Ja n 20 07 Observations of TeV γ-rays from Mrk 421 during Dec . 2005 to Apr . 2006 with the TACTIC telescope

The TACTIC γ-ray telescope has observed Mrk 421 on 66 clear nights from Dec. 07, 2005 to Apr. 30, 2006, totalling ∼ 202 hours of on-source observations. Here, we report the detection of flaring activity from the source at ≥ 1 TeV energy and the time-averaged differential γ-ray spectrum in the energy range 1 − 11 TeV for the data taken between Dec. 27, 2005 to Feb. 07, 2006 when the source was in a relatively higher state as compared to the rest of the observation period. Analysis of this data spell, comprising about ∼97h reveals the presence of a ∼ 12.0σ γ-ray signal with daily flux of > 1 Crab unit on several days. A pure power law spectrum with exponent −3.11 ± 0.11 as well as a power law spectrum with an exponential cutoff (Γ = −2.51 ± 0.26 and E 0 = (4.7 ± 2.1)T eV) are found to provide reasonable fits to the inferred differential spectrum within statistical uncertainties. We believe that the TeV light curve presented here, for nearly 5 months of extensive coverage, as well as the spectral information at γ-ray energies of > 5 TeV provide a useful input for other groups working in the field of γ-ray astronomy.