C K Bhat

TeV observations of Mrk 421 with the TACTIC γ-ray telescope during 2006-8

In this paper, we report on the Markarian 421 results obtained during TeV γ-ray observations from 18 December 2006 to 18 April 2007 and 11 January to 7 May 2008 for 122.68 and 184.82 h, respectively, with the TACTIC γ-ray telescope. During, 2006–7 observations the source was found to be in a low state and we have placed an upper limit of 1.44 × 10−12 photons cm−2 s−1 at the 3σ level on the integrated TeV γ-ray flux above 1 TeV from the source direction. Interestingly, during the 2008 observations the source was detected in a high state. Detailed data analysis revealed the presence of a TeV γ-ray signal with a statistical significance of 9.74σ above Eγ ≥ 1 TeV. We have estimated the time-averaged observed differential energy spectrum of γ-ray-like events in the energy range 1.0–9.77 TeV. The spectrum fits well with the power law function of the form (dF/dE = f0E−Γ) with f0 = (6.8 ± 1.4) × 10−11 cm−2 s−1 TeV−1 and Γ = 3.32 ± 0.22.

TeV gamma-ray observations of Markarian 421 using TACTIC during 2009–10

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 h. Detailed analysis of the data so recorded revealed the presence of a TeV γ-ray signal with a statistical significance of 12.12σ at Eγ ⩾ 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 = f0E−Γ) with f0 = (1.39 ± 0.239) × 10−11 cm−2 s−1 TeV−1 and Γ = 2.31 ± 0.14.