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A space-based detector dedicated to measurements of γ -rays and charged particles has to achieve a balance between different instrumental requirements. A good angular resolution is necessary for the γ -rays, whereas an excellent geometric factor is needed for the charged particles. The tracking reference technique of γ -ray physics is based on a pair-conversion telescope made of passive material (e.g., tungsten) coupled with sensitive layers (e.g., silicon microstrip). However, this kind of detector has a limited acceptance because of the large lever arm between the active layers, needed to improve the track reconstruction capability. Moreover, the passive material can induce fragmentation of nuclei, thus worsening charge reconstruction performances. The Tracker-In-Calorimeter (TIC) project aims to solve all these drawbacks. In the TIC proposal, the silicon sensors are moved inside a highly-segmented isotropic calorimeter with a couple of external scintillators dedicated to charge reconstruction. In principle, this configuration has a good geometrical factor, and the angle of the γ -rays can be precisely reconstructed from the lateral profile of the electromagnetic shower sampled, at different depths in the calorimeter, by silicon strips. The effectiveness of this approach has been studied with Monte Carlo simulations and validated with beam test data of a small prototype. [ABSTRACT FROM AUTHOR]

Ground-based γ-ray astronomy (GRA) using atmospheric Cherenkov techniques has emerged as a mature field during the last three decades in modern high-energy astrophysics. Advent of the imaging atmospheric Cherenkov telescopes (IACTs) for the indirect detection of the cosmic γ -ray photons has played a very important and leading role to understand the non-thermal Universe in the GeV–TeV energy band. In this paper, we present a topical review of the field with a special focus on the two Indian IACTs namely TeV atmospheric Cherenkov telescope with imaging camera and major atmospheric Cherenkov experiment. We also describe the present status and future scope of the GRA using IACTs in India. [ABSTRACT FROM AUTHOR]

A space-based detector dedicated to measurements of γ-rays and charged particles has to achieve a balance between different instrumental requirements. A good angular resolution is necessary for the γ-rays, whereas an excellent geometric factor is needed for the charged particles. The tracking reference technique of γ-ray physics is based on a pair-conversion telescope made of passive material (e.g., tungsten) coupled with sensitive layers (e.g., silicon microstrip). However, this kind of detector has a limited acceptance because of the large lever arm between the active layers, needed to improve the track reconstruction capability. Moreover, the passive material can induce fragmentation of nuclei, thus worsening charge reconstruction performances. The Tracker-In-Calorimeter (TIC) project aims to solve all these drawbacks. In the TIC proposal, the silicon sensors are moved inside a highly-segmented isotropic calorimeter with a couple of external scintillators dedicated to charge reconstruction. In principle, this configuration has a good geometrical factor, and the angle of the γ-rays can be precisely reconstructed from the lateral profile of the electromagnetic shower sampled, at different depths in the calorimeter, by silicon strips. The effectiveness of this approach has been studied with Monte Carlo simulations and validated with beam test data of a small prototype.

The evolution of extensive air shower detection as a technique for γ-ray astronomical instrumentation for the last three decades is reviewed. The first discoveries of galactic PeVatrons by the Large High Altitude Air Shower Observatory demonstrate the importance of this technique in ultra-high energy γ-ray astronomy. Utilizing this technique, the origins of high energy cosmic rays may be discovered in the near future.

The Fermi Gamma-ray Observatory discovered about a thousand extragalactic sources emitting energy from 100 MeV to 100 GeV. The majority of these sources belong to the class of blazars characterized by a quasi-featureless optical spectrum (BL Lac Objects). This hampers the determination of their redshift and therefore hinders the characterization of this class of objects. To investigate the nature of these sources and to determine their redshift, we are carrying out an extensive campaign using the 10 m Gran Telescopio Canarias to obtain high S/N ratio optical spectra. These observations allow us to confirm the blazar nature of the targets, to find new redshifts or to set stringent limits on the redshift based on the minimum equivalent width of specific absorption features that can be measured in the spectrum and are expected from their host galaxy, assuming it is a massive elliptical galaxy. These results are of importance for the multi-frequencies emission models of the blazars, to test their extreme physics, to shed light on their cosmic evolution and abundance in the far Universe. These gamma emitters are also of great importance for the characterization of the extragalactic background light through the absorption by the IR-optical background photons.

The observation of Al is an useful tool for γ-ray astronomy and in studies of galactic chemical evolution. The most likely mechanism for A1 nucleosynthesis is in the hydrogen burning MgAl cycle, and the A1 production reaction Mg( p, γ)Al at the important temperature range below T = 0.2 GK is still not well known. The spectroscopic factor of 58 keV resonance level in A1 is determined with shell model calculation and then used to deduce the resonance strength of the Mg( p, γ)Al reaction. The result provides a reference for the future Mg( p, γ)Al direct measurement at Jinping underground laboratory. [ABSTRACT FROM AUTHOR]

In recent years, observational γ-ray astronomy has witnessed a remarkable range of exciting new results in the high-energy and very high-energy regimes. Coupled with extensive theoretical and phenomenological studies of nonthermal processes in the Universe, these observations have provided deep insights into a number of fundamental problems of high-energy astrophysics and astroparticle physics. Although the main motivations of γ-ray astronomy remain unchanged, recent observational results have contributed significantly toward our understanding of many related phenomena. This article aims to review the most important results in the young and rapidly developing field of γ-ray astrophysics. [ABSTRACT FROM AUTHOR]

An array of seven atmospheric Cherenkov telescopes was commissioned at a high altitude site in Hanle in the Ladakh region of the Himalayas. The array called HAGAR has been designed to observe celestial γ-rays of energy >100 GeV. Each telescope is altitude-azimuth mounted and carries seven parabolic mirrors whose optic axes are co-aligned with the telescope axis. The telescopes point and track a celestial source using a PC-based drive control system. Two important issues in positioning of each HAGAR telescope are pointing accuracy of telescope axis and co-alignment of mirrors' optic axes with the telescope axis. We have adopted a three pronged strategy to address these issues, namely use of pointing models to improve pointing accuracy of the telescopes, RA-DEC scan technique to measure the pointing offsets of the mirrors and mechanical fine-tuning of off-axis mirrors by sighting a distant stationary light source. This paper discusses our efforts in this regard as well as the current status of pointing and monitoring of HAGAR telescopes. [ABSTRACT FROM AUTHOR]

Abstract: We present results of Monte Carlo simulations for the High Altitude Gamma Ray (HAGAR) telescope array which detects very high energy gamma rays from astronomical sources. This telescope array, located at Hanle at an altitude of 4270m in the Ladakh region of the Himalayas in India, is the highest altitude atmospheric Cherenkov experiment in the world. Taking advantage of the high altitude, this experiment could achieve relatively low energy threshold with a modest mirror area coverage. To understand the performance parameters of this telescope system, we have simulated large samples of extensive air showers initiated by gamma rays and various species of cosmic rays, using the CORSIKA package. Cherenkov photons produced in the atmosphere are sampled at ground level. These photons are then passed through the detector simulation program, which takes into account various design details and the data acquisition system of HAGAR. Night sky photons are also considered in the detector simulation program as performance of the telescope depends strongly on the level of night sky background (NSB) at the observation site. We have estimated various performance parameters like energy threshold and effective area for vertically incident showers as well as inclined showers. Details of these parameters, results obtained from simulations and comparison with the observed data are presented. It is shown that the energy threshold of the HAGAR telescope system is about 208GeV, a factor of ∼4 less than for a similar set up at about 1000m altitude, and it is able to detect Crab like sources at significance in 17h of observation without imposing additional criteria, like gamma-hadron separation, for further rejection of cosmic rays. [Copyright &y& Elsevier]

Abstract: HESS is an array of atmospheric Cherenkov telescopes dedicated to GeV–TeV γ-ray astronomy. The original array has been in operation since the beginning of 2004. It is composed of four 12-m diameter telescopes. The installation of a fifth 28-m diameter telescope is being completed. This telescope will operate both in stereoscopic mode and in monoscopic mode i.e. without a coincident detection on the smaller telescopes. A second-level trigger system is needed to suppress spurious triggers of the 28-m telescope when operated in monoscopic mode. This paper gives the motivation and principle of the second-level trigger. The principle of operation is illustrated by an example algorithm. The hardware implementation of the second level trigger system of HESS phase 2 is described and its expected performances are then evaluated. [Copyright &y& Elsevier]

Abstract: At the time large air Cherenkov arrays are being discussed for future γ-ray observatories, we review the relationship between the targeted capabilities and the main design parameters taking into account construction costs. As an example application, we describe a telescope array optimized for observations between 1TeV and a few 100TeV and use detailed simulations to estimate its performances in comparison to science objectives. [Copyright &y& Elsevier]

Flaring activity of Active Galactic Nuclei (AGN) in VHE γ-ray astronomy is observed on timescales from minutes to years and can be explained either by the interaction of relativistic jets with the surrounding material or by imprints of the central engine, like temporal modulation caused by binary systems of supermassive black holes. The key to answer those questions lies in combining 24/7 monitoring with short high sensitivity exposures as provided by the third generation γ-ray astronomy instruments like MAGIC, VERITAS and H.E.S.S. The long-term observations can be provided by a global network of small robotic Cherenkov telescopes.1 As a first step, we are currently setting up a dedicated Cherenkov telescope, which will carry out joint observations with the Whipple 10 m telescope for AGN monitoring. The new telescope will be designed for low costs but high performance by upgrading one of the former HEGRA telescopes, still located at the MAGIC site on the Canary Island of La Palma (Spain). The main novelties will be its robotic operation and a novel camera type, resulting in a greatly improved sensitivity and a lower energy threshold. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this article is to present a single photon counting technique applied to the imaging atmospheric Cherenkov telescopes as an alternative to the charge integration technique used by most experiments in the field. A comparison of the two techniques has been performed, assuming suitable and realistic telescope layouts but different methods to treat and convert the electrical signals at the output of the light sensors. A set of γ-ray induced showers, with primary energies ranging from 300GeV to 10TeV, has been simulated for this comparison. The study has been performed both for a single telescope and for a three-telescope array. The results show, in both cases, considerably higher trigger efficiencies in the single photon counting method, with respect to the analog one, particularly at the lowest energies. Single telescope energy thresholds for the two methods are also reported and discussed. [Copyright &y& Elsevier]

The Major Atmospheric Gamma-ray Imaging Čerenkov Telescope (MAGIC) will be the first ground-based Čerenkov telescope able to detect photons with E > 30 GeV. This region of the electromagnetic spectrum, yet unexplored, is fundamental for pulsar physics since theoretical models foreseen a cut-off in the pulsed emission at tens of GeV. In order to perform timing analysis of pulsars, accuracy below milliseconds in the time stamp of the detected event is needed. In this work, we present the timing system of the MAGIC telescope and the first timing analysis performed on real data. [ABSTRACT FROM AUTHOR]

Abstract: If dark matter is made of neutralinos, annihilation of such Majorana particles in the halos of galaxies should produce high energy cosmic rays in high density regions, like the centres of galaxies. M31 (Andromeda) is the nearest spiral galaxy from us, and both its high mass and its low distance make it a source of interest for the indirect search for dark matter through γ-ray detection. The ground based atmospheric Čerenkov telescope CELESTE has been observing M31 for the last three years, and we present here the limit we get on SUSY models from the analysed data (first observation of M31 in the range 50–1000GeV). [Copyright &y& Elsevier]

The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is the major next generation imaging atmospheric Cherenkov γ-ray telescope in the western hemisphere and will be located in southern Arizona nearby Kitt Peak National Observatory. The VERITAS observatory will provide unprecedented sensitivity to photon energies between 50 GeV and 50 TeV. The first stage is an array of four telescopes to be fully operational in early 2006, with an expansion to seven telescopes envisioned for 2008. The construction of a prototype telescope is underway, for which first light is expected in Fall 2003. The technical concept is outlined and a progress report is given. [Copyright &y& Elsevier]

AGILE is a small γ-ray astronomy satellite, with good spatial resolution, excellent timing capabilities and an unprecedented large field of view (∼1/5 of the sky). It will be the next mission dedicated to high energy astrophysics in the range 30 MeV–50 GeV and will be launched in 2005. Pulsars are a major topic of investigation of AGILE and, besides studying the small sample of known objects, AGILE will offer the first possibility of detecting several young and energetic radio pulsars that have been discovered since the end of the CGRO mission. We provide an estimate of the expected number of detectable γ-ray pulsars and present AGILE capabilities for timing analysis with small counting statistics, based on the analysis of data from simulations, from the EGRET archive and from radio pulsar catalogs. [Copyright &y& Elsevier]

We describe the calibration measurements of the MEGA prototype, a tracking Compton and pair creation telescope. The measurements were performed at the high intensity gamma-ray source (HIγS) facility from April 21 to May 06, 2003. The main goal of this calibration was directed at higher energies, above those available from radioactive lab sources, and at polarization. A total of 15.5×106 triggered events at 10 energies in the range of 0.7–49 MeV and at six angles of incidence (0°–180°) were recorded. [Copyright &y& Elsevier]

A prototype balloon-borne telescope is being constructed for γ-ray observations in the MeV energy range. The Tracking and Imaging Gamma-Ray Experiment (TIGRE) uses multi-layers of thin silicon detectors to track and measure the energy losses of Compton recoil electrons. When combined with the direction and energy of the Compton scattered γ-ray a unique incident direction for each photon event is determined. This facilitates background rejection, improved sensitivity and image reconstruction. The converter/tracker also serves as an electron–positron pair detector for γ-rays up to 100 MeV. The initial continental US flight will be used to determine the sub-orbital atmospheric backgrounds and search for polarized γ-emission for the Crab pulsar. Longer southern hemisphere flights with an enhanced instrument will map out the 26Al emissions from the galactic center region. [Copyright &y& Elsevier]

We describe the development of a new telescope for Medium Energy Gamma-Ray Astronomy (MEGA) for the energy band 0.4–50 MeV. As a successor to COMPTEL and EGRET (low energies), MEGA aims to improve the sensitivity for astronomical sources by at least an order of magnitude. It could thus fill the severe sensitivity gap between scheduled or operating hard-X-ray and high-energy γ-ray missions and open the way for a future Advanced Compton Telescope. MEGA records and images γ-rays by completely tracking Compton and Pair creation events in a stack of double sided Si-strip track detectors surrounded by a pixelated CsI calorimeter. A scaled down prototype has been built and calibrations using radioactive sources and exposures to an accelerator generated γ-ray beam were performed in 2003. A balloon flight is planned for 2004. [Copyright &y& Elsevier]

A review of the main features of the γ-ray emission of individual classical novae is presented. We concentrate on the theoretical predictions of both the long lasting emission, produced by electron captures on 7Be and β+-decay of 22Na, and the prompt emission, produced by electron–positron annihilation (with positrons coming from 13N and 18F β+-decays). Special attention is paid to the observability of novae with the current INTEGRAL observatory and with future missions, like EXIST. [Copyright &y& Elsevier]

As ultra-high energy photons (EeV and beyond) propagate from their sources of production to Earth, radiation-matter interactions can occur, leading to an effective screening of the incident flux. In this energy domain, photons can undergo e + / e − pair production when interacting with the surrounding geomagnetic field, which in turn can produce a cascade of electromagnetic particles called preshower. Such cascade can initiate air showers in the Earth's atmosphere that gamma-ray telescopes, such as the next-generation gamma-ray observatory Cherenkov Telescope Array, can detect through Cherenkov emission. In this paper, we study the feasibility of detecting such phenomenon using Monte-Carlo simulations of nearly horizontal air showers for the example of the La Palma site of the Cherenkov Telescope Array. We investigate the efficiency of multivariate analysis in correctly identifying preshower events initiated by 40 EeV photons and cosmic ray dominated background simulated in the energy range 10 TeV – 10 EeV. The effective areas for such kind of events are also investigated and event rate predictions related to different ultra-high energy photons production models are presented. While the expected number of preshowers from diffuse emission of UHE photons for 30 hours of observation is estimated around 3.3 × 10 − 5 based on the upper limits put by the Pierre Auger Observatory, this value is at the level of 2.7 × 10 − 4 (5.7 × 10 − 5 ) when considering the upper limits of the Pierre Auger Observatory (Telescope Array) on UHE photon point sources. However, UHE photon emission may undergo possible "boosting" due to gamma-ray burst, increasing the expected number of preshower events up to 0.17 and yielding a minimum required flux of ~ 0.2 km − 2 yr − 1 to obtain one preshower event, which is about a factor 10 higher than upper limits put by the Pierre Auger Observatory and Telescope Array (0.034 and 0.019 km − 2 yr − 1 , respectively). [ABSTRACT FROM AUTHOR]

Here we discuss two related aspects of the detection of low energy gammas by IACT's (Imaging Atmospheric Cherenkov Telescope): the effect of the Earth's magnetic field and a possible novel trigger technique. The Earth's magnetic field affects the development of extensive air showers (EAS), spreading the collected photons and, therefore, decreasing the sensitivity of the Cherenkov telescope. The new effects that appear in low energy showers can be partially offset modifying the trigger criteria. The result is a large collection area (> 1000 m2) below 10 GeV for a typical 17 m class telescope, increasing its sensitivity at these energies. [ABSTRACT FROM AUTHOR]