Simulation studies for optimizing the trigger generation criteria for the TACTIC telescope

In this paper, we present the results of Monte Carlo simulations of $\gamma$-ray and cosmic ray proton induced extensive air showers as detected by the TACTIC atmospheric Cherenkov imaging telescope for optimizing its trigger field of view and topological trigger generation scheme. The simulation study has been carried out at several zenith angles. The topological trigger generation uses a coincidence of 2 or 3 nearest neighbour pixels for producing an event trigger. The results of this study suggest that a trigger field of 11$\times$11 pixels ($\sim$ $3.4^o$ $\times$ $3.4^o$) is quite optimum for achieving maximum effective collection area for $\gamma$-rays from a point source. With regard to optimization of topological trigger generation, it is found that both 2 or 3 nearest neighbour pixels yield nearly similar results up to a zenith angle of ~$25^o$ with a threshold energy of $\sim$ 1.5 TeV for $\gamma$-rays. Beyond zenith angle of ~$25^o$, the results suggest that a 2-pixel nearest neighbour trigger should be preferred. Comparison of the simulated integral rates has also been made with corresponding measured values for validating the predictions of the Monte Carlo simulations, especially the effective collection area, so that energy spectra of sources (or flux upper limits in case of no detection) can be determined reliably. Reasonably good matching of the measured trigger rates (on the basis of $\sim$ 207 hours of data collected with the telescope in NN-2 and NN-3 trigger configurations) with that obtained from simulations reassures that the procedure followed by us in estimating the threshold energy and detection rates is quite reliable.