TACTIC Imaging γ-Ray Telescope: A Simulation Study of the Effect of Switched-off Pixels on Its Quality Factor

The 349-pixel Cerenkov Light Imaging Camera of the -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.     

Examining the feasibility of air shower core-location from polarization properties of associated Cherenkov light

Based on this exploratory investigation involving CORSIKA simulation code generated Cherenkov photons and a linearly polarized, hypothetical photon beam, we make a case here for exploiting polarization properties of atmospheric Cherenkov events for providing an independent method for locating air-shower cores by a TACTIC-like array of atmospheric Cherenkov telescopes. Preliminary results based on simulations indicate that for a 3 TeV γ-ray having ∼30% degree of polarization for its associated Cherenkov light at a core distance of ∼100 m, core location can be found with an error of ∼27 m. Deceased This revised version was published online in July 2006 with corrections to the Cover Date.     

Drive-Control System for the TACTIC Gamma-Ray Telescope

A PC-based drive-control system has been developed for the altitude-azimuth mounted TACTIC -ray telescope to control the speed and direction of motion of its 2-axes. Details of various hardware components chosen for the telescope, including hybrid-stepper motor, 16-bit absolute encoder and CAMAC-based programmable stepper motor controller, are discussed in this paper. The telescope-control strategy, based on the position-loop with a proportional type control for the source-seek mode and the on/off type control for the tracking mode, is explained in detail. Some important performance features of the telescope, including its blind-spot size, drive-system backlash and encoder-error compensation, are also presented. The drive system has been extensively field-tested and has been operating satisfactorily during observation campaigns carried out since March 1997 with the TACTIC Imaging Element. A tracking accuracy of ±3 arc-minutes has been achieved. A test report of its performance, with regard to its tracking accuracy on the basis of successful detection of TeV -rays from the active galaxy Markarian 501 in April–May 1997, is also presented.     

Ground-based calibration and characterization of the Fermi gamma-ray burst monitor detectors

One of the scientific objectives of NASA’s Fermi Gamma-ray Space Telescope is the study of Gamma-Ray Bursts (GRBs). The Fermi Gamma-Ray Burst Monitor (GBM) was designed to detect and localize bursts for the Fermi mission. By means of an array of 12 NaI(Tl) (8 keV to 1 MeV) and two BGO (0.2 to 40 MeV) scintillation detectors, GBM extends the energy range (20 MeV to > 300 GeV) of Fermi’s main instrument, the Large Area Telescope, into the traditional range of current GRB databases. The physical detector response of the GBM instrument to GRBs is determined with the help of Monte Carlo simulations, which are supported and verified by on-ground individual detector calibration measurements. We present the principal instrument properties, which have been determined as a function of energy and angle, including the channel-energy relation, the energy resolution, the effective area and the spatial homogeneity.