Radio Galaxies at TeV Energies

Unlike blazars, radio galaxies have jets that are misaligned relative to our line-of-sight. This misaligned geometry provides us with a unique view of both the jet and super massive black hole. To date, four radio galaxies have been detected at TeV energies with an additional two active galactic nuclei shown to exhibit both radio galaxy and BL Lac-type properties. TeV observations of radio galaxies have revealed these objects to be fascinating, displaying ultra-fast variability and often relatively hard spectral energy distributions. This work aims to provide a review of the current state of radio galaxy observations within the context of very-high-energy γ-ray astronomy, while also highlighting that radio galaxies are excellent targets for multi-wavelength observations. A number of motivations for the continued study of radio galaxies are provided, and these are discussed with a focus on the key observational results, including implications for future observations with next-generation instruments soon to be operational

ROBAST: Development of a ROOT-Based Ray-Tracing Library for Cosmic-Ray Telescopes and its Applications in the Cherenkov Telescope Array

We have developed a non-sequential ray-tracing simulation library, ROOT-based simulator for ray tracing (ROBAST), which is aimed to be widely used in optical simulations of cosmic-ray (CR) and gamma-ray telescopes. The library is written in C++, and fully utilizes the geometry library of the ROOT framework. Despite the importance of optics simulations in CR experiments, no open-source software for ray-tracing simulations that can be widely used in the community has existed. To reduce the dispensable effort needed to develop multiple ray-tracing simulators by different research groups, we have successfully used ROBAST for many years to perform optics simulations for the Cherenkov Telescope Array (CTA). Among the six proposed telescope designs for CTA, ROBAST is currently used for three telescopes: a Schwarzschild-Couder (SC) medium-sized telescope, one of SC small-sized telescopes, and a large-sized telescope (LST). ROBAST is also used for the simulation and development of hexagonal light concentrators proposed for the LST focal plane. Making full use of the ROOT geometry library with additional ROBAST classes, we are able to build the complex optics geometries typically used in CR experiments and ground-based gamma-ray telescopes. We introduce ROBAST and its features developed for CR experiments, and show several successful applications for CTA.Comment: Accepted for publication in Astroparticle Physics. 11 pages, 10 figures, 4 table

ROBAST: Development of a Non-Sequential Ray-Tracing Simulation Library and its Applications in the Cherenkov Telescope Array

We have developed a non-sequential ray-tracing simulation library, ROot-BAsed Simulator for ray Tracing (ROBAST), which is aimed for wide use in optical simulations of cosmic-ray (CR) and gamma-ray telescopes. The library is written in C++ and fully utilizes the geometry library of the ROOT analysis framework. Despite the importance of optics simulations in CR experiments, no open-source software for ray-tracing simulations that can be widely used existed. To reduce the unnecessary effort demanded when different research groups develop multiple ray-tracing simulators, we have successfully used ROBAST for many years to perform optics simulations for the Cherenkov Telescope Array (CTA). Among the proposed telescope designs for CTA, ROBAST is currently being used for three telescopes: a Schwarzschild--Couder telescope, one of the Schwarzschild--Couder small-sized telescopes, and a large-sized telescope (LST). ROBAST is also used for the simulations and the development of hexagonal light concentrators that has been proposed for the LST focal plane. By fully utilizing the ROOT geometry library with additional ROBAST classes, building complex optics geometries that are typically used in CR experiments and ground-based gamma-ray telescopes is possible. We introduce ROBAST and show several successful applications for CTA.Comment: In Proceedings of the 34th International Cosmic Ray Conference (ICRC2015), The Hague, The Netherlands. All CTA contributions at arXiv:1508.0589

Atmospheric monitoring for the H.E.S.S. experiment using a single scattering lidar

The High Energy Stereoscopic System (H.E.S.S.) is an array of 4 telescopes located in Namibia, which use the imaging atmospheric Cherenkov technique (IACT) to study astrophysical emission of gamma radiation in the energy window from 100 GeV to 50 TeV. The calorimetric nature of the technique means that the sensitivity and energy resolution of the instrument are highly dependent on atmospheric parameters. This thesis presents the findings of atmospheric measurements taken using a 355 nm single scattering lidar. The lidar wavelength is well matched to the maximum in the Cherenkov spectrum seen by the telescopes. Monte Carlo simulation software is presented which has been developed to calculate the integral vertical lidar ratio (the ratio of extinction to backscatter) for Mie scattering by aerosols assumed to be at the H.E.S.S. site. This is found to be 29 ± 3 steradians. This ratio is used with the Fernald method to derive the probability of transmission profile, and is also compared to other lidar analysis techniques; the Klett method and the multi-angle method. The results of all 3 methods are compared to the lidar manufacturer's closed-source analysis software, with which the Klett method is found to be in strongest agreement. A model that describes the relationship between the lidar ratio and the extinction is presented. Using this with the lidar manufacturer's extinction values provides a vertical lidar ratio profile which, for the first time, provides insight into the aerosol scattering layers present at the H.E.S.S. site in Namibia. Recommendations for improvement of this research, and suggestions for incorporation of data into the H.E.S.S. analysis, have been mad

Performance studies for the Cherenkov Telescope Array (CTA) with prospects for detecting pulsed gamma-ray emission.

Currently in the design stage, the Cherenkov Telescope Array (CTA) is an advanced facility for ground-based high-energy gamma-ray astronomy. This research presents the projected system performance of CTA calculated as part of a large Monte Carlo simulation effort including air shower and telescope response simulations. The integral sensitivity of CTA’s baseline Subarray-E is found to be nearly an order of magnitude more sensitive between 100 GeV and 10 TeV compared to existing ground-based Cherenkov telescope systems. This research finds that this particular subarray achieves CTA’s goal of milli-Crab sensitivity at 1 TeV. In addition, this work uses a multi-layer perceptron neural network to separate the cosmic-ray background from the gamma-ray signal. This includes employing 5 different methods for estimating the neural network response cut, with the energy-scaled signifi- cance method being found to provide the best and most stable performance. Performance measures calculated include: energy resolution, angular resolution, effective collecting area as well as flux sensitivity. In addition, the results of three small studies are also presented. The first includes the performance results of a high-altitude (3700 m) subarray. This re- search confirms a gain in sensitivity at the lowest detectable energies under ∼100 GeV. The second and third are small technical studies on CTA’s dynamic range performance. Finally, to assess CTA’s prospects for detecting pulsed gamma-ray emission data for the Crab and Vela pulsars taken from Fermi Gamma-Ray Space Telescope observations are analysed and compared with the CTA sensitivity performance derived in this research. Results for CTA Subarray-E and Subarray-B suggest these arrays will both have sufficient sensitivity perfor- mance for detecting the Vela pulsar if it behaves as expected on the basis of the published spectra

Monte Carlo Studies of the GCT Telescope for the Cherenkov Telescope Array

The GCT is an innovative dual-mirror solution proposed for the small-size telescopes for CTA, capable of imaging primary cosmic gamma-rays from below a TeV to hundreds of TeV. The reduced plate scale resulting from the secondary optics allows the use of compact photosensors, including multi-anode photomultiplier tubes or silicon photomultipliers. We show preliminary results of Monte Carlo simulations using the packages CORSIKA and Sim_telarray, comparing the relative performance of each photosensor type. We also investigate the effect of the secondary optics in terms of optical performance, image resolution and camera response. With the ongoing commissioning of the prototype structure and camera, we present the preliminary expected performance of GCT.Comment: In Proceedings of the 34th International Cosmic Ray Conference (ICRC2015), The Hague, The Netherlands. All CTA contributions at arXiv:1508.0589

A search for Centaurus A-like features in the spectra of Fermi-LAT detected radio galaxies

Motivated by the detection of a hardening in the γ-ray spectrum of the radio galaxy CentaurusA, we have analysed ∼10 years of Fermi-LAT observations of 26 radio galaxies to search for similar spectral features. We find that the majority of the radio galaxies’ γ-ray spectral energy distributions are best fitted with a simple power-law model, and no spectral hardening similar to that found in CentaurusA was detected. We show that, had there been any such spectral features present in our sample of radio galaxies, they would have been seen, but note that 7 of the radio galaxies (3C111, 3C120, 3C264, IC4516, NGC1218, NGC2892 and PKS0625-35) show evidence for flux variability on 6-month timescales, which makes the detection of any steady spectral features difficult. We find a strong positive correlation (r = 0.9) between the core radio power at 5GHz and the γ-ray luminosity and, using a simple extrapolation to TeV energies, we expect around half of the radio galaxies studied will be detectable with the forthcoming Cherenkov Telescope Array

Design concepts for the Cherenkov Telescope Array CTA: an advanced facility for ground-based high-energy gamma-ray astronomy

Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. CTA is an international initiative to build the next generation instrument, with a factor of 5-10 improvement in sensitivity in the 100 GeV-10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA