Long term performance evaluation of the TACTIC imaging telescope using ∼400 h Crab Nebula observation during 2003–2010

The TeV atmospheric Cherenkov telescope with imaging camera (TACTIC) γ-ray telescope has been in operation at Mt. Abu, India since 2001 to study TeV γ-ray emission from celestial sources. During the last 10 years, apart from consistently detecting a steady signal from the Crab Nebula above ∼1.2 TeV energy, at a sensitivity level of ∼5.0 σ in ∼25 h, the telescope has also detected flaring activity from Mrk 421 and Mrk 501 on several occasions. Although we used Crab Nebula data partially, in some of the reported results, primarily for testing the validity of the full data analysis chain, the main aim of this work is to study the long term performance of the TACTIC telescope by using consolidated data collected between 2003 and 2010. The total on-source data, comprising ∼402 h, yields an excess of ∼(3742±192) γ-ray events with a statistical significance of ∼19.9 σ. The off-source data, comprising ∼107 h of observation, is found to be consistent with a no-emission hypothesis, as expected. The resulting γ-ray rate for the on-source data is determined to be ∼(9.31±0.48) h −1. A power law fit (dΦ/dE=f 0 E −Γ ) with f 0 ∼ (2.66±0.29) × 10−11 cm−2 s−1 TeV−1 and Γ ∼ 2.56±0.10 is found to provide reasonable fit to the inferred differential spectrum within statistical uncertainties. The spectrum matches reasonably well with that obtained by other groups. A brief summary of the improvements in the various subsystems of the telescope carried out recently, which has resulted in a substantial improvement in its detection sensitivity (viz., ∼5 σ in an observation period of ∼13 h as compared to ∼25 h earlier) are also presented in this paper. Encouraged by the detection of strong γ-ray signals from Mrk 501 and Mrk 421 on several occasions, 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.