary masses in the ultra-high-energy regime (E 0 > 10 17 eV) and to provide a practical tool for any other studies depending on X max distributions. Second, the angular distribution of Cherenkov light emitted from extensive air showers is parametrized using simulations performed in a broad energy range: from 100 GeV to 1 EeV. For this, a functional form to describe the angular distribution of Cherenkov photons is built using analytical arguments, and its parameters are constrained to describe the simulated showers. The resulting parametrization can be used to reconstruct the properties of air showers both through fluorescence detectors and Imaging Atmospheric Cherenkov Telescopes (IACT). Third, a technique to measure X max through IACTs is developed. The proposed technique is shown to provide a resolution comparable to that of fluorescence detectors, allowing for mass composition studies around the knee in the cosmic-ray spectrum. Fourth, the problem of modeling hadronic interactions is visited in the context of the CTA observatory. An analysis based on simulations is performed to determine the extent to which image parameters reconstructed by IACTs are influenced by uncertainties in hadronic interaction models. Then, a computation of the uncertainty on the estimated CTA sensitivity curve due to hadronic interaction models is performed.

Night Sky Background (NSB) is a complex phenomenon, consisting of all light detected by Imaging Atmospheric Cherenkov Telescopes (IACTs) not attributable to Cherenkov light emission. Understanding the effect of NSB on cameras for the next-generation Cherenkov Telescope Array (CTA) is important, as it affects the systematic errors on observations, the energy threshold, the thermal control of the cameras and the ability of the telescopes to operate under partial moonlight conditions. This capacity to observe under partial moonlight conditions is crucial for the CTA transient science programme, as it substantially increases the potential observing time. Using tools initially developed for H.E.S.S. (in combination with the prototype CTA analysis package ctapipe) we will present predictions for the NSB present in images taken by the CTA Small Sized Telescope Camera (SSTCAM), showing that SSTCAM will likely be able to meet the associated CTA requirements. Additionally, we calculate the potential observing time gain by operating under high NSB conditions.

Throughout this thesis, the spectral and temporal long-term behavior of the Active Galactic Nucleus (AGN) 1ES1959+650 is studied. Being a blazar, i.e. an AGN with a highly relativistic plasma outflow pointing towards the Earth, and having shown strong hints of hadronic emission processes in the past, 1ES1959+650 is a primary candidate for a source of cosmic rays. The origin of these ultra high energetic charged particles is the central question of astroparticle physics. Therefore, 1ES1959+650 has been monitored with the MAGIC telescope since 2004, measuring very high energy (VHE) gamma-rays. The resulting time-integrated energy spectrum reported here is by far the most exact one determined for this source up to now. Also the temporal variability for different wavelength bands, which should be connected in the case of most non-hadronic emission processes, is examined in this thesis. While strong variations occur in the optical and X-rays, the VHE gamma-ray emission is compatible with a constant flux. Furthermore, the time-integrated spectral energy distribution is compiled from radio to VHE gamma-rays and modeled for hadronic and leptonic emission scenarios to unveil the character of the emitting particles. Unexpected results are obtained on the spectral shape of the observed blazar, especially in high energy gamma-rays. By this, 1ES1959+650 is one of the very first high frequency peaked blazars whose non-thermal emission cannot be explained with a single-zone SSC model.Additionally, in this thesis the idea of long-term monitoring of blazars at VHE gamma-rays is pursued further. The FACT telescope, the first application of G-APDs as photosensors in an imaging atmospheric Cherenkov telescope, has recently been constructed and already during its commissioning phase reported on very promising first results. After the commissioning, it will be devoted to monitoring observations of bright blazars. For the construction of this telescope, the completely reworked aluminum mirrors have been characterized, including measurements of the focal lengths and the spot sizes of a reflected point-like light source.Finally, it is presented that DWARF, a global network of a small number of Cherenkov telescopes dedicated to monitoring observations of TeV-bright blazars, will be well suited to obtain comprehensive, well sampled lightcurves which are needed to study possible temporal correlations between VHE gamma-rays and other wavelengths.

The Cherenkov Telescope Array (CTA) project aims to create a next generation Very High Energy (VHE)γ-ray telescope array, devoted to the observation in a wide band of energy, from a few tens of GeV to more than 100 TeV. Two sites are foreseen to view the whole sky, with the main one in the Southern Hemisphere where about 100 telescopes of three different classes, related to the specific energy region to be investigated, will be installed. Among these, the Small Size class of Telescopes, SSTs, are 4-meter telescopes and are devoted to the highest energy region, from 1 TeV to beyond 100 TeV. Some of these sites considered for CTA exhibit strong seismic constraints. At the Observatoire de Paris, we have designed a prototype of a Small Size Telescope named SST-GATE, based on the dual-mirror Schwarzschild-Couder optical formula, which was never before implemented in the design of a Cherenkov telescope. The integration of this telescope on the site of the Observatoire de Paris is currently in progress. Technical solutions exist in the literature to protect structures from dynamic loads caused by earthquakes without increasing the mass and cost of the structure. This paper presents a state of the art of these techniques by keeping in mind that the operational performance of the telescope should not be compromised. The preliminary seismic analysis of SSTGATE performed by the finite element method is described before.

The High Energy Stereoscopic system, H.E.S.S. is an array of four imaging atmospheric Cherenkov telescopes, designed for the study of non-thermal phenomena in the universe at very high energies (VHE). The sensitivity of telescope systems such as H.E.S.S. can considerably be improved by a better discrimination of the vast number of hadronic cosmic-ray background events against the very rare gamma-ray signal events. In this work, an elaborated discrimination technique – the Boosted Decision Tree method – has been developed and its capabilities in terms of gamma/hadron separation and improved sensitivity are demonstrated. In the second part, the BDT method is applied to data obtained in observations of massive star forming environments, namely the colliding wind binary η Carinae, the massive stellar cluster Westerlund 1 and the Starburst galaxy NGC 253. An upper limit on the gamma-ray flux of the famous colliding wind binary system η Carinae is derived and, for the first time, an alternative model for the high-energy emission observed by the Fermi satellite is presented. The detection of very extended VHE gamma-ray emission from the vicinity of Westerlund 1 is reported and thorough spectral and morphological tests are presented. Large parts of the resolved emission can be explained in a hadronic scenario, however, a decisive conclusion can not be drawn. Finally, the BDT method allowed to detect the first Starburst galaxy, namely NGC253, in VHE gamma rays. Spectral and morphological results are presented and suggest that large parts of the CR energy content are convectively and diffusively transported into the intergalactic medium.

Muon ring images observed with Imaging Atmospheric Cherenkov Telescopes (IACTs) provide a powerful means to calibrate the optical throughput of IACTs and monitor their optical point spread function. We investigate whether muons ring images can be used as the primary optical throughput calibration method for the telescopes of the future Cherenkov Telescope Array (CTA) and find several additional systematic effects in comparison to previous works. To ensure that the method achieves the accuracy required by CTA, these systematic effects need to be taken into account and minor modifications to the hardware and analysis are necessary. We derive analytic estimates for the expected muon data rates to be used for optical throughput calibration, monitor- ing of the optical point spread function, with achievable statistical and systematic uncertainties, and explore the potential of muon ring images as a secondary method of camera pixel flat-fielding.

The Cherenkov Telescope Array (CTA) will consist of two arrays of Imaging Atmospheric Cherenkov Telescopes (IACTs) at the northern and southern hemispheres. CTA will feature IACTs with mirrors of three different sizes optimized to cover different energy ranges. The pro- posed sub-arrays of four Large Size Telescopes (LST) at CTA-North and CTA-South target the lowest energy range between around 20 GeV and 100 GeV. Thanks to their low weight of around 110 tons the LSTs can move by 180 deg in azimuth in 20 seconds for Gamma Ray Burst (GRB) follow-up. An LST has a tessellated parabolic mirror of 23 m diameter equipped with a system of actuators to correct for gravity-induced deformations during data taking. Its low-weight 2 ton camera at the prime focus has a 4.5 deg diameter, 1855 high QE PMTs and an embedded readout with 1 GSps sampling speed designed for data acquisition rates exceeding 10 kHz. A fully equipped LST has been installed at the CTA-North site in 2018 and is expected to be finished commissioning during 2019. The remaining three LSTs in the north will be installed by 2022. We will review the status of the LSTs, describe the installation of the first LST and report on the first results of the commissioning tests.

The Cherenkov Telescope Array (CTA) is the next generation ground-based observatory for very high-energy (E>100 GeV) gamma-ray astronomy. It will integrate several tens of imaging atmospheric Cherenkov telescopes (IACTs) with different apertures into a single astronomical instrument. The US part of the CTA collaboration has proposed and is developing a novel IACT design with a Schwarzschild-Couder (SC) aplanatic two-mirror optical system. In comparison with the traditional single mirror Davies-Cotton IACT the SC telescope, by design, can accommodate a wider field-of-view, with significantly improved imaging resolution. In addition, the reduced plate scale of an SC telescope makes it compatible with highly integrated cameras assembled from silicon photo multipliers. In this submission we report on the status of the development of the SC optical system, which is part of the e ort to construct a full-scale prototype telescope of this type at the Fred Lawrence Whipple Observatory in southern Arizona.

In this thesis a detailed study of the interaction between gamma-rays coming from active galactic nuclei (AGN) and the radiation eld generated by the thermal emission from all the galaxies, the extragalactic background light (EBL), is performed. The development of the imaging air Cherenkov telescope (IACT) technique has been providing well determined spectral energy distributions (SED) of extragalactic sources and, in turn, allowing investigations of the attenuation processes of very high energy (VHE) radiation by the EBL. In 1997, a are state of the blazar Markarian 501 (Mkn 501) provided ux measurements at energies up to 20 TeV. We show that this spectrum is the one with the highest expected attenuation by the EBL dust component among the current known TeV emitters. Starting from an EBL model based on stellar and dust blackbody contributions, we were able to investigate the role of each EBL component in the attenuation of Mkn 501 SED. In such EBL model, dust is divided into three dierent grain types (polycyclic aromatic hydrocabons (PAH), small (SG) and large grains (LG)) and the importance of each one of these components to the opacity of the extragalactic medium for the Mkn 501 SED could be better understood. In particular, PAH molecules have a fundamental importance in the description of Mkn 501 spectrum, because its slope observed around 10 TeV can only be correctly described by this dust component. On the other hand, an eective model containing only PAH as dust can be excluded with a signicance of at least 5, independently of the intrinsic spectrum assumed. Finally, a forecasting study is performed for the third generation of IACT, the Cherenkov Telescope Array (CTA). Through a maximum likelihood estimator we show that, already for a single source analysis, the PAH relative contribution can be determined with a good resolution for 50 hours of observation. The combination of several observations shows that 30 minutes of exposure time is enough to determine the PAH fraction with a reasonable resolution. The impact of the redshift uncertainties is also analyzed considering an a priori probability for this observable when determining the PAH fraction.

When very-high-energy gamma rays interact high in the Earth’s atmosphere, they produce cascades of particles that induce flashes of Cherenkov light. Imaging Atmospheric Cherenkov Telescopes (IACTs) detect these flashes and convert them into shower images that can be analyzed to extract the properties of the primary gamma ray. The dominant background for IACTs is comprised of air shower images produced by cosmic hadrons, with typical noise-to-signal ratios of several orders of magnitude. The standard technique adopted to differentiate between images initiated by gamma rays and those initiated by hadrons is based on classical machine learning algorithms, such as Random Forests, that operate on a set of handcrafted parameters extracted from the images. Likewise, the inference of the energy and the arrival direction of the primary gamma ray is performed using those parameters. State-of-the-art deep learning techniques based on convolutional neural networks (CNNs) have the potential to enhance the event reconstruction performance, since they are able to autonomously extract features from raw images, exploiting the pixel-wise information washed out during the parametrization process. Here we present the results obtained by applying deep learning techniques to the reconstruction of Monte Carlo simulated events from a single, next-generation IACT, the Large-Sized Telescope (LST) of the Cherenkov Telescope Array (CTA). We use CNNs to separate the gamma-ray-induced events from hadronic events and to reconstruct the properties of the former, comparing their performance to the standard reconstruction technique. Three independent implementations of CNN-based event reconstruction models have been utilized in this work, producing consistent results.

Stereoscopic arrays of Imaging Cherenkov Telescopes allow to reconstruct gamma‐ray‐induced showers in 3 dimensions. An analysis method based on a simple 3D‐model of electromagnetic showers and implemented in the framework of the H.E.S.S. experiment was recently improved by an additional quality criterion which reduces the background contamination by a factor of about 2 in the case of extended sources, while hardly affecting gamma‐ray selection efficiency. Moreover, the dramatic flares of PKS 2155‐304 in July 2006, which provided H.E.S.S. data with an almost pure gamma‐ray sample, offered the unique opportunity of a precision test of the 3D‐reconstruction method as well as of the H.E.S.S. simulations used in its calibration. An agreement at a few percent level is found between data and simulations for the distributions of all 3D shower parameters.