Search for extended gamma ray emission in Markarian 421 using VERITAS   observations
  Very high energy (VHE: >100 GeV) gamma rays coming from AGN can pair-produce on the intergalactic background light generating an electromagnetic cascade. If the Intergalactic Magnetic Field (IGMF) is sufficiently strong, this cascade may result in an extended isotropic emission of photons around the source, or halo. Using VERITAS observations of the blazar Markarian 421, we search for extended emission by comparing the source angular distribution (${\theta}^2$) from a quiescent period with one coming from a flare period, which can be considered as halo-free. ${\chi}^2$ test showed no significant statistical differences between the samples, suggesting that the effect is either non-existent or too weak to be detected. We calculated upper limits for the extended flux considering different angle ranges, the most stringent being <8% of the Crab Nebulae flux (C.U), in the range $0\deg \leq {\theta} \leq 0.1\deg$ . 
Mateo Fernandez Alonso (for the VERITAS Collaboration)
astro-ph.HE