Light from a distant blazar, an active galactic nucleus whose jet points our way, travels for billions of years. Along the way, it triggers cascades of charged particles that the intergalactic magnetic field deflects sideways. This creates a blurred gamma-ray glow around the blazar: a secondary halo. The weaker the field, the wider the halo.
But what if magnetic monopoles, particles with a single magnetic pole, exist in space? Theory has allowed them for a long time: they make Maxwell's equations symmetric and explain why electric charge always comes in multiples of a single unit. The authors showed that in an intergalactic field, monopoles oscillate and create a 'magnetic echo.' This echo acts like a lens for charged particles, gathering their trajectories into a narrower beam.
The halo-narrowing effect depends on the number of monopoles. The scientists used data on the blazar 1ES 0229+200, collected by the ground-based H.E.S.S. system and the Fermi-LAT space telescope. Comparing the predicted halo size with the observed one, they obtained a constraint on the flux of light monopoles (with mass below 10⁶ GeV): no more than 6×10⁻²³ particles per square centimeter per second per steradian. This is stronger than all current laboratory and astrophysical limits. If the intergalactic field is stronger than 10⁻¹² G, the monopole signature is lost against ordinary scattering, but then the field itself would need to be reconsidered if monopoles do exist. Thus, blazar halos become not just a beautiful phenomenon but a precise tool for particle physics.
A magnetic monopole the size of a bacterium would possess a field capable of tearing atoms apart at arm's length, so concentrated is its power.
In Frederick Pohl's novel Gateway, the protagonists find an alien spacecraft powered by monopole energy — an idea inspired precisely by the symmetry of Maxwell's equations.