Multiwavelength study of VHE emission from Markarian 501 using TACTIC observations during April–May, 2012

Abstract

We have observed Markarian 501 in Very High Energy (VHE) gamma-ray wavelength band for 70.6 h from 15 April to 30 May, 2012 using TACTIC telescope. Detailed analysis of ∼66.3 h of clean data revealed the presence of a TeV γ-ray signal (686± 77 γ-ray events) from the source direction with a statistical significance of 8.89σ above 850 GeV. Further, a total of 375 ± 47 γ-ray like events were detected in 25.2 h of observation from 22 – 27 May, 2012 with a statistical significance of 8.05σ indicating that the source has possibly switched over to a relatively high gamma-ray emission state. We have derived time-averaged differential energy spectrum of the state in the energy range 850 GeV - 17.24 TeV which fits well with a power law function of the form dF/dE=f0E−Γ with f0=(2.27±0.38)×10−11 photons cm−2 s−1 TeV−1 and Γ=2.57±0.15. In order to investigate the source state, we have also used almost simultaneous multiwavelength observations viz: high energy data collected by Fermi-LAT, X-ray data collected by Swift-XRT and MAXI, optical and UV data collected by Swift-UVOT, and radio data collected by OVRO, and reconstructed broad-band Spectral Energy Distribution (SED). The obtained SED supports leptonic model (homogeneous single zone) for VHE gamma-ray emission involving synchrotron and synchrotron self Compton (SSC) processes.