Possible Discrimination between Gamma Rays and Hadrons using Cerenkov Photon Timing Measurements

Atmospheric \v{C}erenkov Technique is an established methodology to study $TeV$ energy gamma rays. However the challenging problem has always been the poor signal to noise ratio due to the presence of abundant cosmic rays. Several ingenious techniques have been employed to alleviate this problem, most of which are centred around the \v{C}erenkov image characteristics. However there are not many techniques available for improving the signal to noise ratio of the data from wavefront sampling observations. One such possible technique is to use the \v{C}erenkov photon arrival times and identify the species dependent characteristics in them. Here we carry out systematic monte carlo simulation studies of the timing information of \v{C}erenkov photons at the observation level. We have parameterized the shape of the \v{C}erenkov shower front as well as the pulse shapes in terms of experimentally measurable quantities. We demonstrate the sensitivity of the curvature of the shower front, pulse shape parameters as well as the photon arrival time jitter to primary species and show their efficiency in improving the signal to noise ratio. The effect of limiting the \v{C}erenkov telescope opening angle by using a circular focal point mask, onthe efficacy of the parameters has also been studied for each of the parameters. Radius of the shower front, pulse decay time and photon arrival time jitter have been found to be the most promising parameters which could be used to discriminate $\gamma -$ray events from the background. We also find that the efficiency of the first two parameters increases with zenith angle and efficiency of pulse decay time decreases with increasing altitude of observation.Comment: 30 pages, 5 postscript figures, uses elsart.sty; To appear in Astroparticle Physic

Simulation Studies on Arrival Time Distributions of Cherenkov Photons in Extensive Air Showers

Atmospheric Cherenkov technique is an established methodology to study TeV energy gamma rays. Here we carry out systematic monte carlo simulation studies of the timing information of Cherenkov photons. Extensive studies have already been carried out in this regard. Most of these are carried out at higher energies with the aim of studying the elemental composition of cosmic rays. However not much attention is paid to the species dependent signatures at TeV energies. In this work, functional fits have been carried out to the spherical Cherenkov shower fronts and the radii of curvature have been found to be equal to the height of shower maximum irrespective of the species or the observation level. Functional fits have also been carried out to describe the pulse shapes at various core distances in terms of well known probability density distribution functions (PDF). Two types of PDF's have been tried viz. gamma function and lognormal function. The variation of the pulse shape parameters as a function of primary energy, observation height and incident angles have been studied. The possibility of deriving the pulse shape parameters like the rise & decay times, full width at half maximum from the easily measurable quantities like the mean and RMS variation of photon arrival times offers a very important new technique which can be easily applied in an observation.Comment: 34 pages, 14 figures, accepted for publication in Astroparticle Physic

Angular Resolution of the Pachmarhi Array of Cerenkov Telescopes

The Pachmarhi Array of Cerenkov Telescopes consists of a distributed array of 25 telescopes that are used to sample the atmospheric Cerenkov Photon showers. Each telescope consists of 7 parabolic mirrors each viewed by a single photo-multiplier tube. Reconstruction of photon showers are carried out using fast timing information on the arrival of pulses at each PMT. The shower front is fitted to a plane and the direction of arrival of primary particle initiating the shower is obtained. The error in the determination of the arrival direction of the primary has been estimated using the {\it split} array method. It is found to be $\sim 2.4^\prime ~$ for primaries of energy $> 3 \~TeV$. The dependence of the angular resolution on the separation between the telescopes and the number of detectors are also obtained from the data.Comment: 26 pages, 11 Postscript figures; uses elsart.cls. To appear in Astroparticle Physic

Cerenkov Photon Density Fluctuations in Extensive Air Showers

The details of Cerenkov light produced by a gamma ray or a cosmic ray incident at the top of the atmosphere is best studied through systematic simulations of the extensive air showers. Recently such studies have become all the more important in view of the various techniques resulting from such studies, to distinguish gamma ray initiated showers from those generated by much more abundant hadronic component of cosmic rays. We have carried out here such systematic simulation studies using CORSIKA package in order to understand the Cerenkov photon density fluctuations for 5 different energies at various core distances both for gamma ray and proton primaries incident vertically at the top of the atmosphere. Such a systematic comparison of shower to shower density fluctuations for gamma ray and proton primaries is carried out for the first time here. It is found that the density fluctuations are significantly non-Poissonian. Such fluctuations are much more pronounced in the proton primaries than gamma ray primaries at all energies. The processes that contribute significantly to the observed fluctuations have been identified. It has been found that significant contribution to fluctuations comes from photons emitted after shower maximum. The electron number fluctuations and correlated emission of Cerenkov photons are mainly responsible for the observed fluctuations.Comment: 31 pages, latex, 16 figures in ps files, Accepted for publication in "Astroparticle Physics

Long term variability and correlation study of the blazar 3C 454.3 in radio, NIR and optical wavebands

We performed a long-term optical (B, V, R bands), infra-red (J and K bands) and radio band (15, 22, 37 GHz band) study on the flat spectrum radio quasar, 3C 454.3, using the data collected over a period of more than 8 years (MJD 54500–57500). The temporal variability, spectral properties and inter-waveband correlations were studied by dividing the available data into smaller segments with more regular sampling. This helped us constrain the size and the relative locations of the emission regions for different wavebands. Spectral analysis of the source revealed the interplay between the accretion disk and jet emission. The source predominantly showed a redder-when-brighter trend, though we observed a bluer-when-brighter trend at high flux levels which could be signatures of particle acceleration and radiative cooling. Significant correlations with near-zero lag were seen between various optical/infra-red bands, indicating that these emission regions are co-spatial. Correlations with a time lag of about 10–100 days are seen between optical/infra-red and radio bands indicating these emissions arise from different regions. We also observe the DCF peak lag change from year to year. We try to explain these differences using a curved jet model where the different emission regions have different viewing angles resulting in a frequency dependent Doppler factor. This variable Doppler factor model explains the variability timescales and the variation in DCF peak lag between the radio and optical emissions in different segments. Lags of 6-180 days are seen between emissions in various radio bands, indicating a core-shift effect.Peer reviewe

VHE Gamma-ray Observation of Crab Nebula with HAGAR Telescope Array

International audienceHAGAR is a system of seven Non-imaging Atmospheric Cherenkov Telescopes located at Hanle in the Ladakh region of the Indian Himalayas at an altitude of 4270 meters amsl. Since 2008, we have observed the Crab Nebula to assess the performance of the HAGAR telescopes. We describe the analysis technique for the estimation of γ-ray signal amidst cosmic ray background. The consolidated results spanning nine years of the Crab nebula observations show long term performance of the HAGAR telescopes. Based on about 219 hours of data, we report the detection of γ-rays from the Crab Nebula at a significance level of about 20σ, corresponding to a time averaged flux of (1.64±0.09) × 10$^{− 10}$ photons cm$^{− 2}$ sec$^{− 1}$ above 230 GeV. Also, we perform a detailed study of possible systematic effects in our analysis method on data taken with the HAGAR telescopes

Calibration of cameras of the H.E.S.S. detector

H.E.S.S.—the high energy stereoscopic system—is a new system of large atmospheric Cherenkov telescopes for GeV/TeV astronomy. Each of the four telescopes of 107 m2 mirror area is equipped with a 960-pixel photomultiplier-tube camera. This paper describes the methods used to convert the photomultiplier signals into the quantities needed for Cherenkov image analysis. Two independent calibration techniques have been applied in parallel to provide an estimation of uncertainties. Results on the long-term stability of the H.E.S.S. cameras are also presented

Very high energy gamma rays from the direction of Sagittarius A*

We report the detection of a point-like source of very high energy (VHE) gamma-rays coincident within 1' of Sgr A*, obtained with the H.E.S.S. array of Cherenkov telescopes. The gamma-rays exhibit a power-law energy spectrum with a spectral index of -2.2 +/- 0.09 +/- 0.15 and a flux above the 165 GeV threshold of (1.82 +/- 0.22) \times 10^{-7} m^{-2} s^{-1}. The measured flux and spectrum differ substantially from recent results reported in particular by the CANGAROO collaboration