  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.

High Energy Astrophysical Phenomena