Using Muon Rings for the Optical Throughput Calibration of the Cherenkov Telescope Array

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.

The Cherenkov Telescope Array (CTA) observatory will probe the non-thermal universe above 20 GeV up to several hundreds of TeV with a significant improvement in sensitivity and angular resolution compared to current experiments. Its outstanding capabilities will allow to increase the number of extragalactic cosmic accelerators detected at very high energy (VHE) and therefore to better constrain the population of VHE accelerators and the gamma-ray absorption processes in the intergalactic medium. For the first time in the history of imaging atmospheric Cherenkov telescopes (IACTs), CTA will be an open observatory and high-level data will be made available to the astronomical community. Gammapy is an open-source Python package developed by the Cherenkov telescope community that provides tools to simulate the gamma-ray sky and analyse IACT data. The versatile architecture of, and steady user contributions to Gammapy enable a large variety of high-level data analyses. Examples of Gaammapy applications are presented, particularly in the context of extragalactic science with CTA.

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.

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.

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.

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

A Central Laser Facility is a system composed of a laser placed at a certain distance from a light-detector array, emitting fast light pulses,typically in the vertical direction, with the aim to calibrate that array. During calibration runs, all detectors are pointed towards the same portion of the laser beamat a given altitude. Central Laser Facilities are used for various currently operating ultra-high-energy cosmic ray and imaging atmospheric Cherenkov telescope arrays.In view of the future Cherenkov Telescope Array, a similar device could provide a fast calibration of the whole installation at different wavelengths.The relative precision (i.e. each individual telescope with respect to the rest of the array is expected) to be better than 5%, while an absolutecalibration should reach a precisions of 6–11%, if certain design requirements are met.Additionally, a preciser monitoring of the sensitivity of each telescope can be made on time-scales of days to years.

We focus our analysis on 55 BL Lac objects with a hard Fermi gamma-ray spectrum, and for which a redshift or a lower limit to it has been determined by a previous study of ours. We extrapolate the spectral fits given by the 4FGL catalogue to the VHE band (>0.1 TeV), which can be explored by imaging atmospheric Cherenkov telescopes. Furthermore, we take into account the absorption due to the extragalactic background light, strongly depending on the redshift. Finally, we compare our results with publicly available sensitivity curves for a selection of imaging atmospheric Cherenkov telescope arrays currently operating or under construction. From our extrapolations and simulations we find a large number of promising candidates for observation with the forthcoming Cherenkov Telescope Array observatory.

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.

Riccardo Rando∗ab, Daniele Cortia, Francesco Dazzic, Alessandro De Angelisd, Antonios Dettlaffc, Daniela Dornere, David Finkc, Nadia Fouque f , Felix Grundnerc, Werner Habererc, Alexander Hahnc, Richard Hermel f , Samo Korparg, Gasper Kukec Mezekh, Ronald Maierc, Christian Maneaa, Mose Mariottiab, Daniel Mazinci, Fatima Mehrez f , Razmik Mirzoyanc, Sergey Podkladkinc, Ignasi Reichardta, Wolfgang Rhode j, Sylvie Rosier f , Cornelia Schultzab, Carlo Stellak, Masahiro Teshimaci, Holger Wetteskindc and Marko Zavrtanikgh for the CTA Consortium† E-mail: riccardo.rando@pd.infn.it a INFN Section of Padova, Italy; b University of Padova, Italy; c Max Planck Institute for Physics, Munich, Germany; d LIP/IST Lisboa, Portugal; e University of Wurzburg, Germany; f LAPP, CNRS-IN2P3, Annecy, France; g Jožef Stefan Institute, Ljubljana, Slovenia; h University of Nova Gorica, Slovenia; i University of Tokyo and ICRR, Tokyo, Japan; j Technische Universitat, Dortmund, Germany; k University of Udine and INFN, Italy. The Cherenkov Telescope Array (CTA) is the the next generation facility of imaging atmospheric Cherenkov telescopes; two sites will cover both hemispheres. CTA will reach unprecedented sensitivity, energy and angular resolution in very-high-energy gamma-ray astronomy. Each CTA array will include four Large Size Telescopes (LSTs), designed to cover the low-energy range of the CTA sensitivity (∼20 GeV to 200 GeV). In the baseline LST design, the focal-plane camera will be instrumented with 265 photodetector clusters; each will include seven photomultiplier tubes (PMTs), with an entrance window of 1.5 inches in diameter. The PMT design is based on mature and reliable technology. Recently, silicon photomultipliers (SiPMs) are emerging as a competitor. Currently, SiPMs have advantages (e.g. lower operating voltage and tolerance to high illumination levels) and disadvantages (e.g. higher capacitance and cross talk rates), but this technology is still young and rapidly evolving. SiPM technology has a strong potential to become superior to the PMT one in terms of photon detection efficiency and price per square mm of detector area. While the advantage of SiPMs has been proven for high-density, small size cameras, it is yet to be demonstrated for large area cameras such as the one of the LST. We are working to develop a SiPM-based module for the LST camera, in view of a possible camera upgrade. We will describe the solutions we are exploring in order to balance a competitive performance with a minimal impact on the overall LST camera design. The 34th International Cosmic Ray Conference, 30 July6 August, 2015 The Hague, The Netherlands

ASTRI is a flagship project of the Italian Ministry of Education, University and Research, which aims to develop an endto- end prototype of one of the three types of telescopes to be part of the Cherenkov Telescope Array (CTA), an observatory which will be the main representative of the next generation of Imaging Atmospheric Cherenkov Telescopes. The ASTRI project, led by the Italian National Institute of Astrophysics (INAF), has proposed an original design for the Small Size Telescope, which is aimed to explore the uppermost end of the Very High Energy domain up to about few hundreds of TeV with unprecedented sensitivity, angular resolution and imaging quality. It is characterized by challenging and innovative technological solutions which will be adopted for the first time in a Cherenkov telescope: a dual-mirror Schwarzschild-Couder configuration, a modular, light and compact camera based on silicon photomultipliers, and a front-end electronic based on a specifically designed ASIC. The end-to-end project is also including all the data-analysis software and the data archive. In this paper we describe the process followed to derive the ASTRI specifications from the CTA general requirements, a process which had to take into proper account the impact on the telescope design of the different types of the CTA requirements (performance, environment, reliability-availability-maintenance, etc.). We also describe the strategy adopted to perform the specification verification, which will be based on different methods (inspection, analysis, certification, and test) in order to demonstrate the telescope compliance with the CTA requirements. Finally we describe the integration planning of the prototype assemblies (structure, mirrors, camera, control software, auxiliary items) and the test planning of the end-to-end telescope. The approach followed by the ASTRI project is to have all the information needed to report the verification process along all project stages in a single layer. From this unique layer it is possible to, in a semi-automatic way, generate updated project documentation and progress report.

Imaging Atmospheric Cherenkov Telescopes (IACTs) have opened a new astronomical window at photon energies exceeding tens of GeV during the last decades. The technique relies on the detection of Cherenkov light produced by electromagnetic showers in the Earth’s atmosphere using telescope mirrors with diameters of a few to tens of meters. A world-wide community is behind the design and construction of the next generation Cherenkov Telescope Array (CTA), consisting of two arrays of IACTs at the northern and southern hemispheres. I will review the astrophysics that IACTs are revealing at these energies, cover the design characteristics of CTA, the status of the project and the physics prospects of the instrument.

The ctools open-source software package was developed for the scientific analysis of astronomical data from Imaging Air Cherenkov Telescopes (IACTs), such as H.E.S.S., VERITAS, MAGIC, and the future Cherenkov Telescope Array (CTA). To date, the software has been mainly tested using simulated CTA data; however, upon the public release of a small set of H.E.S.S. observations of the Crab nebula, MSH 15–52, RX J1713.7–3946, and PKS 2155–304 validation using real data is now possible. We analysed the data of the H.E.S.S. public data release using ctools version 1.6 and compared our results to those published by the H.E.S.S. Collaboration for the respective sources. We developed a parametric background model that satisfactorily describes the expected background rate as a function of reconstructed energy and direction for each observation. We used that model, and tested all analysis methods that are supported by ctools, including novel unbinned and joint or stacked binned analyses of the measured event energies and reconstructed directions, and classical On-Off analysis methods that are comparable to those used by the H.E.S.S. Collaboration. For all analysis methods, we found a good agreement between the ctools results and the H.E.S.S. Collaboration publications considering that they are not always directly comparable due to differences in the datatsets and event processing software. We also performed a joint analysis of H.E.S.S. and Fermi-LAT data of the Crab nebula, illustrating the multi-wavelength capacity of ctools. The joint Crab nebula spectrum is compatible with published literature values within the systematic uncertainties. We conclude that the ctools software is mature for the analysis of data from existing IACTs, as well as from the upcoming CTA.