Probing for Extended Gamma-ray Emission Within and Beyond the Virial Radius from the Coma Galaxy Cluster

Galaxy Clusters (GCls) are the largest known virialized objects in the Universe. About one-third of GCls exhibit an extended radio halo associated with a population of nonthermal electrons emitting synchrotron radiation in a microgauss level magnetic field. The cooling time for the nonthermal electrons is about two orders of magnitude below the diffusion time needed to explain the radio halo, indicating that the electrons are created locally. The electrons may either be accelerated directly in the turbulent cluster medium or created as secondary products from nonthermal protons colliding with the intracluster medium. Observed gamma-ray emission can therefore resolve the contribution from these two mechanisms. Despite the empirical and theoretical motivation for the existence of gamma-ray emission from GCls, so far it remains undetected. In this talk VERITAS observations are used to analyze a 6x8 degree field of the Coma galaxy cluster and evaluate the existence of possible extended gamma-ray emission. To achieve this, a method is developed which utilizes cosmic rays as a calibration source to determine a background measurement across the full field of view. The resulting excess demonstrates a small significance of 2.7 sigma in the central 1.5 degree radius of the cluster. This observed central excess, while alone is not significant, has an angular extent in agreement with simulations and corresponds to a similar significance level seen by Fermi-LAT in the same region, and hence it is worth further investigation by VERITAS and future instruments such as CTA