We have observed the blazar Markarian 421 (z=0.031 ) with the TACTIC gamma- ray telescope having thresholdenergy of 1.5TeV at Mt. Abu Rajasthan India from January to April 2004. Observations were m ...

A generalized ray-tracing procedure has been developed and used for studying the optical properties of the 21m diameter (f/1 system) tessellated light collector of the MACE telescope, proposed to be set up at Hanle, India (32.8 degrees N, 78.9 degrees E, 4200m asl). The light collector will use 356 panels of size similar to 984 mm x 984 mm, with each panel having 4 or 9 small spherical mirrors fixed on it. The results of the ray tracing simulation studies, carried out for several tight collector designs, suggest that using a paraboloid basket along with mirror panels of graded focal lengths in the range 2109-2245 cm yield the best possible focusing and timing characteristics. The D-95 values of this light collector design are determined to be similar to 29.3 mm ( 0.08 degrees) and similar to 84.7 mm ( 0.23 degrees) at incidence angles of 0 degrees and 1 degrees, respectively and are significantly smaller than those of the conventional paraboloid design using mirror panels of the same focal length. The Davies-Cotton design, on the other hand, although found to yield slightly better off-axis characteristics at incidence angles beyond 0.75 degrees as compared to the graded focal length paraboloid design, introduces a time spread of similar to 9 ns which can lead to a contamination of the genuine Cerenkov signal from unwanted light of night sky background.

The fact that EGRET detected over 250 objects above 100 MeV while only a handful of sources have been detected above 100 GeV indicates that there is a good chance of detecting a significant number of these sources (particularly those which do not show a signature of steepening or cut off up to a few GeV), if the threshold energy of ground based telescopes is lowered to similar to 20 GeV. In the present study, we report the threshold energy estimate of the MACE (Major Atmospheric Cerenkov Experiment) imaging telescope, proposed to be installed in the campus of Indian Astronomical Observatory at Hanle (32.8 degrees N, 78.9 degrees E, 4200m, asl). The results of the Monte Carlo simulations carried out with CORSIKA code, suggest that using a pixel threshold of >= 4pe and a nearest neighbour quadruplet trigger, gamma-ray energy threshold of similar to 15GeV is achievable by the MACE telescope. Details of the simulation work for estimating the threshold energy along with results obtained are presented in this paper.

A generalized ray-tracing procedure has been developed, which facilitates the design of a multimirror-based light collector used in atmospheric Cherenkov telescopes. This procedure has been employed to study the optical characteristics of the 3.5m diameter light collector of the TACTIC Imaging telescope. Comparison of the measured point-spread function of the light collector with the simulated performance of ideal Davies–Cotton and paraboloid designs has been made to determine an optimum arrangement of the 34 spherical mirror facets used in the telescope to obtain the best possible point-spread function. A description of the ray-tracing subroutine used for processing CORSIKA-generated Cherenkov data, required for carrying out Monte-Carlo simulation studies, is also discussed in the paper.

A CAMAC-based programmable trigger generator has been developed for the imaging camera of the TACTIC telescope. The paper describes the design and implementation details of the multi-module trigger generator which selects Cherenkov events on the basis of simultaneous detection by 3 non-collinear nearest-neighbour pixels. The details of its performance evaluation along with its expected behaviour in deciding the threshold energy of the TACTIC telescope are also discussed.

Air particulate samples collected during 1995–96 ata background site situated on the east coast of Thar Desert inRajsthan State of India were analysed for atmospheric dustloads (Suspended Particulate Matter) and elemental composition.The values of SPM ranged from 9 μg M-3 to 97μg M-3 with an average of 43 μg M-3 except a fewepisodic values, which were 3 to 5 times higher than the averageduring summer months. The results for elemental composition ofthe particulate samples showed that the concentrations ofanthropogenic toxic trace elements viz. Br, Cr, Pb, Sb, Se and Znare lower by a factor of 2 to 10 as compared to urban areas. Thehigh enrichment factors for anthropogenic elements viz. Br, Pb,Sb and Zn show an input from coal/wood fuel burning andvehicular pollution at the sampling site. The depletion of Si inSPM samples shows long distance transport of dust to the samplingsite.

Based on a detailed simulation study using the CORSIKA code, we present here results on the γ-ray and cosmic-ray proton trigger-thresholds for the 4-element TACTIC array of atmospheric Cerenkov telescopes, used for several potentially useful configurational modes.

The 349-pixel Cerenkov Light Imaging Camera of the γ-raytelescope array, TACTIC, has been recently commissioned at Mt.Abu, India. This telescope is being used for gamma-ray sourceobservations at photon energies above 0.5 TeV. Comprehensivesimulation studies, using the CORSIKA air-shower code, are inprogress to predict and optimize the performance of the TACTIC.In this short note, we study the expected performance of theImaging Element of the TACTIC under conditions of a pixelswitch-off, forced either by the well-known star-rotationphenomenon or by a defective instrumentation channel. Thechanges in the image parameters, and the resulting telescopequality factor, are studied after simulating two representativecases of pixel shut-down.

We present here CORSIKA-based simulation results with the principal aim of examining the feasibility of operating a MACE-like large-aperture Cerenkov telescope in the near UV-region (lambda similar to 220-300nm). Among other things, this unconventional mode of operation will help to use such a telescope under moonlit conditions, leading to a significantly larger operational duty-cycle at low gamma -ray threshold energies.

The results of our observational campaigns on the two extragalactic sources Mkn 501 and Mkn 421, carried out with the Imaging Element of the TACTIC array, during March-May, 1998 and April-May, 1999, are presented. The results indicate that the two BL Lac objects (Mkn 501 and Mkn 421) were in a `low' gamma-ray emission state during both epochs of our observations.