Magnetic monopole plasma oscillations and implications for TeV blazars
Magnetic monopoles arise in many beyond Standard Model scenarios, symmetrize Maxwell's equations, and their existence would be tied to the quantization of electric charge. It has been argued that, when placed in an astrophysical magnetic field, monopoles can induce a magnetic version of plasma oscillations. In this work, we explore monopole-induced oscillations of the intergalactic magnetic field (IGMF). We show that monopole-induced oscillations of the magnetic field lead to collimation of electrically charged particle trajectories, reducing the usual deflection by the magnetic field. The collimation effect impacts the deflection angle in the electromagnetic cascades of TeV blazars and leads to a decrease in the angular size of blazar secondary GeV halos. Therefore, the constraints on the secondary halo angular size from combined H.E.S.S. and Fermi-LAT observations translate into bounds on the magnetic monopole abundance. The bounds on the magnetic monopole flux obtained in this work from blazar 1ES 0229+200, depending on the IGMF strength, can be as strong as $F \lesssim 6 \times 10^{-23}\, \text{cm}^{-2} \text{s}^{-1} \text{str}^{-1}$ for low-mass monopoles $m \lesssim 10^6\, \text{GeV}$, stronger than existing laboratory and astrophysical bounds. The bound becomes subdominant to current constraints if the present-day IGMF value is stronger than $B \gtrsim 10^{-12}\, \text{G}$. At the same time, in the case of non-zero monopole abundance, the IGMF lower bound from TeV observations itself should be revised, resulting in a stronger lower bound at higher monopole number density.