Search

Very-high-energy gamma rays produce electron positron pairs in interactions with low-energy photons of extragalactic background light during propagation through the intergalactic medium. The electron-positron pairs generate secondary gamma rays detectable by gamma-ray telescopes. This secondary emission can be used to detect intergalactic magnetic fields (IGMF) in the voids of large-scale structure. A new gamma-ray observatory, namely, Cherenkov Telescope Array (CTA), will provide an increase in sensitivity for detections of these secondary gamma-ray emission and enable the measurement of its properties for sources at cosmological distances. The interpretation of the CTA data, including detection of IGMF and study of its properties and origins, will require precision modeling of the primary and secondary gamma-ray fluxes. We asses the precision of the modeling of the secondary gamma-ray emission using model calculations with publicly available Monte-Carlo codes CRPropa and ELMAG and compare their predictions with theoretical expectations and with model calculations of a newly developed CRbeam code. We find that model predictions of different codes differ by up to 50% for low-redshift sources, with discrepancies increasing up to order-of-magnitude level with the increasing source redshifts. We identify the origin of these discrepancies and demonstrate that after eliminating the inaccuracies found, the discrepancies between the three codes are reduced to 10% when modeling nearby sources with z~0.1. We argue that the new CRbeam code provides reliable predictions for spectral, timing and imaging properties of the secondary gamma-ray signal for both nearby and distant sources with z~1. Thus, it can be used to study gamma-ray sources and IGMF with a level of precision that is appropriate for the prospective CTA study of the effects of gamma-ray propagation through the intergalactic medium., V2: 15 pages, 12 figures, version accepted by A&A. While the paper was under review, new versions of the codes CRPropa3-3.2 and ELMAG 3.03 were released in which many of the comments presented in this paper were taken into account (see text for the details)

Dwarf spheroidal galaxies are among the best environments that can be studied with Cherenkov telescopes for indirect searches of $\gamma$-ray signals coming from dark matter self-interaction (annihilation or decay), due to their proximity and negligible background emission. We present new determinations of the dark-matter amount - i.e. the astrophysical factors $J$ and $D$ - in dwarf-galaxy halos obtained through the MCMC Jeans analysis of their brightness and kinematic data. Such factors are of great importance to test the performances of the next-generation $\gamma$-ray instruments such as the Cherenkov Telescope Array in detecting dark-matter signals from astronomical environments, or constraining the limits to dark-matter physics parameters (particle mass and lifetime, annihilation cross section)., Comment: 4 pages, 2 figures, 1 table, proceedings of the "RICAP-18 - 7th Roma International Conference on Astroparticle Physics" held on 2018 Sep 04-07 at Universit\`a di "Roma Tre", Via V. Volterra 62, I-00182 Roma, Italy (to appear on EPJ Web of Conferences)

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 Cherenkov Telescope Array (CTA) will bring a whole new insight to the gamma-ray Universe. In order to fulfill its performance requirements, we need to understand and correct the atmospheric effects that influence the acquired instrument data. One such systematic effect is due to the varying molecular density profile with time. We have studied such profiles for both CTA sites using publicly available historical data assimilation archives. Our study reveals that we can distinguish at least three differentiated seasonal periods at the northern site and at least two at the southern site, that allow to model the molecular part of the atmosphere using average profiles, as done with current Cherenkov telescope projects. Seasonal transitions are smoother at the southern site than at the northern one. Moreover, the latter shows a greater amplitude in density variations at an altitude of 15 km. We also explored deviations of the molecular profiles with respect to their mean values using a 5-years data set and concluded that they are always found within specifications.

The Large Magellanic Cloud (LMC) is a spiral galaxy, satellite of the Milky way with a high star formation activity. It represents a unique laboratory for studying an extended and spatially resolved star-forming galaxy through gamma-ray observatories. Therefore, the LMC survey is one of the key science projects for the Cherenkov Telescope Array (CTA), the next-generation ground-based gamma-ray observatory. In this document we present the work performed over the last year by the CTA working group dedicated to the LMC, in order to offer a first characterization of the LMC at TeV energies. We have performed detectability forecasts based on the expected CTA performance for all sources in the region of interest of the LMC with known emission at GeV energies and above. Based on previous observations made by Fermi-LAT and H.E.S.S. we have characterized all point sources, extended sources and diffuse emission produced by cosmic-ray propagation, extrapolating their spectra to CTA energies. Finally, we have characterized the signal expected by different annihilation mechanisms of dark matter (DM) particles within the LMC, computing the detection sensitivity curve for this target in the cross-section-to-mass plane.

A FRAM (F/(Ph)otometric Robotic Atmospheric Monitor) telescope is a system of a robotic mount, a large-format CCD camera and a fast telephoto lens that can be used for atmospheric monitoring at any site when information about the atmospheric transparency is required with high spatial or temporal resolution and where continuous use of laser-based methods for this purpose would interfere with other observations. The original FRAM has been operated at the Pierre Auger Observatory in Argentina for more than a decade, while three more FRAMs are foreseen to be used by the Cherenkov Telescope Array (CTA). The CTA FRAMs are being deployed ahead of time to characterize the properties of the sites prior to the operation of the CTA telescopes; one FRAM has been running on the planned future CTA site in Chile for a year while two others are expected to become operational before the end of 2018. We report on the hardware and current status of operation and/or deployment of all the FRAM instruments in question as well as on some of the preliminary results of integral aerosol measurements by the FRAMs in Argentina and Chile, Comment: Proceedings of AtmoHEAD 2018

The Cherenkov Telescope Array (CTA) is the major ground-based gamma-ray observatory planned for the next decade and beyond. Consisting of two large atmospheric Cherenkov telescope arrays (one in the southern hemisphere and one in the northern hemisphere), CTA will have superior angular resolution, a much wider energy range, and approximately an order of magnitude improvement in sensitivity, as compared to existing instruments. The CTA science programme will be rich and diverse, covering cosmic particle acceleration, the astrophysics of extreme environments, and physics frontiers beyond the Standard Model. This paper outlines the science goals for CTA and covers the current status of the project., 7 page, 5 figures, Proceedings of RICAP 2018

The ASTRI (Astrofisica con Specchi a Tecnologia Replicante Italiana) program was born as a collaborative international effort led by the Italian National Institute for Astrophysics (INAF) to design and realize, within the Cherenkov Telescope Array (CTA) framework, an end-to-end prototype of the Small-Sized Telescope (SST) in a dual-mirror configuration (2M). While the activities concerning the characterization of the prototype are under completion, the program entered a new phase. With the final aim of contributing at the production of the SST telescopes for the CTAO Southern site, we started the development of nine telescopes based on the evolution of the ASTRI prototype design to work as pathfinder for the CTAO. Furthermore, together with the CHEC (Compact High Energy Camera) collaboration, the ASTRI team presented a proposal, that will be evaluated with other proposals, to deliver to CTAO the complete set of SST telescopes.

The idea of using stellar photometry for atmospheric monitoring for optical experiments in high-energy astrophysics is seemingly straightforward, but reaching high precision of the order of 0.01 in the determination of the vertical aerosol optical depth (VAOD) has proven difficult. Wide-field photometry over a large span of altitudes allows a fast determination of VAOD independently of the absolute calibration of the system, while providing this calibration as a useful by-product. Using several years of data taken by the FRAM (F/(Ph)otometric Robotic Atmospheric Monitor) telescope at the Pierre Auger Observatory in Argentina and about a year of data taken by a similar instrument deployed at the planned future Southern site of the Cherenkov Telescope Array in Chile, we have developed methods to improve the precision of this measurement technique towards and possibly beyond the 0.01 mark. Detailed laboratory measurements of the response of the whole system to both the spectrum and intensity of incoming light have proven indispensable in this analysis as the usual assumption of linearity of the CCD detectors is not valid anymore for the conditions of the observations., Accepted for AtmoHEAD 2018 proceedings

The future ground-based gamma-ray observatory, the Cherenkov Telescope Array (CTA) will require reliable monitoring of the atmosphere which is an inherent part of the detector. We discuss here the implementation of the extended method of the Cherenkov Transparency Coeffcient for the atmospheric calibration for the CTA. The method estimates the atmospheric transmission of Cherenkov light, relying on the measurement of the rates of cosmic ray-induced air showers that trigger different pairs of telescopes. We examine the performance of our approach utilizing Monte Carlo simulations assuming various atmospheric conditions and CTA observation configurations.

The Computing Center of the Institute of Physics (CC IoP) of the Czech Academy of Sciences provides compute and storage capacity to several physics experiments. Most resources are used by two LHC experiments, ALICE and ATLAS. In the WLCG, which coordinates computing activities for the LHC experiments, the computing center is Tier-2. The rest of computing resources is used by astroparticle experiments like the Pierre Auger Observatory (PAO) and the Cherenkov Telescope Array (CTA) or particle experiments like NOvA and DUNE. Storage capacity is distributed to several locations. DPM servers used by the ATLAS and the PAO are all in the same server room. ALICE uses several xrootd servers located at the Nuclear Physics Institute in Rez, about 10 km away. The storage capacity for the ATLAS and the PAO is extended by resources of the CESNET (the Czech National Grid Initiative representative) located in Ostrava, more than 100 km away from the CC IoP. Storage is managed by dCache instance, which is published in the CC IoP BDII. ATLAS users can use these resources using the standard ATLAS tools in the same way as the local storage without noticing this geographical distribution. The computing center provides about 8k CPU cores which are used by the experiments based on fair-share. The CPUs are distributed amongst server rooms in the Institute of Physics, in the Faculty of Mathematics and Physics of the Charles University, and in CESNET. For the ATLAS experiment, the resources are extended by opportunistic usage of the Salomon HPC provided by the Czech national HPC center IT4Innovations in Ostrava. The HPC provides 24-core nodes. The maximum number of allowed single-node jobs in the batch system is 200. The contribution of the HPC to the CPU consumption by the ATLAS experiment is about 15% on average.

Cumulative optical and infrared emission from galaxies accumulated over cosmological time scales, the extragalactic background light (EBL), could be probed by complementary techniques of direct observations and source counting in the visible and infrared as well as via its imprint on the signal of distant active galactic nuclei in gamma rays. We compare the visible and infrared measurements with the gamma-ray constraints and study if the discrepancies of the measurements with different methods could be due to the presence of features in the EBL spectrum that are localised in the micron wavelength range. We combined data on time-averaged spectra of selected blazars that were obtained by Fermi and ground-based gamma-ray telescopes. We also modelled the effect of absorption on EBL while allowing for the existence of a previously unaccounted spectral feature. We show that a previously reported 'excess' in EBL flux in the $\sim 1$~micron wavelength range is consistent with gamma-ray measurements, that is, if the excess has the form of a narrow feature of the width $\delta \lambda < \lambda$ and an overall flux of up to 15 nW/(m$^2$ sr) above the 'minimal' EBL, which is estimated from the visible and infrared source counts. Such 'bump-like' spectral features could originate, for example, from decaying dark-matter particles, or either axions or peculiar astrophysical processes in the course of star-formation history. We discuss the possibilities for the search of spectral features in the EBL with the Cherenkov Telescope Array (CTA)., Comment: V2: 7 pages, 4 figures. Considerable changes in the blazar sample, results unchanged; discussions extended - version accepted by A&A

We simulate the spectral behaviour of the non-transient gamma-ray binaries using archival observations as a reference. With this we test the CTA capability to measure the sources' spectral parameters and detect variability on various time scales. We review the known properties of gamma-ray binaries and the theoretical models that have been used to describe their spectral and timing characteristics. We show that CTA is capable of studying these sources on time scales comparable to their characteristic variability time scales. For most of the binaries, the unprecedented sensitivity of CTA will allow the spectral evolution to be studied on a time scale as short as 30 min. This will enable a direct comparison of the TeV and lower energy (radio to GeV) properties of these sources from simultaneous observations. We also review the source-specific questions that can be addressed with such high-accuracy CTA measurements., Accepted to A&A

International audience; The High Energy Stereoscopic System (H.E.S.S.) is an array of imaging atmospheric Cherenkov telescopes (IACTs) located in Namibia. It was built to detect Very High Energy (VHE, >100 GeV) cosmic gamma rays, and consists of four 12 m diameter Cherenkov telescopes (CT1-4), built in 2003, and a larger 28 m telescope (CT5), built in 2012. The larger mirror surface of CT5 permits to lower the energy threshold of the array down to 30 GeV. The cameras of CT1-4 are currently undergoing an extensive upgrade, with the goals of reducing their failure rate, reducing their readout dead time and improving the overall performance of the array. The entire camera electronics has been renewed from ground-up, as well as the power, ventilation and pneumatics systems, and the control and data acquisition software. Technical solutions forseen for the next-generation Cherenkov Telescope Array (CTA) observatory have been introduced, most notably the readout is based on the NECTAr analog memory chip. The camera control subsystems and the control software framework also pursue an innovative design, increasing the camera performance, robustness and flexibility. The CT1 camera has been upgraded in July 2015 and is currently taking data; CT2-4 will upgraded in Fall 2016. Together they will assure continuous operation of H.E.S.S at its full sensitivity until and possibly beyond the advent of CTA. This contribution describes the design, the testing and the in-lab and on-site performance of all components of the newly upgraded H.E.S.S. camera.

The standard model of cosmology indicates that approximately 27% of the energy density of the Universe is in the form of dark matter. The nature of dark matter is an open question in modern physics. Indirect dark matter searches with imaging atmospheric Cherenkov telescopes are playing a crucial role in constraining the nature of the dark matter particle through the study of their potential annihilation that could produce very high energy gamma rays from different astrophysical structures. The Cherenkov Telescope Array will provide an unprecedented sensitivity over a range of dark matter mass from ~100 GeV to ~30 TeV. In this contribution we review the status of indirect dark matter searches at dwarf spheroidal galaxies.

This report presents a prototype Cherenkov telescope, the observation method of Cherenkov light from extensive air showers (EAS) generated by cosmic rays (CRs) above 1016 eV and preliminary observations. The interest in Cherenkov light differential detectors of EAS is caused by the possibility to measure the depth of the cascade maximum, X max , and/or the shower age via angular and temporal distributions of the Cherenkov signal. In particular, it was shown, using EAS model simulations, that the pulse width measured at the periphery of the shower, r > 300 m, at sea level is pronouncedly connected with X max . The Cherenkov detector is a wide-angle telescope working in coincidence with scintillation detectors, integral and differential Cherenkov detectors at the Yakutsk complex EAS.

Atmospheric monitoring is a field of special importance for astroparticle physics, especially for Imaging Atmospheric Cherenkov Telescopes (IACTs) as clouds will absorb and scatter the Cherenkov photons of air showers. Conventional tools used for atmospheric monitoring (e.g. LIDAR) are very expensive and monitor only a small part of the sky at once. Therefore, they are not suitable to perform a wide scan of the sky which is necessary to detect clouds in advance. This article gives a short overview about a method that uses an all sky camera with a 180 ° field of view to identify the cloud distribution by measuring the absorption of star light. It can be used to assign a sky quality rating to single spots, arbitrary regions or the whole sky at once within a 1 min exposure time. A cloud map can be created from the available data that can be used to determine shape and dimension of clouds and to predict their movement. The resulting data can be used by a scheduling algorithm or the operating crew to point the telescope to a different source before the current source gets covered by clouds. The all sky cameras used so far are located on La Palma at the observatory Roque de los Muchachos close to the telescopes FACT and MAGIC and the planned northern CTA site.

The Italian National Institute for Nuclear Physics (INFN) is involved in the development of a prototype for a camera based on Silicon Photomultipliers (SiPMs) for the Cherenkov Telescope Array (CTA), a new generation of telescopes for ground-based gamma-ray astronomy. In this framework, an R&D program within the ‘Progetto Premiale TElescopi CHErenkov made in Italy (TECHE.it)’ for the development of SiPMs suitable for Cherenkov light detection in the Near-Ultraviolet (NUV) has been carried out. The developed device is a NUV High-Density (NUV-HD) SiPM based on a micro cell of 30 μ m × 30 μ m and an area of 6 mm × 6 mm, produced by Fondazione Bruno Kessler (FBK). A full characterization of the single NUV-HD SiPM will be presented. A matrix of 8 × 8 single NUV-HD SiPMs will be part of the focal plane of the Schwarzschild- Couder Telescope prototype (pSCT) for CTA. An update on recent tests on the detectors arranged in this matrix configuration and on the front-end electronics will be given.

The Cherenkov Telescope Array (CTA) is a project to build a new generation ground-based gammaray observatory. The FRAM – F(/Ph)otometric Robotic Atmospheric Monitor – is a device, which can provide it with the Vertical Aerosol Optical Depth (VAOD) maps in the field of view of the CTA in almost real time (delay in minutes). The FRAM is a small fast robotic astronomical telescope with a large field of view and a sensitive CCD camera. It uses stellar photometry to measure atmospheric extinction across the field of view and uses it to calculate the VAOD. In this contribution, results of first few months of operations of the prototype and outlook for deployment at the CTA site will be presented.

The Cherenkov Telescope Array (CTA) in the southern hemisphere will be installed at Armazones 2k site in northern Chile. Scarce atmospheric observations are available in the region, particularly radiosonde data. This study analyzes radiosondes launched at Paranal observatory, located at about 21 km from the CTA site, from 24 October and 4 November 2011, to understand the behavior of density in the atmosphere near the CTA site. High-resolution numerical simulations with the Weather Research and Forecasting (WRF) model are validated with Paranal radiosondes to quantify its ability to represent the atmospheric conditions in the region. In addition, the seasonal and diurnal evolution of atmospheric density at the CTA site were studied during 2011 using the high-resolution weather forecasts from the WRF model.

In addition to its relevant astrophysical and cosmological significance, the Extragalactic Background Light (EBL) is a fundamental source of opacity for cosmic high energy photons, as well as a limitation for the propagation of high-energy particles in the Universe. We review our previously published determinations of the EBL photon density in the Universe and its evolution with cosmic time, in the light of recent surveys of IR sources at long wavelengths. We exploit deep survey observations by the Herschel Space Observatory and the Spitzer telescope, matched to optical and near-IR photometric and spectroscopic data, to re-estimate number counts and luminosity functions longwards of a few microns, and the contribution of resolved sources to the EBL. These new data indicate slightly lower photon densities in the mid- and far-infrared and sub-millimeter compared to previous determinations. This implies slightly lower cosmic opacity for photon-photon interactions. The new data do not modify previously published EBL modeling in the UV-optical and near-IR up to several microns, while reducing the photon density at longer wavelengths. This improved model of the EBL alleviates some tension that had emerged in the interpretation of the highest-energy TeV observations of local \textit{blazar}s, reducing the case for new physics beyond the standard model (like violations of the Lorenz Invariance, LIV, at the highest particle energies), or for exotic astrophysics, that had sometimes been called for to explain it. Applications of this improved EBL model on current data are considered, as well as perspectives for future instrumentation, the Cherenkov Telescope Array (CTA) in particular., Comment: 19 pages, tables included, to appear in Astronomy and Astrophysics

A proposed new method for measuring the cherenkov light from extensive air showers (EAS) of cosmic rays (CR), which allows to determine not only the primary particle energy and angle of arrival, but also the parameters of the shower in the atmosphere – the maximum depth and “age”. For measurements it is proposed to use Cherenkov light produced by EAS in a ground network of wide-angle telescopes which are separated from each other by a distance 100–300 m depending on the total number of telescopes operating in coincidence, acting autonomously, or includes a detector of the charged components, radio waves, etc. as part of the EAS. Results of such an array should be developed. The energy measurement and CR angle of the arrival data on the depth of the maximum and the associated mass of the primary particle generating by EAS. This is particularly important in the study of galactic cosmic ray for E > 1014 eV, where currently there are no direct measurements of the maximum depth of the EAS.

Silicon Photomultipliers (SiPMs) are excellent devices to detect the faint and short Cherenkov light emitted in high energy atmospheric showers, and therefore suitable for use in imaging air Cherenkov Telescopes. The high density Near Ultraviolet Violet SiPMs (NUV-HD3) produced by Fondazione Bruno Kessler (FBK) in collaboration with INFN were used to equip optical modules for a possible upgrade of the Schwarzschild-Couder Telescope camera prototype, in the framework of the Cherenkov Telescope Array project. SiPMs are 6×6 mm2 devices based on 40×40 μm2 microcells optimized for photo-detection at the NUV wavelengths. More than 40 optical modules, each composed by a 4×4 array of SiPMs, were assembled. In this contribution we report on the development and on the assembly of the optical modules, their validation and integration in the camera.

The Cherenkov Telescope Array (CTA) is a large collaborative effort aimed at the design and operation of an observatory dedicated to very high-energy gamma-ray astrophysics in the energy range from a few tens of GeV to above 100 TeV, which will yield about an order of magnitude improvement in sensitivity with respect to the current major arrays (H.E.S.S., MAGIC, and VERITAS). Within this framework, the Italian National Institute for Astrophysics is leading the ASTRI project, whose main goals are the design and installation on Mt. Etna (Sicily) of an end-to-end dual-mirror prototype of the CTA small size telescope (SST) and the installation at the CTA Southern site of a dual-mirror SST mini-array composed of nine units with a relative distance of about 300 m. The innovative dual-mirror Schwarzschild-Couder optical solution adopted for the ASTRI Project allows us to substantially reduce the telescope plate-scale and, therefore, to adopt silicon photo-multipliers as light detectors. The ASTRI mini-array is a wider international effort. The mini-array, sensitive in the energy range 1-100 TeV and beyond with an angular resolution of a few arcmin and an energy resolution of about 10-15%, is well suited to study relatively bright sources (a few $\times 10^{-12}$erg cm$^{-2}$s$^{-1}$ at 10 TeV) at very high energy. Prominent sources such as extreme blazars, nearby well-known BL Lac objects, Galactic pulsar wind nebulae, supernovae remnants, micro-quasars, and the Galactic Center can be observed in a previously unexplored energy range. The ASTRI mini-array will extend the current IACTs sensitivity well above a few tens of TeV and, at the same time, will allow us to compare our results on a few selected targets with those of current (HAWC) and future high-altitude extensive air-shower detectors., Proceedings of the "The Roma International Conference on Astroparticle Physics (RICAP) 2014". Submitted to EPJ Web of Conferences. 5 pages, 5 figures. ((1) INAF/IASF Palermo, (2) http://www.brera.inaf.it/astri/, (3) https://portal.cta-observatory.org)

Very high energy gamma-ray astronomy is now bringing an invaluable contribution to the understanding of violent phenomena in the Universe, as well as the search for exotic physics such as indirect detection of dark matter or a test of Lorentz invariance violation. The current Imaging Arrays of Cherenkov Telescopes (IACT) show that this technique is mature. In Europe, the community is gathering around the Cherenkov Telescope Array consortium, to design and build the next generation ground-based array. It should reach an order of magnitude in sensitivity in a wide energy band, ranging from 10 GeV to more than 100 TeV. This goal can be achieved with an array of 50–100 telescopes of various sizes at various spacings. With about 2000 channels per camera, a specific effort has to be made to design front-end electronics with a lower cost and better performances. A gain in cost and performances can be obtained by maximising the integration of the front-end electronics in an ASIC. The amplifiers, analogue memories, digitization and first level buffering can be embedded in the same component. We present here the NECTAr project aiming at building a demonstrator element of a generic camera built around this component.

The Cherenkov Telescope Array (CTA) is the next generation facility of Imaging Atmospheric Cherenkov Telescopes. It will reach unprecedented sensitivity and energy resolution in very-high-energy gamma-ray astronomy. CTA will detect Cherenkov light emitted within an atmospheric shower of particles initiated by cosmic-gamma rays or cosmic rays entering the Earth's atmosphere. From the combination of images the Cherenkov light produces in the telescopes, one is able to infer the primary particle energy and direction. A correct energy estimation can be thus performed only if the local atmosphere is well characterized. The atmosphere not only affects the shower development itself, but also the Cherenkov photon transmission from the emission point in the particle shower, at about 10-20 km above the ground, to the detector. Cherenkov light on the ground is peaked in the UV-blue region, and therefore molecular and aerosol extinction phenomena are important. The goal of CTA is to control systematics in energy reconstruction to better than 10%. For this reason, a careful and continuous monitoring and characterization of the atmosphere is required. In addition, CTA will be operated as an observatory, with data made public along with appropriate analysis tools. High-level data quality can only be ensured if the atmospheric properties are consistently and continuously taken into account. In this contribution, we concentrate on discussing the implementation strategy for the various atmospheric monitoring instruments currently under discussion in CTA. These includes Raman lidars and ceilometers, stellar photometers and others available both from commercial providers and public research centres., (6 pages, 2 figures, Proceedings of the 2nd AtmoHEAD Conference, Padova, Italy May 19-21, 2014)

The Automatic Telescope for Optical Monitoring (ATOM) for H.E.S.S. is an 75 cm optical telescope which operates fully automated. As there is no observer present during observation, an auxiliary all-sky camera serves as weather monitoring system. This device takes an all-sky image of the whole sky every three minutes. The gathered data then undergoes live-analysis by performing astrometric comparison with a theoretical night sky model, interpreting the absence of stars as cloud coverage. The sky monitor also serves as tool for a meteorological analysis of the observation site of the the upcoming Cherenkov Telescope Array. This overview covers design and benefits of the all-sky camera and additionally gives an introduction into current efforts to integrate the device into the atmosphere analysis programme of H.E.S.S.

The All Sky Camera (ASC) for CTA Site characterization is an automated optical instrument providing optical observation of the night sky conditions above the future Cherenkov Telescope Array observatory. It provides the basic sky condition analysis like cloud coverage, cloud position, nightsky background in visible light wave range and basic raw photometry. The result will serve for further Monte Carlo design studies and calculations. The ASC and additional auxiliary instruments (e.g. Ceilometer) will provide input data for smart scheduler during the standard measurement procedure of the CTA observatory.

The Cherenkov Telescope Array (CTA) is an observatory for ground-based gamma-ray astronomy currently under construction, which will observe photons with very high energies (20 GeV – 300 TeV). One of the main contributions to the systematic uncertainties stems from the uncertainty on the atmospheric density profile, of molecules and aerosols. To minimize these systematics a full calibration of the atmospheric properties is important as well as a calibration of the detector response. In the paper we introduce the strategy for atmospheric simulations, use of Monte Carlo simulations and available CTA computing resources. We also describe in more detail realized and planned atmospheric simulations as well as the Czech contribution to this effort.

We describe a White Rabbit Facility for time synchronization we are preparing for the future Cherenkov Telescope Array observatory

The main contribution to the systematic uncertainties of Atmospheric Imaging Cherenkov Telescopes (IACTs) currently stems from the uncertainty in the determination of atmospheric properties for a given moment of observation. This technique uses the atmosphere as a calorimeter of gamma-ray induced atmospheric air showers and measures the amount of Cherenkov light collected by large mirrors and focused towards a pixellized camera. Atmospheric conditions affect the measured Cherenkov light yield in several ways: the air-shower development itself, the variation of the Cherenkov angle with altitude and the loss of photons due to scattering and absorption of Cherenkov light. Although IACTs use to be located at astronomical sites, characterized by extremely clear atmospheric conditions, the local atmosphere should be continuously monitored, in terms of molecular density profiles, aerosol extinction profiles, and clouds. Moreover, a general understanding of the site and particularly aerosol climatology is desirable for a robust interpretation of the sensing instruments’ data. Here, the strategy of the Cherenkov Telescope Array (CTA) on atmospheric monitoring is presented, designed to ensure that the related systematic uncertainties are brought down by at least a factor of two with respect to current installations. Additionally, a more intelligent scheduling scheme is aimed at, where source observations are selected using dedicated auxiliary atmospheric monitoring instruments and taking into account the feasibility of a later analysis according to the underlying scientific case. This plan should reduce data loss during offline data selection and enhance the effective duty cycle of the CTA.

We present the current knowledge and activities to assess the climatology of the “Observatorio del Roque de los Muchachos” (ORM), selected to host the Northern observatory of the CTA, with particular emphasis on molecular density profiles and aerosol extinction.

The Large Area Telescope (LAT) on the Fermi satellite is expected to publish a catalogue with more than 100 Gamma-Ray Bursts (GRBs) detected above 100 MeV thanks to a new detection algorithm and a new event reconstruction. This work aims at revising the prospects for GRB alerts with the Cherenkov Telescope Array (CTA) based on the new LAT results. We start considering the simulation of the observations with the full CTA of two extremely bright events, the long GRB 130427A and the short GRB 090510, then we investigate how these GRBs would be observed by a particular configuration of the array with the telescopes pointing to different directions in what is called the "coupled divergent mode".

The long gamma-ray burst (GRB) 100621A, at the time the brightest X-ray transient ever detected by Swift-XRT in the $0.3\textrm{--}10$ keV range, has been observed with the H.E.S.S. imaging air Cherenkov telescope array, sensitive to gamma radiation in the very-high-energy (VHE, $>100$ GeV) regime. Due to its relatively small redshift of $z\sim0.5$, the favourable position in the southern sky and the relatively short follow-up time ($, 8 pages, 2 figures. Published in A&A

About 40% of the observation time of the High Energy Stereoscopic System (H.E.S.S.) is dedicated to studying active galactic nuclei (AGN), with the aim of increasing the sample of known extragalactic very-high-energy (VHE, E>100 GeV) sources and constraining the physical processes at play in potential emitters. H.E.S.S. observations of AGN, spanning a period from April 2004 to December 2011, are investigated to constrain their gamma-ray fluxes. Only the 47 sources without significant excess detected at the position of the targets are presented. Upper limits on VHE fluxes of the targets were computed and a search for variability was performed on the nightly time scale. For 41 objects, the flux upper limits we derived are the most constraining reported to date. These constraints at VHE are compared with the flux level expected from extrapolations of Fermi-LAT measurements in the two-year catalog of AGN. The H.E.S.S. upper limits are at least a factor of two lower than the extrapolated Fermi-LAT fluxes for 11 objects. Taking into account the attenuation by the extragalactic background light reduces the tension for all but two of them, suggesting intrinsic curvature in the high-energy spectra of these two AGN. Compilation efforts led by current VHE instruments are of critical importance for target-selection strategies before the advent of the Cherenkov Telescope Array, CTA., 10 pages, 5 figures, 5 tables, accepted in A&A

A new method is presented for an absolute energy calibration of imaging atmospheric Cherenkov telescopes as a whole instrument at all relevant wave- lengths. For this study the Cherenkov telescope #1 of the HEGRA 1 experiment was used. The calibration was carried out by detecting the light of dierent selected stars during one night. From the data a simple linear correlation was derived between the anode current of the Cherenkov camera photomultiplier tubes and the incident photon flux. This result is important for the imaging air Cherenkov technique in several aspects: The method enables to determine the response function of Cherenkov telescopes in the energetic range of Cherenkov light quickly and reliably. Therefore it can be used as an atmospheric calorimeter by measuring the absolute integrated flux of Cherenkov photons. Moreover, the signal of a point run can be predicted and a simulation of the apparatus response can be carried out more reliably.

Very high-energy gamma-ray emission from PKS 0447-439 was detected with the H.E.S.S. Cherenkov telescope array in December 2009. This blazar is one of the brightest extragalactic objects in the Fermi Bright Source List and has a hard spectrum in the MeV to GeV range. In the TeV range, a photon index of 3.89 +- 0.37 (stat) +- 0.22 (sys) and a flux normalisation at 1 TeV, Phi(1 TeV) = (3.5 +- 1.1 (stat) +- 0.9 (sys)) x 10^{-13} cm^{-2} s^{-1} TeV^{-1}, were found. The detection with H.E.S.S. triggered observations in the X-ray band with the Swift and RXTE telescopes. Simultaneous UV and optical data from Swift UVOT and data from the optical telescopes ATOM and ROTSE are also available. The spectrum and light curve measured with H.E.S.S. are presented and compared to the multi-wavelength data at lower energies. A rapid flare is seen in the Swift XRT and RXTE data, together with a flux variation in the UV band, at a time scale of the order of one day. A firm upper limit of z < 0.59 on the redshift of PKS 0447-439 is derived from the combined Fermi-LAT and H.E.S.S. data, given the assumptions that there is no upturn in the intrinsic spectrum above the Fermi-LAT energy range and that absorption on the Extragalactic Background Light (EBL) is not weaker than the lower limit provided by current models. The spectral energy distribution is well described by a simple one-zone Synchrotron Self-Compton (SSC) scenario, if the redshift of the source is less than z, Comment: 14 pages, 10 figures, accepted for publication in A&A

The field of gamma-ray astronomy has seen important progress during the last decade, yet there exists so far no common software framework for the scientific analysis of gamma-ray telescope data. We propose to fill this gap by means of the GammaLib software, a generic library that we have developed to support the analysis of gamma-ray event data. GammaLib has been written in C++ and all functionality is available in Python through an extension module. On top of this framework we have developed the ctools software package, a suite of software tools that enables building of flexible workflows for the analysis of Imaging Air Cherenkov Telescope event data. The ctools are inspired by science analysis software available for existing high-energy astronomy instruments, and they follow the modular ftools model developed by the High Energy Astrophysics Science Archive Research Center. The ctools have been written in Python and C++, and can be either used from the command line, via shell scripts, or directly from Python. In this paper we present the GammaLib and ctools software versions 1.0 that have been released end of 2015. GammaLib and ctools are ready for the science analysis of Imaging Air Cherenkov Telescope event data, and also support the analysis of Fermi-LAT data and the exploitation of the COMPTEL legacy data archive. We propose to use ctools as the Science Tools software for the Cherenkov Telescope Array Observatory., Comment: 19 pages, 10 figures, accepted for publication in A&A, corrected X axis units in Figure 10

Several projects planned or proposed can significantly expand our knowledge of the high-energy Universe in gamma rays. Construction of the Cherenkov telescope array CTA is started, and other detectors are planned which will use the reconstruction of extensive air showers. This report explores the near future, and possible evolutions in a longer term., Comment: Invited talk at VLVnT 2015, Roma La Sapienza, 14-16 September 2015

Context. Accreting black holes in galactic X-ray sources are surrounded by hot plasma. The innermost part of these systems is likely a corona with different temperatures for ions and electrons. In the so-called low-hard state, hot electrons Comptonize soft X-ray photons from the disk that partially penetrates the corona, producing emission up to ∼150 keV, well beyond the expectations for an optically thick disk of maximum temperature ∼107 K. However, sources such as Cygnus X-1 produce steady emission up to a few MeV, which is indicative of a non-thermal contribution to the spectral energy distribution. Aims. We study the radiative output produced by the injection of non-thermal (both electron and proton) particles in a magnetized corona around a black hole. Methods. Energy losses and maximum energies are estimated for all types of particles in a variety of models, characterized by different kinds of advection and relativistic proton content. Transport equations are solved for primary and secondary particles, and spectral energy distributions are determined and corrected by internal absorption. Results. We show that a local injection of non-thermal particles can account for the high energy excess observed in some sources, and we predict the existence of a high-energy bump at energies above 1 TeV, and typical luminosities of ∼1033 erg s-1. Conclusions. High-energy instruments such as the future Cherenkov Telescope Array (CTA) can be used to probe the relativistic particle content of the coronae around galactic black holes., Facultad de Ciencias Astronómicas y Geofísicas

Context. Blazars are the dominant type of extragalactic sources at microwave and at γ-ray energies. In the most energetic part of the electromagnetic spectrum (E ≳ 100 GeV) a high fraction of high Galactic latitude sources are blazars of the high synchrotron peaked (HSP) type, that is BL Lac objects with synchrotron power peaking in the UV or in the X-ray band. Building new large samples of HSP blazars is key to understand the properties of jets under extreme conditions, and to study the demographics and the peculiar cosmological evolution of these sources. Aims: High synchrotron peaked blazars are remarkably rare, with only a few hundreds of them expected to be above the sensitivity limits of currently available surveys, some of which include hundreds of millions of sources. To find these very uncommon objects, we have devised a method that combines ALLWISE survey data with multi-frequency selection criteria. Methods: The sample was defined starting from a primary list of infrared colour-colour selected sources from the ALLWISE all sky survey database, and applying further restrictions on IR-radio and IR-X-ray flux ratios. Using a polynomial fit to the multi-frequency data (radio to X-ray), we estimated synchrotron peak frequencies and fluxes of each object. Results: We assembled a sample including 992 sources, which is currently the largest existing list of confirmed and candidates HSP blazars. All objects are expected to radiate up to the highest γ-ray photon energies. In fact, 299 of these are confirmed emitters of GeV γ-ray photons (based on Fermi-LAT catalogues), and 36 have already been detected in the TeV band. The majority of sources in the sample are within reach of the upcoming Cherenkov Telescope Array (CTA), and many may be detectable even by the current generation of Cherenkov telescopes during flaring episodes. The sample includes 425 previously known blazars, 151 new identifications, and 416 HSP candidates (mostly faint sources) for which no optical spectra is available yet. The full 1WHSP catalogue is online at http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/579/A34 and also available at http://www.asdc.asi.it/1whsp/

The All Sky Camera (ASC) is a passive non-invasive imaging system for rapid night sky atmosphere monitoring. By design, the operation of the ASC will not affect the measurement procedure of the CTA observatory, for which we discuss its application in this report. The data collected should enable improved productivity and increased measurement time for the CTA observatory. The goal of ASC is to identify cloud position, atmosphere attenuation and time evolution of the sky condition, working within the CTA Central Calibration Facilities (CCF) group. Clouds and atmosphere monitoring may allow near-future prediction of the night-sky quality, helping scheduling. Also, in the case of partly cloudy night sky the cameras will identify the uncovered regions of the sky during the operation time, and define potential observable sources that can be measured. By doing so, a higher productivity of the CTA observatory measurements may be possible.

The effective observation time with the Cherenkov telescopes arrays is limited to clear sky conditions due to considerable absorption of Cherenkov light by the possible presence of clouds. However below the cloud altitude the primary particles with high energies can still produce enough Cherenkov photons to allow the detection by the large telescopes. In this paper, using the standard CORSIKA code, we investigate the changes of image parameters due to the absorption of Cherenkov radiation by the cloud (for γ-ray and proton showers with various energies – from 2 TeV to 100 TeV and from 10 TeV to 200 TeV, respectively). We consider the clouds with different transmissions located at various altitudes above the ground level (between 8 km and 3 km). We show that, for both simulated primary particles at fixed energy, the WIDTH and the DIST distributions are shifted towards larger values in the presence of clouds in comparison to the clear sky simulations. This shift decreases with the cloud altitude. The LENGTH distributions are shifted towards smaller values for images of primary γ-rays, while for primary protons this shift is not observed. We conclude that the large Cherenkov telescopes with large camera FOV could be used for observation of γ-ray showers with high energies in the presence of clouds.

The Cherenkov Telescope Array (CTA) is a project to build a new generation ground-based gamma-ray observatory. Among other goals, the project aims to achieve high precision of gamma-ray energy measurement while maximizing the use of observation time. These objectives require detailed and fast information about atmospheric conditions, particularly the transparency which includes cloudiness (including thin high-altitude clouds) and aerosol content. This knowledge is required not only to select and calibrate data after observation, but also to make on-the-fly scheduling decisions. To provide such data without interfering with the observation (as would be the case when using laser-based methods), we propose to use the “FRAM” (F(/Ph)otometric Robotic Atmospheric Monitor) device, which is a small robotic astronomical telescope with a large field of view and a sensitive CCD camera. FRAM will use stellar photometry to measure atmospheric extinction across the field of view of the CTA. The fast robotic mount of the telescope allows quick observation of multiple fields when the array is split and even a check of the conditions in the directions of the upcoming observations. The FRAM concept is built upon experience gained with a similar device operated at the Pierre Auger Observatory.

The supernova remnant (SNR) RX J0852.0-4622 (Vela Jr., G266.6-1.2) is one of the most important SNRs for investigating the acceleration of multi-TeV particles and the origin of Galactic cosmic rays because of its strong synchrotron X-ray and TeV γ-ray emission, which show a shell-like morphology similar to each other. Using the XMM-Newton archival data consisting of multiple pointing observations of the northwestern rim of the remnant, we investigate the spatial properties of the nonthermal X-ray emission as a function of distance from an outer shock wave. All X-ray spectra are well reproduced by an absorbed power-law model above 2 keV. It is found that the spectra show gradual softening from a photon index Γ= 2.56 in the rim region to Γ= 2.96 in the interior region. We show that this radial profile can be interpreted as a gradual decrease of the cutoff energy of the electron spectrum due to synchrotron cooling. By using a simple spectral evolution model that includes continuous synchrotron losses, the spectral softening can be reproduced with the magnetic field strength in the post-shock flow to less than several tens of μG. If this is a typical magnetic fieldin theSNRshell, γ-rayemission would be accounted for by inverse Compton scattering of high-energy electrons that also produce the synchrotron X-ray emission. Future hard X-ray imaging observations with Nustar and ASTRO-H and TeV γ-ray observations with the Cherenkov Telescope Array (CTA) will allow us to explore other possible explanations of the systematic softening of the X-ray spectra.