A method of measurement of extragalactic magnetic fields by TeV gamma ray telescopes

We propose a method of measurement of extragalactic magnetic fields in observations of TeV $\gamma$ rays from distant sources. Multi-TeV $\gamma$ rays from these sources interact with the infrared photon background producing secondary electrons and positrons, which can be just slightly deflected by extragalactic magnetic fields before they emit secondary $\gamma$ rays via inverse Compton scattering of cosmic microwave background photons. Secondary $\gamma$ rays emitted toward an observer on the Earth can be detected as extended emission around initially point source. Energy dependent angular profile of extended emission is related to the strength of extragalactic magnetic field along the line of sight. Small magnetic fields $B\leq10^{-12}\,$G in the voids of the large scale structure can be measured in this way.