Fractal and entropy studies of Cherenkov arrival times

In this paper, time structure of simulated Cherenkov arrival times is explored by studying its multifractal/fractal and entropy properties. It is observed that the simulated Cherenkov arrival times are multifractal in nature. Its multifractal features do not show any dependence on the nature of the initiating particle and on the energy of arriving Cherenkov photons. However, the computed values of the entropy and Hausdorff dimension show consistent energy dependence on the energy of incoming γ-ray showers both for time arrivals as well for number of photons. Our results get partially validated by applying them to light curve of a gamma ray burst.     

Status and Results from the HEGRA Air Shower Experiment

The HEGRA experiment investigates the relativistic (non thermal) universe by measuring air showers initiated in the atmosphere by photons and charged particles from the cosmos, using the imaging atmospheric Cherenkov light technique (> 500 GeV) and the showerfront sampling technique (> 20 TeV). New results concerning the search and study of gamma ray sources above 500 GeV, the measurement of the energy spectrum and chemical composition of cosmic rays above 1 PeV and the search of TeV counterparts of Gamma Ray Bursts are briefly described. This revised version was published online in July 2006 with corrections to the Cover Date.     

Capabilities of a giant hybrid air shower detector

Recent reports of superhigh energy cosmic rays beyond the expected spectral cutoff have intensified interest in the unknown origin of the highest energy cosmic rays. There is a need for a much larger data base of more precisely measured air showers. This requires new sensitive detectors of enormous aperture. Combining a ground array of particle counters with an optical detector of atmospheric fluorescence yields a detector of outstanding capability. Such a hybrid detector provides far more accurate measurements of energies, arrival directions, and primary particle atomic masses than can be achieved by either type of detector separately.     

Stereoscopic observations of gamma rays at the Whipple observatory

Observations of photons at E ≥ 550 GeV from the Crab Nebula are presented and used to assess the potential of multi-telescope systems for γ-ray astronomy.

The Whipple observatory 10 m and 8 m imaging atmospheric erenkov telescopes have been used to provide a stereoscopic view of air showers to make a more complete measurement of air shower parameters. Here we present a measurement of the spread in the arrival direction of primary γ-rays originating from a point source. The data show that the shower arrival direction can be reconstructed with an accuracy of σ = 0.°14.     

New method of analysis for TeV gamma-ray data collected by the TACTIC telescope

We describe a new fractal and wavelet based framework for analysing atmospheric Cerenkov images of extensive air showers. The methodology developed using simulated data has been tested with the data collected from Mrk421 and Crab nebula directions during 10 October 2009–16 May 2010 by the TACTIC telescope. The telescope detected flaring activity in TeV gamma-rays from Mrk421 during 15th-17th February 2010. Hillas parameter analysis of Cerenkov images showed presence of TeV γ-ray signal with statistical significance of 12.5 σ beyond 1TeV energy for the 265 hours data set. Based on a Monte-Carlo simulated database of Cerenkov images recorded by the TACTIC telescope, the wavelet dimension B₆ can be exploited as a useful parameter to segregate gamma-ray initiated events from hadron initiated events with better efficiency. We derive a preliminary value of the relevant significance for gamma initiated events using wavelet parameter B₆ for the 265 hours long observation spell of Mrk421 including the flaring episode during 15th–17th February 2010 and 66 hours data of observations on Crab nebula. Wavelet analysis approach has also been tested on Crab OFF- source data of 27 hours. Application of wavelet parameter B₆ on the recorded Cerenkov images of the TACTIC telescope reveals better results in terms of segregation efficiency and enhancement of the determined value of statistical significance as compared to the Hillas parameters approach.     

Air shower Cerenkov radiation as an instrument for very high energyγ-astronomy (I)

A method for calculating average characteristics of Cerenkov light from very high energy (1–100 TeV) air showers is described. Simple approximation of lateral distribution of light is presented which might be useful for experiment as well as for simulations.     

The shape of the radio wavefront of extensive air showers as measured with LOFAR

Extensive air showers, induced by high energy cosmic rays impinging on the Earth’s atmosphere, produce radio emission that is measured with the LOFAR radio telescope. As the emission comes from a finite distance of a few kilometers, the incident wavefront is non-planar. A spherical, conical or hyperbolic shape of the wavefront has been proposed, but measurements of individual air showers have been inconclusive so far. For a selected high-quality sample of 161 measured extensive air showers, we have reconstructed the wavefront by measuring pulse arrival times to sub-nanosecond precision in 200 to 350 individual antennas. For each measured air shower, we have fitted a conical, spherical, and hyperboloid shape to the arrival times. The fit quality and a likelihood analysis show that a hyperboloid is the best parameterization. Using a non-planar wavefront shape gives an improved angular resolution, when reconstructing the shower arrival direction. Furthermore, a dependence of the wavefront shape on the shower geometry can be seen. This suggests that it will be possible to use a wavefront shape analysis to get an additional handle on the atmospheric depth of the shower maximum, which is sensitive to the mass of the primary particle.     

Software timing calibration of the ARGO-YBJ detector

The ARGO-YBJ experiment is mainly devoted to search for astronomical gamma sources. The arrival direction of air showers is reconstructed thanks to the times measured by the pixels of the detector. Therefore, the timing calibration of the detector pixels is crucial in order to get the best angular resolution and pointing accuracy. Because of the large number of pixels a hardware timing calibration is practically impossible. Therefore an off-line software calibration has been adopted. Here, the details of the procedure and the results are presented.     

Cherenkov τ shower earth-skimming method for PeV–EeV ντ observation with Ashra

We describe a method of observation for PeV–EeV τ neutrinos using Cherenkov light from the air showers of decayed τs produced by τ neutrino interactions in the Earth. Aiming for the realization of neutrino astronomy utilizing the Earth-skimming τ neutrino detection technique, highly precise determination of arrival direction is key due to the following issues: (1) clear identification of neutrinos by identifying those vertices originating within the Earth’s surface and (2) identification of very high energy neutrino sources. The Ashra detector uses newly developed light collectors which realize both a 42°-diameter field-of-view and arcminute resolution. Therefore, it has superior angular resolution for imaging Cherenkov air showers. In this paper, we estimate the sensitivity of and cosmic-ray background resulting from application of the Ashra-1 Cherenkov τ shower observation method. Both data from a commissioning run and a long-term observation (with fully equipped trigger system and one light collector) are presented. Our estimates are based on a detailed Monte Carlo simulation which describes all relevant shower processes from neutrino interaction to Cherenkov photon detection produced by τ air showers. In addition, the potential to determine the arrival direction of Cherenkov showers is evaluated by using the maximum likelihood method. We conclude that the Ashra-1 detector is a unique probe into detection of very high energy neutrinos and their accelerators.     

Time structure of individual extensive air showers

Extensive air showers detected by the GREX array have been sampled by means of highly segmented 8 m² bakelite RPC in the GREX/COVER_PLASTEX experiment. Delay distributions of particles with respect to the first arriving particle in the EAS front at PeV energies have been analysed for individual events in the core distance range of 0–100 m. It is shown that both mean arrival time and EAS front thickness in individual showers fluctuate strongly and cannot be a good measure of the distance from the EAS axis in a 0–100 m core distance interval.

Individual distributions have been compared with integrated inclusive distributions measured in the same experiment. Results indicate that the width of the individual distribution is systematically less than that of the inclusive distribution. It means that the bulk of particles in individual showers arrive as a relatively compact group delayed by different time intervals from the first arriving particle. Such fluctuations of the arrival time for most of the shower particles may be the consequence of large fluctuations in the shower longitudinal development.

Comparison with CORSIKA Monte Carlo simulations confirmed the difference between the mean width of inclusive and individual arrival time distribution. It revealed also the presence in the experiment of the excessive train of delayed particles near the shower core. This train is obviously due to the non-relativistic low energy hadrons most abundant in the shower core region.     

VHE gamma ray astronomy with HAGAR telescope array

HAGAR, an array of seven atmospheric Cherenkov telescopes located at Hanle in Himalayas, has been observing VHE gamma ray sources since September 2008. Taking advantage of the high altitude location, HAGAR could achieve an energy threshold of about 200 GeV. Several astronomical sources, mostly pulsars and blazar class active galactic nuclei, have been observed in the last nine years. Pulsations from Crab pulsar and emission from blazars Mkn 421 and Mkn 501 has been detected successfully. Details of HAGAR telescope array will be given and some important results will be discussed. Also the future plans will be described briefly.     

Methods for point source analysis in high energy neutrino telescopes

Neutrino telescopes are moving steadily toward the goal of detecting astrophysical neutrinos from the most powerful galactic and extragalactic sources. Here we describe analysis methods to search for high energy point-like neutrino sources using detectors deep in the ice or sea. We simulate an ideal cubic kilometer detector based on real world performance of existing detectors such as AMANDA, IceCube, and ANTARES. An unbinned likelihood ratio method is applied, making use of the point spread function and energy distribution of simulated neutrino signal events to separate them from the background of atmospheric neutrinos produced by cosmic ray showers. The unbinned point source analyses are shown to perform better than binned searches and, depending on the source spectral index, the use of energy information is shown to improve discovery potential by almost a factor of two.     

Gamma–hadron separation methods for the VERITAS array of four imaging atmospheric Cherenkov telescopes

Ground-based arrays of imaging atmospheric Cherenkov telescopes have emerged as the most sensitive γ-ray detectors in the energy range of about 100 GeV and above. The strengths of these arrays are a very large effective collection area on the order of 10⁵ m², combined with excellent single photon angular and energy resolutions. The sensitivity of such detectors is limited by statistical fluctuations in the number of Cosmic-ray initiated air showers that resemble γ-ray air showers in many ways. In this paper, we study the performance of simple event reconstruction methods when applied to simulated data of the Very Energetic Radiation Imaging Telescope Array System (VERITAS) experiment. We review methods for reconstructing the arrival direction and the energy of the primary photons, and examine means to improve on their performance. For a software threshold energy of 300 GeV (100 GeV), the methods achieve point source angular and energy resolutions of σ_63% = 0.1° (0.2°) and σ_68% = 15% (22%), respectively. The main emphasis of the paper is the discussion of γ–hadron separation methods for the VERITAS experiment. We find that the information from several methods can be combined based on a likelihood ratio approach and the resulting algorithm achieves a γ–hadron suppression with a quality factor that is substantially higher than that achieved with the standard methods used so far.     

Cascades in the Earth’s magnetosphere initiated by photons with the parameters of the highest energy AGASA events

We investigate the cascading effects of extremely high energy (EHE) photons in the Earth’s magnetosphere assuming that these photons arrive with the parameters of the highest energy AGASA events (energies, arrival directions). For the location of the AGASA Observatory, we determine the directions in the sky from which photons can cascade with a high (low) probability. In the case of the primary photons with the parameters of the events with the energies >10²⁰ eV, we compute the average cascade spectra of secondary photons entering the Earth’s atmosphere, and estimate their fluctuations around these average values by selecting the events with the largest and smallest number of secondary cascade photons. It is shown that most photons with the parameters of the highest energy AGASA events should initiate cascades in the Earth’s magnetosphere with a high probability even though they tend to arrive from directions in the sky for which the perpendicular component of the magnetic field is weaker. On the other hand, if these events are caused by the photons with lower energies, then the fluctuations in their shower development in the magnetosphere and the atmosphere should be higher than in the case of photons with the energies estimated by the AGASA experiment.     

Calibration of the Cherenkov telescope array using cosmic ray electrons

Cosmic ray electrons represent a background for gamma-ray observations with Cherenkov telescopes, initiating air-showers which are difficult to distinguish from photon-initiated showers. This similarity, however, and the presence of cosmic ray electrons in every field observed, makes them potentially very useful for calibration purposes. Here we study the precision with which the relative energy scale and collection area/efficiency for photons can be established using electrons for a major next generation instrument such as CTA. We find that variations in collection efficiency on hour timescales can be corrected to better than 1%. Furthermore, the break in the electron spectrum at ∼ 0.9 TeV can be used to calibrate the energy scale at the 3% level on the same timescale. For observations on the order of hours, statistical errors become negligible below a few TeV and allow for an energy scale cross-check with instruments such as CALET and AMS. Cosmic ray electrons therefore provide a powerful calibration tool, either as an alternative to intensive atmospheric monitoring and modelling efforts, or for independent verification of such procedures.     

Gamma-Hadron Separation using Čerenkov Photon Density Fluctuations

In the atmospheric Čerenkov technique γ-rays are detected against the abundant background produced by hadronic showers. In order to improve the signal to noise ratio of theexperiment, it is necessary to reject a significant fraction of hadronic showers. Traditional background rejection methods based on image shape parameters have been extensively used for the data from imaging telescopes. However, non-imaging Čerenkov telescopes have to develop very different means of statistically identifying and removing cosmic ray events. Some of the parameters, which could be potentially important for non-imaging arrays, are the temporal and spectral differences, the lateral distributions and density fluctuations of Čerenkov photons generated by γ-ray and hadron primaries. Here we study the differences in fluctuations of Čerenkov photon density in the light pool at the observation level from showers initiated by photons and those initiated by protons or heavier nuclei. The database of simulated events for the PACT array has been used to evaluate the efficiency of the new technique. Various types of density fluctuations like the short range and medium range fluctuations as well as flatness parameter are studied. The estimated quality factors reflect the efficiencies with which the hadrons can be rejected from the data. Since some of these parameters are independent, the cuts may be applied in tandem and we demonstrate that the proton rejection efficiency of ∼90% can be achieved. Use of density fluctuations is particularly suited for wavefront sampling observations and it seems to be a good technique to improve the signal to noise ratio. This revised version was published online in July 2006 with corrections to the Cover Date.     

Inverse Compton Gamma Rays from Dark Matter Annihilation in the Dwarf Galaxies

Dwarf spheroidal (dSph) galaxies are thought to be good candidates for dark matter search due to their high mass-to-light (M/L) ratio. One of the most favored dark matter candidates is the lightest neutralino (neutral χ particle) as predicted in the Minimal Supersymmetric Standard Model (MSSM). In this study, we model the gamma ray emission from dark matter annihilation coming from the nearby dSph galaxies Draco, Segue 1, Ursa Minor and Willman 1, taking into account the contribution from prompt photons and photons produced from inverse Compton scattering off starlight and Cosmic Microwave Background (CMB) photons by the energetic electrons and positrons from dark matter annihilation. We also compute the energy spectra of electrons and positrons from the decay of dark matter annihilation products. Gamma ray spectra and fluxes for both prompt and inverse Compton emission have been calculated for neutralino annihilation over a range of masses and found to be in agreement with the observed data. It has been found that the ultra faint dSph galaxy Segue 1 gives the largest gamma ray flux limits while the lowest gamma ray flux limits has been obtained from Ursa Minor. It is seen that for larger M/L ratio of dwarf galaxies the intensity pattern originating from e ⁺ e ^? pairs scattering off CMB photons is separated by larger amount from that off the starlight photons for the same neutralino mass. As the e ⁺ e ^? energy spectra have an exponential cut off at high energies, this may allow to discriminate some dark matter scenarios from other astrophysical sources. Finally, some more detailed study about the effect of inverse Compton scattering may help constrain the dark matter signature in the dSph galaxies.     

The potential of the ground based arrays of imaging atmospheric Cherenkov telescopes. II. Gamma ray flux sensitivities

We discuss the capability of ‘100 GeV’ class imaging atmospheric Cherenkov telescope (IACT) arrays as future powerful instruments of ground-based gamma-ray astronomy. It is assumed that the array is gathered from individually triggered quadrangular 4-IACT ‘cells’ with a linear size of about 100 m. The multi-cell concept allows coverage of large detection areas economically, and at the same time the effective exploitation of the stereoscopic approach of determination of the shower parameters using information obtained by several IACTs simultaneously. Determination of arrival directions of γ-ray primaries on an event-by-event basis with accuracy δθ ≤ 0.1° combined with high suppression efficiency (at both the hardware and software levels) of the background hadronic showers by a factor of ≈ 10³, and large, up to 1 km² collection areas, can provide minimum detectable energy fluxes of ≥ 100 GeV γ-rays from point sources down to 10⁻¹³ erg/cm² s which is about 3 orders of magnitude lower than the current sensitivities achieved by the satellite-borne detectors at MeV and GeV energies. High sensitivities of multi-IACT arrays would partially compensate the limited efficiency of the technique for all-sky surveys, as well as allow study of moderately extended (≤ 1°) γ-ray sources. IACT arrays with minimum detectable fluence of ≥ 100 GeV γ-rays S_γ < 10⁻⁸ erg/cm² are well suited for effective exploration of highly sporadic nonthermal phenomena from different classes of astrophysical objects on time-scales from ≤ 1 s to several minutes.