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 hours from 15 April to 30 May, 2012 using TACTIC telescope. Detailed analysis of $\sim$66.3 hours of clean data revealed the presence of a TeV $\gamma$-ray signal (686$\pm$77 $\gamma$-ray events) from the source direction with a statistical significance of 8.89$\sigma$ above 850 GeV. Further, a total of 375 $\pm$ 47 $\gamma$-ray like events were detected in 25.2 hours of observation from 22 - 27 May, 2012 with a statistical significance of 8.05$\sigma$ 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 $dF/dE=f_{0}E^{-\Gamma}$ with $f_{0}= (2.27 \pm 0.38) \times 10 ^{-11} $ photons cm$^{-2}$ s$^{-1}$ TeV$^{-1}$ and $\Gamma=2.57 \pm 0.15$. In order to investigate the source state, we have also used almost simultaneous multiwavelength observations viz: high energy data collected by $\it{Fermi}$-LAT, X-ray data collected by $\it{Swift}$-XRT and MAXI, optical and UV data collected by $\it{Swift}$-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.