K. Noda

ROBAST: Development of a ROOT-based ray-tracing library for cosmic-ray telescopes and its applications in the Cherenkov Telescope Array

Abstract 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.

First study of Mrk501 through the eyes of NuSTAR, VERITAS and the

The blazar Mrk 501 is among the brightest X-ray and TeV sources in the sky, and among the few sources whose spectral energy distributions can be characterized by current instruments with relatively short observations (minutes to hours). In 2013, we organized an extensive multi-instrument campaign including Fermi-LAT, MAGIC, VERITAS, F-GAMMA, Swift, GASP-WEBT, and other groups and instruments which provided the most detailed temporal and energy coverage on Mrk 501 to date. This campaign included, for the first time, observations with the Nuclear Stereoscopic Telescope Array (NuSTAR), a satellite mission launched in 2012. NuSTAR provides unprecedented sensitivity in the hard X-ray range 3-79 keV, which, together with very high energy (VHE; >100 GeV) observations, is crucial to probe the highest energy electrons in Mrk 501. The campaign covered a few day long flaring activity in July 2013 which could be studied with strictly simultaneous NuSTAR and MAGIC observations. A large fraction of the MAGIC data during this activity was affected by hazy atmospheric conditions, due to a sand layer from the Saharan desert. These data would have been removed in any standard Cherenkov Telescope data analysis. MAGIC has developed a technique to correct for adverse atmospheric conditions, making use of information from the LIDAR facility at the MAGIC site, and applies an event-by-event correction to recover data affected by adverse weather conditions. This is the first time that LIDAR information has been used to produce a physics result with Cherenkov Telescope data taken during adverse atmospheric conditions, and hence sets a precedent for current and future ground-based gamma-ray instruments. In this contribution we report the observational results, focusing on the LIDAR-corrected MAGIC data and the strictly simultaneous NuSTAR and MAGIC/VERITAS data, and discuss the scientific implications.

\textit{LIDAR-corrected}

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.

eyesight of MAGIC

The Large Size Telescope (LST) of the Cherenkov Telescope Array (CTA) is designed to achieve a threshold energy of 20GeV. The LST optics is composed of one parabolic primary mirror 23m in diameter and 28m focal length. The reflector dish is segmented in 198 hexagonal, 1.51m flat to flat mirrors. The total effective reflective area, taking into account the shadow of the mechanical structure, is about 368m 2 . The mirrors have a sandwich structure consisting of a glass sheet of 2.7mm thickness, aluminum honeycomb of 60mm thickness, and another glass sheet on the rear, and have a total weight about 47kg. The mirror surface is produced using a sputtering deposition technique to apply a 5-layer coating, and the mirrors reach a reflectivity of � 94% at peak. The mirror facets are actively aligned during operations by an active mirror control system, using actuators, CMOS cameras and a reference laser. Each mirror facet carries a CMOS camera, which measures the position of the light spot of the optical axis reference laser on the target of the telescope camera. The two actuators and the universal joint of each mirror facet are respectively fixed to three neighboring joints of the dish space frame, via specially designed interface plate.

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

Gamma-ray bursts (GRBs) are the most luminous explosions in the Universe, yet many of their basic properties remain poorly understood, particularly for short GRBs with durations less than ~ 2 sec. Fermi/LAT has shown that some GRBs emit at high-energy (100 MeV to ~ 100 GeV) gamma-rays with a hard (index of 100 GeV) gamma-rays. In particular MAGIC telescopes were designed to explore this particular physics case. Although no firm detection has been reported so far, the MAGIC Collaboration reported a hint of a VHE gamma-ray emissions from a short, very nearby (z = 0.16) GRB 160821B. Even if it is only a hint, this creates doubts on the the standard expectations for gamma-ray emissions from GRBs: low energy (~ 30 GeV) over a short period (< 100 s). Moreover GRB 160821B showed a clear extended emission in the X-ray band, which can be generated by a ms pulsar after a NS-NS or NS-BH merger. In this picture a long activity with multiple Lorenz factors is well expected, which can consistently explain a possible long VHE emission. In this contribution we will briefly report on the hint of the signal and on possible interpretations of the data assuming that the hint is real.

The Optical System for the Large Size Telescope of the Cherenkov Telescope Array

We have developed a prototype hexagonal light concentrator for the Large-Sized Telescopes of the Cherenkov Telescope Array. To maximize the photodetection efficiency of the focal-plane camera pixels for atmospheric Cherenkov photons and to lower the energy threshold, a specular film with a very high reflectance of 92–99% has been developed to cover the inner surfaces of the light concentrators. The prototype has a relative anode sensitivity (which can be roughly regarded as collection efficiency) of about 95 to 105% at the most important angles of incidence. The design, simulation, production procedure, and performance measurements of the light-concentrator prototype are reported.