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This book summarizes the science to be carried out by the upcoming Cherenkov Telescope Array, a major ground-based gamma-ray observatory that will be constructed over the next six to eight years. The major scientific themes, as well as core program of key science projects, have been developed by the CTA Consortium, a collaboration of scientists from many institutions worldwide.CTA will be the major facility in high-energy and very high-energy photon astronomy over the next decade and beyond. CTA will have capabilities well beyond past and present observatories. Thus, CTA's science program is expected to be rich and broad and will complement other major multiwavelength and multimessenger facilities. This book is intended to be the primary resource for the science case for CTA and it thus will be of great interest to the broader physics and astronomy communities. The electronic version (e-book) is available in open access.
In this paper, we present studies for optimizing the next generation of ground-based imaging atmospheric Cherenkov telescopes (IACTs). Results focus on mid-sized telescopes (MSTs) for CTA, detecting very high energy gamma rays in the energy range from a few hundred GeV to a few tens of TeV. We describe a novel, flexible detector Monte Carlo package, FAST (FAst Simulation for imaging air cherenkov Telescopes), that we use to simulate different array and telescope designs. The simulation is somewhat simplified to allow for efficient exploration over a large telescope design parameter space. We investigate a wide range of telescope performance parameters including optical resolution, camera pixel size, and light collection area. In order to ensure a comparison of the arrays at their maximum sensitivity, we analyze the simulations with the most sensitive techniques used in the field, such as maximum likelihood template reconstruction and boosted decision trees for background rejection. Choosing telescope design parameters representative of the proposed Davies–Cotton (DC) and Schwarzchild–Couder (SC) MST designs, we compare the performance of the arrays by examining the gamma-ray angular resolution and differential point-source sensitivity. We further investigate the array performance under a wide range of conditions, determining the impact of the number of telescopes, telescope separation, night sky background, and geomagnetic field. We find a 30–40% improvement in the gamma-ray angular resolution at all energies when comparing arrays with an equal number of SC and DC telescopes, significantly enhancing point-source sensitivity in the MST energy range. Finally, we attribute the increase in point-source sensitivity to the improved optical point-spread function and smaller pixel size of the SC telescope design.
The Cherenkov Telescope Array (CTA) is a ground-based observatory for very high-energy (10 GeV to 100 TeV) gamma rays, planned for operation starting in 2018. It will be an array of dozens of optical telescopes, known as Atmospheric Cherenkov Telescopes (ACTs), of 8 m to 24 m diameter, deployed over an area of more than 1 square km, to detect flashes of Cherenkov light from showers initiated in the Earth's atmosphere by gamma rays. CTA will have improved angular resolution, a wider energy range, larger fields of view and an order of magnitude improvement in sensitivity over current ACT arrays such as H.E.S.S., MAGIC and VERITAS. Several institutions have proposed a research and development program to eventually contribute 36 medium-sized telescopes (9 m to 12 m diameter) to CTA to enhance and optimize its science performance. The program aims to construct a prototype of an innovative, Schwarzschild-Couder telescope (SCT) design that will allow much smaller and less expensive cameras and much larger fields of view than conventional Davies-Cotton designs, and will also include design and testing of camera electronics for the necessary advances in performance, reliability and cost. We report on the progress of the mid-sized SCT development program.
Since the creation of the first telescope in the 17th century, every major discovery in astrophysics has been the direct consequence of the development of novel observation techniques, opening new windows in the electromagnetic spectrum. After Karl Jansky discovered serendipitously the first radio source in 1933, Grote Reber built the first parabolic radio telescope in his backyard, planting the seed of a whole new field in astronomy. Similarly, new technologies in the 1950s allowed the establishment of other fields, such as the infrared, ultraviolet or the X-rays. The highest energy end of the electromagnetic spectrum, the gamma-ray range, represents the last unexplored window for astronomers and should reveal the most extreme phenomena that take place in the Universe. Given the technical complexity of gamma-ray detection and the extremely relative low fluxes, gamma-ray astronomy has undergone a slower development compared to other wavelengths. Nowadays, the great success of consecutive space missions together with the development and refinement of new detection techniques from the ground, has allowed outstanding scientific results and has brought gamma-ray astronomy to a worthy level in par with other astronomy fields. This work is devoted to the study and improvement of the future Cherenkov Telescope Array (CTA), the next generation of ground based gamma-ray detectors, designed to observe photons with the highest energies ever observed from cosmic sources. These results on the sensitivity studies performed for the CTA collaboration evaluate the observatory performance through the analysis of large-scale Monte Carlo (MC) simulations, along with an estimation of its future potential on specific physics cases. Together with the testing and development of the analysis tools employed, these results are critical to understand CTA's future capabilities, the efficiency of different telescope placement approaches and the effect on performance of the construction site, related to parameters such as the altitude or the geomagnetic field. The Northern Hemisphere proposed construction sites were analyzed and evaluated, providing an accurate estimation of their capabilities to host the observatory. As for the CTA layout candidates, an unbiased comparison of the different arrays proposed by the collaboration was performed, using Fermi-LAT catalogs to forecast the performance of each array over specific scientific cases. In addition, the application of machine learning algorithms on gamma-ray astronomy was studied, comparing alternative methods for energy reconstruction and background suppression and introducing new applications to these algorithms, such as the determination of gamma-ray source types through the training of their spectral features. The analysis presented here of both CTA-N and CTA-S candidates represents the most comprehensive study of CTA capabilities performed by the collaboration to date. Experience gained with the improvement of this software will guide the future gls{cta} analysis pipelines by comparing the attained sensitivity by alternative analysis chains. From these results, both CTA-N and CTA-S candidates "2N" and "2Q" fulfill the sensitivity, angular and energy resolution, effective area and off-axis performance requirements. MC simulations provide an useful test-bench for the different designs within the CTA project, and these results demonstrate their correct implementation would attain the desired performance and potential scientific output.
The problems encountered in obtaining a wide field-of-view with large, space-based direct imaging phased telescope arrays were considered. After defining some of the critical systems issues, previous relevant work in the literature was reviewed and summarized. An extensive list was made of potential error sources and the error sources were categorized in the form of an error budget tree including optical design errors, optical fabrication errors, assembly and alignment errors, and environmental errors. After choosing a top level image quality requirment as a goal, a preliminary tops-down error budget allocation was performed; then, based upon engineering experience, detailed analysis, or data from the literature, a bottoms-up error budget reallocation was performed in an attempt to achieve an equitable distribution of difficulty in satisfying the various allocations. This exercise provided a realistic allocation for residual off-axis optical design errors in the presence of state-of-the-art optical fabrication and alignment errors. Three different computational techniques were developed for computing the image degradation of phased telescope arrays due to aberrations of the individual telescopes. Parametric studies and sensitivity analyses were then performed for a variety of subaperture configurations and telescope design parameters in an attempt to determine how the off-axis performance of a phased telescope array varies as the telescopes are scaled up in size. The Air Force Weapons Laboratory (AFWL) multipurpose telescope testbed (MMTT) configuration was analyzed in detail with regard to image degradation due to field curvature and distortion of the individual telescopes as they are scaled up in size. Harvey, James E. Unspecified Center...
This thesis presents work towards the design of a new array of Image Atmospheric Cherenkov Telescopes (IACTs) to detect multi-TeV (E > 101 2 eV) [gamma]-ray sources. The array consists of 5 telescopes in a square layout with one central telescope, known as the Pevatron eXplorer or PeX. PeX is a PeV (101 5 eV) cosmic ray explorer that aims to study and discover [gamma]-ray sources in the 1 to 500 TeV range. The initial PeX design has been influenced by the HEGRA CT-System and H.E.S.S. configurations. One important feature of multi-TeV air showers is their ability to trigger telescopes at large core distance (> 400 m). PeX will utilise large core distance events to improve the performance and illustrate the viability of a sparse array for multi-TeV [gamma]-ray astronomy. In Chapter 1, I will discuss the astrophysical motivation behind multi-TeV observations. A number of [gamma]-ray sources have shown emission that extends above 10 TeV, for example unidentified source HESS J1908-063. A new multi-TeV detector can provide a new look at the Galactic plane and work towards uncovering the origin of Galactic cosmic ray acceleration. In Chapter 2, I will look at the physics of air showers, which involves the interaction of protons and [gamma]-rays with the atmosphere to form a cascade of particles. I will discuss the lateral distribution for [gamma]-rays and show the importance of large core distance shower for multi-TeV events. Gamma-ray showers with an image size > 60pe can be detected up to 700 m away from PeX for 500 TeV showers. In Chapter 3, I introduce PeX in detail along with the simulation programs used to model it. I discuss the standard shower reconstruction algorithm (Algorithm 1) and an advanced shower reconstruction algorithm (Algorithm 3). I also introduce the image parameters that I will investigate while optimising PeX, which include; site altitude, image triggering conditions, image cleaning conditions, telescope separation and image size cut. In Chapter 4, I have optimised the PeX cell for a low altitude (0.22 km) observational site using Algorithm 1. Parameters such as telescope separation, triggering combination, cleaning combination and image size cut have been varied over a range of values to provide the optimum results for PeX. In Chapter 5, I have optimised the PeX cell for a higher altitude (1.8 km) observational site using Algorithm 1. The same parameter variations considered in Chapter 4 have been used in Chapter 5. It appears that scaling the H.E.S.S. values to appropriate values for PeX provides the near optimum results. A comparison between the site altitudes suggests that a 0.22 km altitude provides the slightly better performance for energy > 10 TeV. In Chapter 6, a new time cleaning cut has been investigated. The arrival time between photons in two adjacent pixels in the camera is used to apply an extra cut which helps mitigate night sky background. To illustrate the robustness of the time cleaning cut, various level of night sky background have been considered. These levels include: off-Galactic plane, on-Galactic plane and towards the Galactic centre. The most important result is that PeX performance with a time cleaning cut improves results when a high level of night sky background is present. For a Galactic centre level of night sky background there is a factor of 1.5 improvement in angular resolution, effective area and quality factor when a time cleaning cut is applied compared to using no time cleaning cut. In Chapter 7, Algorithm 3 has been considered. A smaller sample of parameter variations has been simulated to confirm that the same trends found in Chapters 4 and 5 appear for Algorithm 3. The site altitude and time cleaning cut have also been considered. Algorithm 3 provides a direction reconstruction improvement over Algorithm 1 especially for large core distance events which are important for PeX. In Chapter 8, I consider some possible enhancements to PeX. These enhancements include: varying pixel size and pixel arrangement in the camera, further cuts to rejection proton events and possible separation between proton and [gamma]-ray pulses. Chapter 8 also provides the flux sensitivity results for multiple PeX configurations. The final configuration and flux sensitivity for PeX is presented in this Chapter. This work shows the value of a sparse array of Cherenkov telescopes to open up the > 10 TeV energy regime.