The TACTIC γ-ray telescope, equipped with a light collector of area ∼9.5m 2 and a medium resolution imaging camera of 349-pixels, has been in operation at Mt.Abu, India since 2001. This paper describes the main features of its various subsystems and its overall performance with regard to (a) tracking accuracy of its 2-axes drive system, (b) spot size of the light collector, (c) back-end signal processing electronics and topological trigger generation scheme, (d) data acquisition and control system and (e) relative and absolute gain calibration methodology. Using a trigger field of view of 11×11 pixels (∼ 3.4 • ×3.4 • ), the telescope records a cosmic ray event rate of ∼2.5 Hz at a typical zenith angle of 15 • . Monte Carlo simulation results are also presented in the paper for comparing the expected performance of the telescope with actual observational results. The consistent detection of a steady signal from the Crab Nebula above ∼1.2 TeV energy, at a sensitivity level of ∼5.0σ in ∼25 h, alongwith excellent matching of its energy spectrum with that obtained by other groups, reassures that the performance of the TACTIC telescope is quite stable and reliable. Furthermore, encouraged by the detection of strong γ-ray signals from Mrk 501 (during 1997(during and 2006(during observations) and Mrk 421 (during 2001(during and 2005(during -2006 observations), we believe that there is considerable scope for the TACTIC telescope to monitor similar TeV γ-ray emission activity from other active galactic nuclei on a long term basis.

Abstract. We have observed the blazar Markarian 421 with the TACTIC γ-ray telescope at Mt. Abu, India, from 22 November 2009 to 16 May 2010 for 265 hours. Detailed analysis of the data so recorded revealed presence of a TeV γ-ray signal with a statistical significance of 12.12σ at E γ ≥ 1 TeV. We have estimated the time averaged differential energy spectrum of the source in the energy range 1.0 -16.44 TeV. The spectrum fits well with the power law function of the form (dF/dE = f 0 E −Γ ) with f 0 = (1.39 ± 0.239) × 10 −11 cm −2 s −1 T eV −1 and Γ = 2.31 ± 0.14.

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.

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