ALTAI: Computational code for the simulations of TeV air showers as observed with the ground-based imaging atmospheric Cherenkov telescopes

Ground-based atmospheric Cherenkov telescopes are proven to be effective instruments for observations of very high energy (VHE) gamma-radiation from celestial objects. For effective use of such technique one needs detailed Monte Carlo simulations of gamma-ray- and proton/nuclei-induced air showers in Earth atmosphere. Here we discuss in detail the algorithms used in the numerical code ALTAI developed particularly for the simulations of Cherenkov light emission from air showers of energy below 50 TeV. The specific scheme of sampling the charged particle transport in the atmosphere allows the performance of very fast and accurate simulations used for interpretation of the VHE gamma-ray observations.Comment: 11 pages, 4 figures, Nuclear Instruments & Methods in Physics Research (Section A), in pres

5@5 - a 5 GeV energy threshold array of imaging atmospheric Cherenkov telescopes at 5 km altitude

We discuss the concept and the performance of a powerful future ground-based astronomical instrument - a stereoscopic array of several large imaging atmospheric Cherenkov telescopes installed at a very high mountain elevation of about 5 km a.s.l. or more - for the study of the gamma-ray sky at energies from several GeV to 100 GeV.Comment: 33 pages, 25 figures, the revised version accepted for publication in Astroparticle Physic

Observations of TeV gamma-rays from Mrk 421 during Dec. 2005 to Apr. 2006 with the TACTIC telescope

The TACTIC $\gamma$-ray telescope has observed Mrk 421 on 66 clear nights from Dec. 07, 2005 to Apr. 30, 2006, totalling $\sim$ 202 hours of on-source observations. Here, we report the detection of flaring activity from the source at $\geq$ 1 TeV energy and the time-averaged differential $\gamma$-ray spectrum in the energy range 1-11 TeV for the data taken between Dec. 27, 2005 to Feb. 07, 2006 when the source was in a relatively higher state as compared to the rest of the observation period. Analysis of this data spell, comprising about $\sim$97h reveals the presence of a $\sim 12.0 \sigma$ $\gamma$-ray signal with daily flux of $>$ 1 Crab unit on several days. A pure power law spectrum with exponent $-3.11\pm0.11$ as well as a power law spectrum with an exponential cutoff $(\Gamma = -2.51\pm0.26$ and $E_0=(4.7\pm2.1) TeV)$ are found to provide reasonable fits to the inferred differential spectrum within statistical uncertainties. We believe that the TeV light curve presented here, for nearly 5 months of extensive coverage, as well as the spectral information at $\gamma$-ray energies of $>$ 5 TeV provide a useful input for other groups working in the field of $\gamma$-ray astronomy.Comment: 13pages,4figures; Accepted for publication in Astroparticle Physic

Evidence of intergalactic absorption in the TeV Gamma-ray spectrum of Markarian 501

The recent High-Energy Gamma-Ray Array (HEGRA) observations of the blazar Mrk 501 show strong curvature in the very high energy 7-ray spectrum. Applying the γ-ray opacity derived from an empirically based model of the intergalactic infrared background radiation field to these observations, we find that the intrinsic spectrum of this source is consistent with a power law: dNγldE ∝ E-α, with α = 2.00 ± 0.03 over the range 500 GeV-20 TeV. Within current synchrotron self-Compton scenarios, the fact that the TeV spectral energy distribution of Mrk 501 does not vary with luminosity, combined with the correlated, spectrally variable emission in X-rays as observed by the BeppoSAX and Rossi X-Ray Timing Explorer instruments, also independently implies that the intrinsic spectrum must be close to α = 2. Thus, the observed curvature in the spectrum is most easily understood as resulting from intergalactic absorption

The first imaging air Cherenkov telescope of the HEGRA collaboration

Available from TIB Hannover: RR 2916(94-24) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman