C. L. Bhat

Programmable topological trigger generator for the 349-pixel imaging camera of the TACTIC telescope

Abstract 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.

Drive-Control System for the TACTIC Gamma-Ray Telescope

A PC-based drive-control system has been developed for the altitude-azimuth mounted TACTIC γ-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 γ-rays from the active galaxy Markarian 501 in April–May 1997, is also presented.

On a single-counts rate stabilization scheme employed in the imaging camera of the TACTIC γ-ray telescope

A cost-effective methodology for operating the 349-pixel Cerenkov light imaging camera of the imaging element of the TACTIC γ-ray telescope array at stable single-counts rate and safe anode current values has successfully been tried out despite variations in the light of the night sky experienced by these pixels from time to time. Important details of this unconventional scheme are discussed and reference is made to the methods evolved for in situ absolute and relative gain calibrations of the camera pixels, which are particularly required for this mode of operation.

Semi-intelligent trigger-generation scheme for Cherenkov light imaging cameras

Abstract We propose here an improved trigger-generation scheme for TeV gamma-ray imaging telescopes. Based on a memory-based Majority Coincidence Circuit, this scheme involves deriving two- or three-pixel nearest-neighbour coincidences as against the conventional approach of generating prompt coincidences using any two photomultiplier detector pixels of an imaging-camera. As such, the new method can discriminate better against shot-noise-generated triggers and, to a significant extent, also against cosmic-ray and local-muon-generated background events, without compromising on the telescope response to events of γ-ray origin. An optional feature of the proposed scheme is that a suitably scaled-up value of the chance-trigger rate can be independently derived, thereby making it possible to use this parameter reliably for keeping a log of the “health” of the experimental system.

A computer-programmable delay-generator scheme for the TACTIC gamma-ray telescope

Abstract A programmable multi-channel digital-delay generator, characterised by negligible dead-time count losses, has been developed for deployment in the high-sensitivity TeV γ-ray telescope, TACTIC. This telescope comprises an array of 4 × 10 m 2 of light collectors, of which three are placed at the corners of an equilateral triangle of side ≥ 20 m and are operated in a prompt-coincidence mode with a resolving time of ∼10 ns to detect atmospheric Cherenkov events produced by very high energy γ-ray and cosmic-ray proton primaries. The delay generator helps to line up these events in real time whenever they are detected at different relative times by these elements. The basic circuit used in this scheme can be readily adapted for utilisation in several other areas, including experimental nuclear physics.

A new upper limit on optical bursts from primordial black hole explosions

Two wide-angle photomultiplier systems were simultaneously operated over a baseline of nearly 30 km in a search for cosmic optical bursts of fractional microsecond time-scale. In 74 hours of overlapping observation, one event was recorded coincident to 1 ms as against the corresponding accidental coincidence value of 0.14 from Čerenkov light pulses from unrelated cosmic ray showers. The possible cosmic origin of this event, including that from primordial black-hole explosions, is discussed and the corresponding upper limits derived.

Hybrid Zener-based VDN - a better choice for Cerenkov light imaging cameras

A Zener-based voltage divider network has been used for the photomultiplier tubes (PMTs) employed in the multi-pixel camera of a Cherenkov imaging telescope. This hybrid network ensures a linear photomultiplier response at a chain current nearly an order of magnitude less than that required for a purely resistive voltage divider network of conventional design. The network has been validated for the Thorn-EMI photomultiplier tube 9135B and the results discussed here are also representative of other PMT makes.

A Search for the millisecond pulsar in Cygnus X-3

Atmospheric Cerenkov data, recorded at Gulmarg, India, during the 2-year period 1988–1989 in the direction of Cygnus X-3, have been searched for the presence of a periodic pulsar modulation, employing both the Rayleigh power and the epoch-folding (χ2) techniques. No evidence is found for pulsations at either ∼ 12.59 ms or ∼ 9.22 ms, the two period values around which the detection of pulsed signals has been reported by three atmospheric Cerenkov groups, but not by at least two other groups, including one using a higher-sensitivity Cerenkov imaging telescope.

Possible transient emission from Cygnus X-3 at TeV energies during October 1985 radio flare

The Gulmarg gamma-ray telescope (threshold energy ∼6 TeV) was tracking Cygnus X-3 on four days in October 1985, around the time the source produced one of its most outstanding radio flares. Examined here is the behaviour of the event rates recorded during this period with a time-resolution of 216 s. Two episodes of higher event rates, each several minutes in duration and having a Poissonian significance of ∼4σ, were recorded on 10 and 12 October, suggesting possible activity at TeV energies during the radio flare period of Cygnus X-3.

Artificial Neural Network Approach for Reconstruction of Event Arrival-Direction in Wide-Angle Atmospheric Cerenkov Detector Arrays: A Feasibility Study

A feasibility study has been carried out to assess the potentialof an Artificial Neural Network (ANN) for determiningthe direction of incidence of an Atmospheric Cerenkov Event(ACE) from the arrival-time information registered by aspaced-array of wide-angle Cerenkov detectors. Theresults obtained so far, using both, simulated and experimental data, indicate that a properly-trained net can yield a degree of accuracy which is comparable with what is achieved through the conventional χ2-minimization, wavefront-fitting procedure.