Limits on the TeV flux of diffuse gamma rays as measured with the HEGRA air shower array

Possible discrimination between gamma rays and hadrons using Čerenkov photon timing measurements

The Cygnus X region contains a number of powerful sources of radio and X-ray radiation which are also considered as potential sources of high and ultrahigh-energy radiation. One of such sources is the massive binary system Cyg X-3 systematically observed by the SHALON telescope from 1995 to the present. The results of Cyg X-3 observations are presented by γ-ray spectra and images at energies of 800 GeV-85 TeV. In the field of the SHALON telescope, an ultrahigh-energy γ-ray source was detected, whose position is ∼2° from Cyg X-3; its coordinates coincide with coordinates of the known source of radio and X-ray radiation, i.e., the γCygni SNR supernova remnant. The γCygni SNR spectra, spectral energy distribution, and image according to the SHALON telescope data are presented in the energy range of 800 GeV-50 TeV.

Long-term observations of the Cyg X-3 and λCygni SNR at ultrahigh energies by SHALON mirror Cherenkov telescopes

Monte Carlo simulations of cosmic-ray-induced showers in the atmosphere have been used to develop three new methods for measuring the energy of the primary particles in the - eV range independently of the particle species. Both the electron number and Cherenkov light density near and beyond the hump in the lateral distributions are found to accurately indicate the primary energy, provided an appropriate correction based on the depth of the shower maximum is applied. The depth of the shower maximum can be estimated from the slope of the radial distribution of the Cherenkov light. In addition, a simultaneous measurement of the muon number and Cherenkov light intensity also provides a very accurate measure of the primary energy. All of these parameters can be determined by using existing particle detector arrays. A very precise measurement of the cosmic-ray all-particle spectrum regardless of chemical composition is thus shown to be practicable.

http://iopscience.iop.org/article/10.1088/0954-3899/23/11/020/pdf

Methods for determining the primary energy of cosmic-ray showers

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.

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

Data from the HEGRA air shower array are used to set an upper limit on the emission of gamma-radiation above 25 (18) TeV from the direction of the radio bright region DR4 within the SNR G78.2 + 2.1 of 2.5 (7.1). 10^-13 cm^-2 sec^-1. The shock front of SNR G78.2 + 2.1 probably recently overtook the molecular cloud Gong 8 which then acts as a target for the cosmic rays produced within the SNR, thus leading to the expectation of enhanced gamma-radiation. Using a model of Drury, Aharonian and Volk which assumes that SNRs are the sources of galactic cosmic rays via first order Fermi acceleration, we calculated a theoretical prediction for the gamma-ray flux from the DR4 region and compared it with our experimental flux limit. Our 'best estimate' value for the predicted flux lies a factor of about 18 above the upper limit for gamma-ray energies above 25 TeV. Possible reasons for this discrepancy are discussed.

Search for very high energy gamma radiation from the radio bright region DR4 of the SNR G78.2+2.1

The discrimination between air showers initiated by γ rays and by hadrons is one of the fundamental problems in experimental cosmic-ray physics. The physics of this ‘ γ /hadron separation’ is discussed in this paper. We restrict ourselves to the energy range from about 20 to 500 TeV, and take only the information contained in the lateral Cerenkov light distribution and the number of electrons at the detector level into consideration. An understanding of the differences between air showers generated by γ rays and those due to hadrons leads us to formulate suitable observables for the separation process. Angle integrating Cerenkov arrays (AICA) offer a promising new approach to ground-based γ -ray astronomy in the energy region from about 20 to 500 TeV. In order to establish this technique, an efficient suppression of the overwhelming hadronic background radiation is required. As an example for our general discussion, we present one method for γ /hadron separation in AICAs called ‘LES’. It is based on the simultaneous determination of the shower size and some characteristic parameters of the lateral distribution of the Cerenkov light. The potential inherent within this technique is demonstrated in quantitative detail for the existing ‘AIROBICC’ AICA. We also propose an objective measure of the intrinsic sensitivity of a detection scheme in ground-based γ -ray astronomy, the ‘reduced quality factor’. It is shown that AICAs may reach a sensitivity to γ -ray point sources in the high VHE range similar to that of the Cerenkov-telescope imaging technique in the low VHE region.

Separation of gamma and hadron initiated air showers with energies between 20 and 500 TeV.

Separation of gamma and hadron initiated air showers with energies between 20 and 500 TeV

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Search for very high energy gamma radiation from the radio bright region DR4 of the SNR G78.2+2.1

Separation of gamma and hadron initiated air showers with energies between 20 and 500 TeV.

Separation of gamma and hadron initiated air showers with energies between 20 and 500 TeV