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 8.89 σ 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 8.05 σ 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 d F / d E = f 0 E − Γ with f 0 = ( 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.