TeV γ-ray emission from Mrk 421 observed with TACTIC during December 2014-February 2015

We observed the TeV blazar Mrk421 in the very high energy (VHE) region with TACTIC from 21 December, 2014 to 21 February, 2015. The VHE γ-ray signal is detected from the source at a statistical significance of  ∼ 9σ with 648  ±  74 γ-ray like events using a total observation time of 65.6 h of clean data. To denote the active period of the source, the entire observation has been divided into two spells: Spell I (21-30 December, 2014; MJD: 57012-57021) and Spell II (15 January-21 February, 2015; MJD: 57037-57074). The Spell I corresponds to relatively higher activity state wherein 337  ±  47 γ-rays are detected at a statistical significance of 7.39σ in 21.7 h. During Spell II, 311  ±  57 γ-ray events have been detected in  ∼ 44c at a statistical significance of 5.6σ. The analysis of high energy (HE) γ-ray data from Fermi-LAT has been carried out for the period of TACTIC observations. In addition, near simultaneous multi-wavelength data from Swift-BAT and MAXI in the X-ray region, SPOL in optical V-band and OVRO at 15 GHz radio band have also been considered. The HE γ-ray and optical observations indicate variability during both the spells whereas hard X-ray observations are consistent with the average emission through out the TACTIC observations. The broad-band spectral energy distributions of the source are reproduced by single zone homogeneous synchrotron self Compton process for the two spells. The model parameters estimated for the two spells are similar except for the particle energy density which is obtained to be slightly higher during Spell I.