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

Search for extended gamma ray emission in Markarian 421 using VERITAS observations