Study of short term enhanced TeV γ- ray emission from Mrk 421 observed with TACTIC on December 28, 2014

Abstract

In this work, we report on the detection of enhanced TeV γ- ray emission from the high synchrotron-peaked blazar Mrk 421 with the TACTIC telescope on the night of December 28, 2014 (MJD 57019). We use data from the TACTIC observations of Mrk 421 for one week during December 25–31, 2014 (MJD 57016–57022) in this study. The TACTIC observation on December 28, 2014 (MJD 57019) alone results in the detection of 86 ± 17 γ- ray like events from Mrk 421 with a statistical significance of 5.17σ in a livetime of ∼ 2.2 hours above an energy threshold of 0.85 TeV. The high statistics (higher than three Crab Units) of TeV photons enables us to study the very high energy (VHE) γ- ray emission from the source at shorter timescales. A minimum variability timescale of ∼ 0.72 days is obtained for the TeV γ- ray emission from Mrk 421 during the above flaring activity of the source. The intrinsic VHE spectrum is described by a power law with spectral index of 2.99 ± 0.38 in the energy range 0.85–8.5 TeV. The integral VHE γ- ray flux above 0.85 TeV is determined to be (3.68±0.64)×10−11 ph cm−2 s−1 from the TACTIC observations of Mrk 421 on the night of December 28, 2014 (MJD 57019). Near simultaneous measurements by the HAWC observatory give an integral flux of (2.90±0.40)×10−11 ph cm−2 s−1 above 2 TeV from Mrk 421 observations on December 29, 2014 (MJD 57020.33–57020.58). We have also analysed the contemporaneous data from Fermi-LAT to study the high energy (HE) γ–ray emission during the high activity state of the source. The HE γ–ray emission is observed to be increasing after the TeV flaring activity detected with the TACTIC. We also use other near simultaneous archival data available from the Swift-BAT in hard X-rays and from SPOL at Steward Observatory in optical V and R bands to characterize the multi-wavelength emission of Mrk 421 during the high activity state observed at TeV energies. The TeV γ- ray emission observed on December 28, 2014 (MJD 57019) is found to be more prominent than the emissions in lower energy bands during the same period. The TeV γ- ray observation of Mrk 421 in high activity state with the TACTIC telescope is also used to understand the physical mechanism for blazar emission under the frame work of the leptonic single zone synchrotron self Compton process.