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

## TL;DR

This study reports the detection of VHE gamma-ray emission from Markarian 501 during 2012, analyzes its spectral properties, and supports a leptonic emission model based on multiwavelength observations.

## Contribution

First detailed multiwavelength analysis of Markarian 501's VHE emission during a high state in 2012, confirming leptonic processes as the emission mechanism.

## Key findings

- Detected VHE gamma-ray signal with 8.89σ significance above 850 GeV.
- Observed a high emission state with 8.05σ significance in May 2012.
- Derived a power-law energy spectrum with index 2.57 ± 0.15.

## 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.