Towards a model of photon-axion conversion in the host galaxy of GRB 221009A

Abstract: GRB 221009A was the brightest gamma-ray burst ever detected on Earth. In its early afterglow phase, photons with exceptional energies above 10 TeV were observed by LHAASO, and a photon-like air shower above 200 TeV was detected by Carpet-2. Gamma rays of very high energies can hardly reach us from the distant GRB because of pair production on cosmic background radiation. Though final results on the highest-energy photons from this GRB have not been published yet, a number of particle-physics solutions to this problem were discussed in recent months. One of the most popular ones invokes the mixing of photons with axion-like particles (ALPs). Whether this is a viable scenario, depends crucially on the magnetic fields along the line of sight, which are poorly known. Here, we use the results of recent Hubble Space Telescope observations of the host galaxy of GRB 221009A, combined with magnetic-field measurements and simulations for other galaxies, to construct a toy model of the host-galaxy magnetic field and to estimate the rate of the photon-axion conversion there. Thanks, in particular, to the exceptional edge-on orientation of the host galaxy, strong mixing appears to be natural, both for LHAASO and Carpet-2 energy bands, for a wide range of ALP masses m<10^{-5} eV and photon couplings g>10^{-11}/GeV.