The extragalactic magnetic field could be detected by searching for signatures of the electromagnetic cascade initiated by high-energy photons on the intergalactic radiation and deflected by the field. This process produces a time delay and an extended gamma-ray halo around the source, which are looked for. We propose a new signature of electromagnetic echoes: the asymmetry of the gamma-ray distribution around blazars. As a measure of asymmetry, we use the offset of the gamma-ray distribution to the location of the blazar. This offset is due to the tilt of the jet of the blazar relative to the line of sight. Using a subsample of the 10 brightest BL Lacs, we exclude the range of extragalactic magnetic fields from 10 –16 to 10 –14 G, assuming that these objects have maintained a constant average luminosity over hundreds of thousands of years.
Stern et al. (Mon,) studied this question.