MACHETE: A transit imaging atmospheric Cherenkov telescope to survey half of the very high energy γ-ray sky

Current imaging atmospheric Cherenkov telescopes for very high energy γ-ray astrophysics are pointing instruments with a field of view up to a few tens of sq deg. We propose to build an array of two non-steerable (drift) telescopes. Each of the telescopes would have a camera with a FOV of 5 × 60 sq deg oriented along the meridian. About half of the sky drifts through this FOV in a year. We have performed a Monte Carlo simulation to estimate the performance of this instrument. We expect it to survey this half of the sky with an integral flux sensitivity of ∼0.77% of the steady flux of the Crab Nebula in 5 years, an analysis energy threshold of ∼150 GeV and an angular resolution of ∼0.1°. For astronomical objects that transit over the telescope for a specific night, we can achieve an integral sensitivity of 12% of the Crab Nebula flux in a night, making it a very powerful tool to trigger further observations of variable sources using steerable IACTs or instruments at other wavelengths.     

TeV γ-ray fluxes from the long campaigns on Mrk 421 as constraints on the emission of TeV–PeV neutrinos and UHECRs

Long TeV γ-ray campaigns have been carried out to study the spectrum, variability and duty cycle of the BL Lac object Markarian 421. These campaigns have given some evidence of the presence of protons in the jet: (i) Its spectral energy distribution which shows two main peaks; one at low energies (∼1 keV) and the other at high energies (hundreds of GeV), has been described by using synchrotron proton blazar model. (ii) The study of the variability at GeV γ-rays and X-rays has indicated no significant correlation. (iii) TeV γ-ray detections without activity in X-rays, called “orphan flares” have been observed in this object.Recently, The Telescope Array Collaboration reported the arrival of 72 ultra-high-energy cosmic rays with some of them possibly related to the direction of Markarian 421. The IceCube Collaboration reported the detection of 37 extraterrestrial neutrinos in the TeV–PeV energy range collected during three consecutive years. In particular, no neutrino track events were associated with this source. In this paper, we consider the proton–photon interactions to correlate the TeV γ-ray fluxes reported by long campaigns with the neutrino and ultra-high-energy cosmic ray observations around this blazar. Considering the results reported by The IceCube and Telescope Array Collaborations, we found that only from ∼25% to 70% of TeV fluxes described with a power law function with exponential cutoff can come from the proton–photon interactions.     

Perspective of monochromatic gamma-ray line detection with the High Energy cosmic-Radiation Detection (HERD) facility onboard China’s space station

HERD is the High Energy cosmic-Radiation Detection instrument proposed to operate onboard China’s space station in the 2020s. It is designed to detect energetic cosmic ray nuclei, leptons and photons with a high energy resolution ( ∼1% for electrons and photons and 20% for nuclei) and a large geometry factor (>3 m² sr for electrons and diffuse photons and > [2]m² sr for nuclei). In this work we discuss the capability of HERD to detect monochromatic γ-ray lines, based on simulations of the detector performance. It is shown that HERD will be one of the most sensitive instruments for monochromatic γ-ray searches at energies between ∼ 10 to a few hundred GeV. Above hundreds of GeV, Cherenkov telescopes will be more sensitive due to their large effective area. As a specific example, we show that a good portion of the parameter space of a supersymmetric dark matter model can be probed with HERD.     

The major upgrade of the MAGIC telescopes,Part II: A performance study using observations of the Crab Nebula

MAGIC is a system of two Imaging Atmospheric Cherenkov Telescopes located in the Canary island of La Palma, Spain. During summer 2011 and 2012 it underwent a series of upgrades, involving the exchange of the MAGIC-I camera and its trigger system, as well as the upgrade of the readout system of both telescopes. We use observations of the Crab Nebula taken at low and medium zenith angles to assess the key performance parameters of the MAGIC stereo system. For low zenith angle observations, the standard trigger threshold of the MAGIC telescopes is ∼ 50  GeV. The integral sensitivity for point-like sources with Crab Nebula-like spectrum above 220 GeV is (0.66 ± 0.03)% of Crab Nebula flux in 50 h of observations. The angular resolution, defined as the σ of a 2-dimensional Gaussian distribution, at those energies is ≲ 0.07°, while the energy resolution is 16%. We also re-evaluate the effect of the systematic uncertainty on the data taken with the MAGIC telescopes after the upgrade. We estimate that the systematic uncertainties can be divided in the following components: < 15% in energy scale, 11%–18% in flux normalization and ± 0.15 for the energy spectrum power-law slope.     

Imaging very high energy gamma-ray telescopes

The technique of γ-ray astronomy at very high energies (VHE:>?100 GeV) with ground-based imaging atmospheric Cherenkov telescopes is described, the H.E.S.S. array in Namibia serving as example. Mainly a discussion of the physical principles of the atmospheric Cherenkov technique is given, emphasizing its rapid development during the last decade. The present status is illustrated by two examples: the spectral and morphological characterization in VHE γ-rays of a shell-type supernova remnant together with its theoretical interpretation, and the results of a survey of the Galactic Plane that shows a large variety of non-thermal sources. The final part is devoted to an overview of the ongoing and future instrumental developments.     

The extragalactic background light and the gamma-ray opacity of the universe

The extragalactic background light (EBL) is one of the fundamental observational quantities in cosmology. All energy releases from resolved and unresolved extragalactic sources, and the light from any truly diffuse background, excluding the cosmic microwave background (CMB), contribute to its intensity and spectral energy distribution. It therefore plays a crucial role in cosmological tests for the formation and evolution of stellar objects and galaxies, and for setting limits on exotic energy releases in the universe. The EBL also plays an important role in the propagation of very high energy γ-rays which are attenuated en route to Earth by pair producing γ–γ interactions with the EBL and CMB. The EBL affects the spectrum of the sources, predominantly blazars, in the ∼10 GeV–10 TeV energy regime. Knowledge of the EBL intensity and spectrum will allow the determination of the intrinsic blazar spectrum in a crucial energy regime that can be used to test particle acceleration mechanisms and very high energy (VHE) γ-ray production models. Conversely, knowledge of the intrinsic γ-ray spectrum and the detection of blazars at increasingly higher redshifts will set strong limits on the EBL and its evolution. This paper reviews the latest developments in the determination of the EBL and its impact on the current understanding of the origin and production mechanisms of γ-rays in blazars, and on energy releases in the universe. The review concludes with a summary and future directions in Cherenkov Telescope Array techniques and in infrared ground-based and space observatories that will greatly improve our knowledge of the EBL and the origin and production of very high energy γ-rays.     

High energy γ-ray emission from compact galactic sources in the context of observations with the next generation Cherenkov Telescope Arrays

The observational progress in the γ-ray astronomy in the last few years has led to the discovery of more than a thousand sources at GeV energies and more than a hundred sources at TeV energies. A few different classes of compact objects in the Galaxy have been established. They show many unexpected features at high energies the physics of which remains mainly unknown. At present it is clear that detailed investigation of these new phenomena can be performed only with the technical equipment which offer an order of magnitude better sensitivity, and a few times better energy, angular and time resolution in the broad energy range staring from a few tens of GeV up to a few hundreds TeV. Such facilities can be realized by the next generation of instruments such as the planned Cherenkov Telescope Array (CTA).The aim of this report is to summarize up to date observational results on the compact galactic sources in the GeV–TeV γ-ray energy range, discuss their theoretical implications, and indicate which hypothesis considered at present might be verified with the next generation of telescopes. We point out which of the observational features of the γ-ray sources are important to investigate with special care with the planned CTA in order to throw new light on physical processes involved. Their knowledge should finally allow us to answer the question on the origin of energetic particles in our Galaxy.     

Systematic search for molecular clouds near supernova remnants as sources of very-high-energy γ-ray emission

Supernova remnants accelerate particles up to energies of at least 100 TeV as established by observations in very-high-energy γ-ray astronomy. Molecular clouds in their vicinity provide an increased amount of target material for proton-proton interaction and subsequent neutral pion decay into γ-rays of accelerated hadrons escaping the remnant. Therefore, these molecular clouds are potential γ-ray sources. The γ-ray emission from these clouds provides a unique environment to derive information on the propagation of very-high-energy particles through the interstellar medium as well as on the acceleration of hadrons in supernova remnants. Current Imaging Atmospheric Cherenkov Telescope systems are suitable to explore a large parameter space of the propagation properties depending on the age of the supernova remnant and the distance between the remnant and the nearby molecular cloud.In this paper we present our strategy and results of a systematic search for γ-ray emitting molecular clouds near supernova remnants which are potentially detectable with current experiments in the TeV energy range and explore the prospects of future experiments.     

Pointing of HAGAR telescope mirrors

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.     

Study of terrestrial γ-ray background in presence of variable radioactivity from rain water

A number of groups have reported significant reduction in the flux of low energy (0.1–3 MeV) γ-rays in observations carried out during the past total solar eclipses. However, the contribution of the radon induced radioactivity to the overall γ-ray background can become substantial, especially during episodes of rain. Depending upon the pattern of the rainfall radon induced γ-ray background may vary significantly on time scales of ∼10 min, making the interpretation of the data in terms of an extraterrestrial effect such as a total solar eclipse rather difficult. A reliable estimate of the low energy terrestrial γ-ray (TGR) background is necessary before attempting to measure the possible contribution of any extraterrestrial phenomenon. The knowledge of the precise energies and branching ratios of radon and other radio-isotope induced γ-rays was exploited to accurately reproduce the TGR background, even in the presence of a large and variable contribution from radon induced radioactivity from fresh rain water. The measurement of the TGR background has paved the way for studying the variation of the soft γ-ray flux during the long duration total solar eclipse that occurred on 22 July 2009 in the middle of the Monsoon season in India.     

Gamma rays from the Galactic Centre region: A review

During the last decades, increasingly precise astronomical observations of the Galactic Centre (GC) region at radio, infrared, and X-ray wavelengths laid the foundations to a detailed understanding of the high energy astroparticle physics of this most remarkable location in the Galaxy. Recently, observations of this region in high energy (HE, 10 MeV–100 GeV) and very high energy (VHE, > 100 GeV) γ-rays added important insights to the emerging picture of the Galactic nucleus as a most violent and active region where acceleration of particles to very high energies – possibly up to a PeV – and their transport can be studied in great detail. Moreover, the inner Galaxy is believed to host large concentrations of dark matter (DM), and is therefore one of the prime targets for the indirect search for γ-rays from annihilating or decaying dark matter particles. In this article, the current understanding of the γ-ray emission emanating from the GC is summarised and the results of recent DM searches in HE and VHE γ-rays are reviewed.     

Crab pulsar spectrum: A non-extensive approach

Non extensive statistical physics has been applied to various problems in physics including astrophysics. In this paper we explore the possibility of using non-extensive approach to explain the recently observed pulsed γ-ray from Crab pulsar above 100 GeV observed by VERITAS γ-ray telescope.     

The Lateral Trigger Probability function for the Ultra-High Energy Cosmic Ray showers detected by the Pierre Auger Observatory

In this paper we introduce the concept of Lateral Trigger Probability (LTP) function, i.e., the probability for an Extensive Air Shower (EAS) to trigger an individual detector of a ground based array as a function of distance to the shower axis, taking into account energy, mass and direction of the primary cosmic ray. We apply this concept to the surface array of the Pierre Auger Observatory consisting of a 1.5 km spaced grid of about 1600 water Cherenkov stations. Using Monte Carlo simulations of ultra-high energy showers the LTP functions are derived for energies in the range between 10¹⁷ and 10¹⁹ eV and zenith angles up to 65°. A parametrization combining a step function with an exponential is found to reproduce them very well in the considered range of energies and zenith angles. The LTP functions can also be obtained from data using events simultaneously observed by the fluorescence and the surface detector of the Pierre Auger Observatory (hybrid events). We validate the Monte Carlo results showing how LTP functions from data are in good agreement with simulations.     

All-particle cosmic ray energy spectrum measured with 26 IceTop stations

We report on a measurement of the cosmic ray energy spectrum with the IceTop air shower array, the surface component of the IceCube Neutrino Observatory at the South Pole. The data used in this analysis were taken between June and October, 2007, with 26 surface stations operational at that time, corresponding to about one third of the final array. The fiducial area used in this analysis was 0.122 km². The analysis investigated the energy spectrum from 1 to 100 PeV measured for three different zenith angle ranges between 0° and 46°. Because of the isotropy of cosmic rays in this energy range the spectra from all zenith angle intervals have to agree. The cosmic-ray energy spectrum was determined under different assumptions on the primary mass composition. Good agreement of spectra in the three zenith angle ranges was found for the assumption of pure proton and a simple two-component model. For zenith angles θ < 30°, where the mass dependence is smallest, the knee in the cosmic ray energy spectrum was observed at about 4 PeV, with a spectral index above the knee of about −3.1. Moreover, an indication of a flattening of the spectrum above 22 PeV was observed.     

Small-scale anisotropy of cosmic rays with energies above 3×1018 eV based on data from the Yakutsk EAS facility

We analyze the directions of the arrival of cosmic rays with energies E ₀≥3×10¹⁸ eV and zenith angles θ≤45° recorded by the Yakutsk extensive air shower (EAS) facility during 1974–2000. They are shown to have a small-scale structure with scale sizes of 5°–10°. Enhanced particle fluxes compared to the expected levels for random distributions at (4–5)σ are observed from the Galactic and Supergalactic planes.     

Performance of the MAGIC stereo system obtained with Crab Nebula data

MAGIC is a system of two Imaging Atmospheric Cherenkov Telescopes located in the Canary island of La Palma. Since autumn 2009 both telescopes have been working together in stereoscopic mode, providing a significant improvement with respect to the previous single-telescope observations. We use observations of the Crab Nebula taken at low zenith angles to assess the performance of the MAGIC stereo system. The trigger threshold of the MAGIC telescopes is 50 − 60 GeV. Advanced stereo analysis techniques allow MAGIC to achieve a sensitivity as good as (0.76 ± 0.03)% of the Crab Nebula flux in 50 h of observations above 290 GeV. The angular resolution at those energies is better than ∼0.07°. We also perform a detailed study of possible systematic effects which may influence the analysis of the data taken with the MAGIC telescopes.     

TeV γ-ray emission from Mrk 421 observed with TACTIC during December 2014–February 2015

We observed the TeV blazar Mrk421 in the very high energy (VHE) region with TACTIC from 21 December, 2014 to 21 February, 2015. The VHE γ-ray signal is detected from the source at a statistical significance of ∼ 9σ with 648  ± 74 γ-ray like events using a total observation time of 65.6 h of clean data. To denote the active period of the source, the entire observation has been divided into two spells: Spell I (21–30 December, 2014; MJD: 57012–57021) and Spell II (15 January-21 February, 2015; MJD: 57037–57074). The Spell I corresponds to relatively higher activity state wherein 337  ±  47 γ-rays are detected at a statistical significance of 7.39σ in 21.7 h. During Spell II, 311  ±  57 γ-ray events have been detected in ∼ 44c at a statistical significance of 5.6σ. The analysis of high energy (HE) γ-ray data from Fermi-LAT has been carried out for the period of TACTIC observations. In addition, near simultaneous multi-wavelength data from Swift-BAT and MAXI in the X-ray region, SPOL in optical V-band and OVRO at 15 GHz radio band have also been considered. The HE γ-ray and optical observations indicate variability during both the spells whereas hard X-ray observations are consistent with the average emission through out the TACTIC observations. The broad-band spectral energy distributions of the source are reproduced by single zone homogeneous synchrotron self Compton process for the two spells. The model parameters estimated for the two spells are similar except for the particle energy density which is obtained to be slightly higher during Spell I.     

Influence of cosmological expansion on the threshold effects in the annihilation reaction of hard photons with CMB photons

The redshift (z) dependence of the dispersion relations for free particles is analyzed by taking into account the Lorentz invariance violation. A nonlinear algebraic equation is derived for the momenta of the particles involved in the annihilation reaction of a hard photon from a γ-ray source with a soft cosmic microwave background (CMB) photon near the threshold of this reaction. The solutions of this threshold equation are constructed and analyzed as a function of the redshift. We show that the threshold of the reaction under consideration tends to decrease with increasing z; the energy spectra of γ-ray sources at energies of ～10 TeV must be cut off in accordance with the calculated z dependence. We also calculate the time delay of the light signals from γ-ray sources that corresponds to the Lorentz invariance violation for photons. We discuss the possibility of improving the standard constraints on the Lorentz invariance violation parameters for fields of various physical natures.