The influence of systematic errors on the calculation of the statistical significance of a $γ$-ray signal with the frequently invoked Li and Ma method is investigated. A simple criterion is derived to decide whether the Li and Ma method can be applied in the presence of systematic errors. An alternative method is discussed for cases where systematic errors are too large for the application of the original Li and Ma method. This alternative method reduces to the Li and Ma method when systematic errors are negligible. Finally, it is shown that the consideration of systematic errors will be important in many analyses of data from the planned Cherenkov Telescope Array.
Compilation of CTA contributions to the proceedings of the 2012 Heidelberg Symposium on High Energy Gamma-Ray Astronomy (Gamma 2012), which took place in July 9-13, 2012, in Heidelberg, Germany
It has been suggested that underwater neutrino telescopes could detect muons from gamma ray showers. Antares' ability to detect high energy muons produced by TeV photons is discussed in the light of a full Monte Carlo study. It is shown that currently known sources would be hardly detectable
Recent detections of gravitational wave signals and neutrinos from gamma-ray sources have ushered in the era of multi-messenger astronomy, while highlighting the importance of gamma-ray observations for this emerging field. AMEGO-X, the All-sky Medium Energy Gamma-Ray Observatory eXplorer, is an MeV gamma-ray instrument that will survey the sky in the energy range from hundreds of keV to one GeV with unprecedented sensitivity. AMEGO-X will detect gamma-ray photons both via Compton interactions and pair production processes, bridging the "sensitivity gap" between hard X-rays and high-energy gam…
The present issue is the first of of a two-volume review devoted to gamma-ray astronomy above 100 MeV which has witnessed considerable progress over the last 20 years. The motivations for research in this area are explained, the follow-on articles of these two thematic issues are introduced and a brief history of the field is given.
The field of TeV gamma-ray astronomy has produced many exciting results over the last decade. Both the source catalogue, and the range of astrophysical questions which can be addressed, continue to expand. This article presents a topical review of the field, with a focus on the observational results of the imaging atmospheric Cherenkov telescope arrays. The results encompass pulsars and their nebulae, supernova remnants, gamma-ray binary systems, star forming regions and starburst and active galaxies.
Transient and variable phenomena in astrophysical sources are of particular importance to understand the underlying gamma-ray emission processes. In the very-high energy gamma-ray domain, transient and variable sources are related to charged particle acceleration processes that could for instance help understanding the origin of cosmic-rays. The imaging atmospheric Cherenkov technique used for gamma-ray astronomy above $\sim 100$ GeV is well suited for detecting such events. However, the standard analysis methods are not optimal for such a goal and more sensitive methods are specifically devel…
We describe a straightforward modification of frequently invoked methods for the determination of the statistical significance of a gamma-ray signal observed in a counting process. A simple criterion is proposed to decide whether a set of measurements of the numbers of photons registered in the source and background regions is consistent with the assumption of a constant source activity. This method is particularly suitable for immediate evaluation of the stability of the observed gamma-ray signal. It is independent of the exposure estimates, reducing thus the impact of systematic inaccuracies…
The field of gamma-ray astronomy has experienced impressive progress over the last decade. Thanks to the advent of a new generation of imaging air Cherenkov telescopes (H.E.S.S., MAGIC, VERITAS) and thanks to the launch of the Fermi-LAT satellite, several thousand gamma-ray sources are known today, revealing an unexpected ubiquity of particle acceleration processes in the Universe. Major scientific challenges are still ahead, such as the identification of the nature of Dark Matter, the discovery and understanding of the sources of cosmic rays, or the comprehension of the particle acceleration …
Very High Energy (VHE) gamma rays constitute one of the main pillars of high energy astrophysics. Gamma rays are produced under extreme relativistic conditions in the Universe. VHE gamma$ rays can be detected indirectly on the ground. Detection of these energetic photons poses several technological challenges. Firstly, even though gamma rays are highly penetrative, the Earth's atmosphere is opaque to them. Secondly, these gamma rays are to be detected against the overwhelming background of cosmic rays. When a VHE gamma ray arrives at the top of the atmosphere it produces charged secondaries. T…
Following the discovery of the cosmic rays by Victor Hess in 1912, more than 70 years and numerous technological developments were needed before an unambiguous detection of the first very-high-energy gamma-ray source in 1989 was made. Since this discovery the field on very-high-energy gamma-ray astronomy experienced a true revolution: A second, then a third generation of instruments were built, observing the atmospheric cascades from the ground, either through the atmospheric Cherenkov light they comprise, or via the direct detection of the charged particles they carry. Present arrays, 100 tim…
In recent years, Imaging Atmospheric Cherenkov Telescopes (IACTs) have discovered a rich diversity of very high energy (VHE, > 100 GeV) gamma-ray emitters in the sky. These instruments image Cherenkov light emitted by gamma-ray induced particle cascades in the atmosphere. Background from the much more numerous cosmic-ray cascades is efficiently reduced by considering the shape of the shower images, and the capability to reduce this background is one of the key aspects that determine the sensitivity of a IACT. In this work we apply a tree classification method to data from the High Energy Stere…
Eventdisplay is a software package for the analysis and reconstruction of data and Monte Carlo events from ground-based gamma-ray observatories such as VERITAS and CTA. It was originally developed as a display tool for data from the VERITAS prototype telescope, but evolved into a full analysis package with routines for calibration, FADC trace integration, image and stereo parameter analysis, response function calculation, and high-level analysis steps. Eventdisplay makes use of an image parameter analysis combined with gamma-hadron separation methods based on multivariate algorithms. An overvi…
The High Altitude Water Cherenkov (HAWC) observatory is an air-shower array located in Mexico. It is sensitive to the highest energy photons we detect at the Earth, reaching energies of several tens of TeV. The observatory was completed more than one year ago and we are presenting in this contribution the first results about its performance. We also show the results of the first-year survey, the first flaring events detected by the observatory, its sensitivity to extended sources and the plans for the upgrade that is currently taking place.
An advanced Compton telescope appears to be the best instrument concept for the next generation gamma-ray space observatory in the MeV range. A first prototype of advanced Compton telescope is being developed to match the constraints of a nano satellite mission, with the scientific objective of measuring gamma-ray burst prompt emission polarization. Our instrumental developments for this project are focusing on the position-sensitive calorimeter module, made of a monolithic inorganic CeBr$_3$ scintillator read by a pixelated photodetector. 3D position reconstruction is obtained by deep-learnin…
The operation of the future Cherenkov Telescope Array (CTA), the next generation of Imaging Atmospheric Cherenkov Telescopes (IACTs), as an Open Observatory requires data products and analysis tools to be accessible and usable by the scientific community. This requirement has impelled gamma-ray astronomers to develop open-source science tools and standardised data formats. The objective of this presentation is to provide a perspective on the current effort to define a prototypical format for high-level gamma-ray astronomical data. The initiative, started by the IACT community in 2016, has gain…
In gamma-ray astronomy, a variety of data formats and proprietary software have been traditionally used, often developed for one specific mission or experiment. Especially for ground-based imaging atmospheric Cherenkov telescopes (IACTs), data and software are mostly private to the collaborations operating the telescopes. However, there is a general movement in science towards the use of open data and software. In addition, the next-generation IACT instrument, the Cherenkov Telescope Array (CTA), will be operated as an open observatory. We have created a Github organisation at https://github.c…
ARGO-YBJ is a full coverage air shower array located at the YangBaJing Cosmic Ray Laboratory (Tibet, P.R. China, 4300 m a.s.l., 606 g/cm^2) recording data with a duty cycle $\geq$85% and an energy threshold of a few hundred GeV. In this paper the latest results in Gamma-Ray Astronomy are summarized.
Thanks to the success of current gamma-ray telescopes (Fermi, H.E.S.S., MAGIC, VERITAS), and in view of the prospects of planned observatories such as the Cherenkov Telescope Array (CTA) or the High-Altitude Water Cherenkov Observatory (HAWC), gamma-ray astronomy is becoming an integral part of modern astrophysical research. Analysis today relies on a large diversity of tools and software frameworks that were specifically and independently developed for each instrument. With the aim of unifying the analysis of gamma-ray data, we are currently developing GammaLib (http://sourceforge.net/project…
During the last two decades Gamma-Ray Astronomy has emerged as a powerful tool to study cosmic ray physics. In fact, photons are not deviated by galactic or extragalactic magnetic fields so their directions bring the information of the production sites and are easier to detect than neutrinos. Thus the search for $γ$ primarily address in the framework of the search of cosmic ray sources and to the investigation of the phenomena in the acceleration sites. This note is not a place for a review of ground-based gamma-ray astronomy. We will introduce the experimental techniques used to detect photon…
The Imaging Air Cherenkov Telescopes (IACTs), like, HESS, MAGIC and VERITAS well demonstrated their performances by showing many exciting results at very high energy gamma ray domain, mainly between 100 GeV and 10 TeV. It is important to investigate how much we can improve the sensitivity in this energy range, but it is also important to expand the energy coverage and sensitivity towards new domains, the lower and higher energies, by extending this IACT techniques. For this purpose, we have carried out the optimization of the array of large IACTs assuming with new technologies, advanced photod…
We are developing a concept of a "thin" detector as a high-angular-precision telescope and polarimeter for cosmic gamma-rays above the pair-creation threshold.
Ground-level particle detection is now a well-established approach to TeV gamma-ray astronomy. Detection of Cherenkov light produced in water-filled detection units is a proven and cost-effective method. Here we discuss the optimization of the units towards the future Southern Wide-field Gamma-ray Observatory (SWGO). In this context, we investigate a new type of configuration in which each water Cherenkov detector (WCD) unit in the array comprises two chambers with black or reflective walls and a single photomultiplier tube (PMT) in each chamber. We find that this is a cost-effective approach …
In the past decade imaging atmospheric Cherenkov telescope arrays such as H.E.S.S., MAGIC, VERITAS, as well as the Fermi-LAT space telescope have provided us with detailed images and spectra of the gamma-ray universe for the first time. Currently the gamma-ray community is preparing to build the next-generation Cherenkov Telecope Array (CTA), which will be operated as an open observatory. Gammapy (available at https://github.com/gammapy/gammapy under the open-source BSD license) is a new in-development Astropy affiliated package for high-level analysis and simulation of astronomical gamma-ray …
All-Sky-ASTROGAM is a gamma-ray observatory operating in a broad energy range, 100 keV to a few hundred MeV, recently proposed as the "Fast" (F) mission of the European Space Agency for a launch in 2028 to an L2 orbit. The scientific payload is composed of a unique gamma-ray imaging monitor for astrophysical transients, with very large field of view (almost 4$π$ sr) and optimal sensitivity to detect bright and intermediate flux sources (gamma-ray bursts, active galactic nuclei, X-ray binaries, supernovae and novae) at different timescales ranging from seconds to months. The mission will operat…
The High Energy Stereoscopic System (H.E.S.S.) is one of the currently operating Imaging Atmospheric Cherenkov Telescopes. H.E.S.S. operates in the broad energy range from a few tens of GeV to more than 50 TeV reaching its best sensitivity around 1 TeV. In this contribution, we present an analysis technique, which is optimised for the detection at the highest energies accessible to H.E.S.S. and aimed to improve the sensitivity above 10 TeV. It includes the employment of improved event direction reconstruction and gamma-hadron separation. For the first time, also extensive air showers with even…
A 'knee-like' approximation of Cherenkov light Lateral Distribution Functions, which we developed earlier, now is used for the actual tasks of background rejection methods for high energy (tens and hundreds of TeV) gamma-ray astronomy. In this work we implement this technique to the HiSCORE wide angle timing array consisting of Cherenkov light detectors with spacing of 100 m covering 0.2 km$^2$ presently and up to 5 km$^2$ in future. However, it can be applied to other similar arrays. We also show that the application of a multivariable approach (where 3 parameters of the knee-like approximati…
We review gamma-ray observations that constrain antimatter - both baryonic and leptonic - in the Universe. Antimatter can be probed through ordinary matter, with the resulting annihilation gamma-rays providing indirect evidence for its presence. Although it is generally accepted that equal amounts of matter and antimatter have been produced in the Big Bang, gamma-rays have so far failed to detect substantial amounts of baryonic antimatter in the Universe. Conversely, positrons are abundantly observed through their annihilation in the central regions of our Galaxy and, although a wealth of astr…
The gamma-ray energy range from a few hundred keV to a few hundred MeV has remained largely unexplored, mainly due to the challenging nature of the measurements, since the pi- oneering, but limited, observations by COMPTEL on the Compton Gamma-Ray Observatory (1991-2000). This energy range is a transition region between thermal and nonthermal processes, and accurate measurements are critical for answering a broad range of astrophysical questions. We are developing a MIDEX-scale wide-aperture discovery mission, ComPair (Compton-Pair Production Space Telescope), to investigate the energy range f…
The analysis and combination of data from different gamma-ray instruments involves the use of collaboration proprietary software and case-by-case methods. The effort of defining a common data format for high-level data, namely event lists and instrument response functions (IRFs), has recently started for very-high-energy gamma-ray instruments, driven by the upcoming Cherenkov Telescope Array (CTA). In this work we implemented this prototypical data format for a small set of MAGIC, VERITAS, FACT, and H.E.S.S. Crab nebula observations, and we analyzed them with the open-source gammapy software p…
We propose a "thin" detector as a high-angular-precision telescope and polarimeter for cosmic gamma-rays above the pair-creation threshold. We have built a demonstrator based on a gaseous TPC. We are presently characterizing the detector with charged cosmic rays in the laboratory. Here we present some of its properties.
A conceptual design of an advanced Imaging Air Cherenkov Telescope with a 23 m diameter mirror and of 50 tons weight will be presented. A system photon detection efficiency of 15-17%, averaged over 300-600 nm, is aimed at to lower the threshold to 10-20 GeV. Prospects for a second generation camera with Geiger-mode Avalanche Photo Diodes will be discussed.
We consider the problem of estimation of a bivariate density function with support $\Re\times[0,\infty)$, where a classical bivariate kernel estimator causes boundary bias due to the non-negative variable. To overcome this problem, we propose four kernel density estimators and compare their performances in terms of the mean integrated squared error. Simulation study shows that the estimator based on the proposed normal-gamma kernel performs best. Two astronomical data sets are used to demonstrate the applicability of this estimator.
The physical motivations, present status, main results in study of cosmic rays and in the field of gamma-ray astronomy as well future plans of the TAIGA-1 (Tunka Advanced Instrument for cosmic ray physics and Gamma Astronomy) project are presented. The TAIGA observatory addresses ground-based gamma-ray astronomy and astroparticle physics at energies from a few TeV to several PeV, as well as cosmic ray physics from 100 TeV to several EeV. The pilot TAIGA-1 complex is located in the Tunka valley, ~50 km west from the southern tip of the lake Baikal.
Composite mirrors for gamma-ray astronomy have been developed to fulfill the specifications required for the next generation of Cherenkov telescopes represented by CTA (Cherenkov Telescope Array). In addition to the basic requirements on focus and reflection efficiency, the mirrors have to be stiff, lightweight, durable and cost efficient. In this paper, the technology developed to produce such mirrors is described, as well as some tests that have been performed to validate them. It is shown that these mirrors comply with the needs of CTA, making them good candidates for use on a significant p…
We pursue a novel morphometric analysis to detect sources in very-high-energy gamma-ray counts maps by structural deviations from the background noise without assuming any prior knowledge about potential sources. The rich and complex structure of the background noise is characterized by Minkowski functionals from integral geometry. By extracting more information out of the same data, we aim for an increased sensitivity. In the first two papers, we derived accurate estimates of the joint distribution of all Minkowski functionals. Here, we use this detailed structure characterization to detect s…
The SST-1M is a 4-m diameter mirror Davies-Cotton gamma-ray telescope. It has been designed to cover the energy range above 500 GeV and to be part of an array of telescopes separated by 150-200 m. Its innovative camera is featuring large area hexagonal silicon photo-multipliers as photon detectors and a fully digital trigger and readout system. Here, the strategy and the methods for its calibration are presented, together with the obtained results. In particular, the off and on-site calibration strategies are demonstrated on the first camera prototype. The performances of the camera in terms o…
The CoMET is an R$\&$D project aiming to design a very-high-energy (VHE) gamma-ray observatory sensitive to energies above $\sim$ 200 GeV. The science goals include continuous observation of soft-spectrum VHE gamma-ray sources such as Active Galactic Nuclei (AGNs) and transients like Gamma-Ray Bursts (GRBs). With these objectives, CoMET is designed to have a low energy threshold with a wide field-of-view of about 2 sr, at a high altitude, and combines ALTO particle detectors with CLiC air-Cherenkov detectors. In this contribution, we focus on the ALTO particle detector array performance only. …
CTLearn is a new Python package under development that uses the deep learning technique to analyze data from imaging atmospheric Cherenkov telescope (IACT) arrays. IACTs use the Cherenkov light emitted from air showers, initiated by very-high-energy gamma rays, to form an image of the longitudinal development of the air shower on the camera plane. The spatial, temporal, and calorimetric information of the originating high-energy particle is then recorded electronically. The sensitivity of IACTs to astrophysical sources depends strongly on the efficient rejection of the background of much more …
This paper summarizes the development of a successful project, LAUE, supported by the Italian Space Agency (ASI) and devoted to the development of long focal length (up to 100 m) Laue lenses for hard X--/soft gamma--ray astronomy (80-600 keV). The apparatus is ready and the assembling of a prototype lens petal is ongoing. The great achievement of this project is the use of bent crystals. From measurements obtained on single crystals and from simulations, we have estimated the expected Point Spread Function and thus the sensitivity of a lens made of petals. The expected sensitivity is a few $\t…
Most major scientific results produced by ground-based gamma-ray telescopes in the last 30 years have been obtained by expert members of the collaborations operating these instruments. This is due to the proprietary data and software policies adopted by these collaborations. However, the advent of the next generation of telescopes and their operation as observatories open to the astronomical community, along with a generally increasing demand for open science, confront gamma-ray astronomers with the challenge of sharing their data and analysis tools. As a consequence, in the last few years, th…
Aims. H.E.S.S. observes an increasing number of large extended sources. A new technique based on the structure of the sky map is developed to account for these additional structures by comparing them with the common point source analysis. Methods. Minkowski functionals are powerful measures from integral geometry. They can be used to quantify the structure of the counts map, which is then compared with the expected structure of a pure Poisson background. Gamma-ray sources lead to significant deviations from the expected background structure. The standard likelihood ratio method is exclusively …
We examine the performance of a TPC as a gamma-ray telescope above the pair-creation threshold. The contributions to the photon angular resolution are studied and their dependence on energy is obtained. The effective area per detector unit mass for such a thin detector is the conversion mass attenuation coefficient. The differential sensitivity for the detection of a point-like source is then derived. Finally, the measurement of track momentum from deflections due to multiple scattering is optimized. These analytical results are exemplified numerically for a few sets of detector parameters. TP…
Among the main disadvantages of using silicon photomultipliers (SiPMs) in large experiments are their limited physical area (increasing the cost and the complexity of the readout of a camera) and their sensitivity to unwanted wavelengths. This explains why photomultiplier tubes (PMTs) are still selected for the largest cameras of present and future Very High Energy (VHE) gamma-ray telescopes. These telescopes require photosensors that are sensitive to the fast and dim optical/near-UV Cherenkov radiation emitted due to the interaction of gamma rays with the atmosphere. Here we introduce a low-c…
We pursue a novel morphometric analysis to detect sources in very-high-energy gamma-ray counts maps by structural deviations from the background noise. Because the Minkowski functionals from integral geometry quantify the shape of the counts map itself, the morphometric analysis includes unbiased structure information without prior knowledge about the source. Their distribution provides access to intricate geometric information about the background. We combine techniques from stochastic geometry and statistical physics to determine the joint distribution of all Minkowski functionals. We achiev…
Very-high energy (VHE) gamma quanta contribute only a minuscule fraction - below one per million - to the flux of cosmic rays. Nevertheless, being neutral particles they are currently the best "messengers" of processes from the relativistic/ultra-relativistic Universe because they can be extrapolated back to their origin. The window of VHE gamma rays was opened only in 1989 by the Whipple collaboration, reporting the observation of TeV gamma rays from the Crab nebula. After a slow start, this new field of research is now rapidly expanding with the discovery of more than 150 VHE gamma-ray emitt…
