Interpretation of multi-TeV photons from GRB221009A

The nearby GRB221009A at redshift $z=0.1505$ has been observed up to a maximum energy of 18 TeV with the LHAASO air shower array. The expected optical depth for a photon with energy $E_\gamma=18$ TeV varies between 9.4 and 27.1 according to existing models of the extra-galactic background light (EBL) in the relevant mid infra-red range. The resulting suppression of the flux in several (but not all) EBL models makes it for these EBL models unlikely that this photon could have been observed at the claimed energy. If the photon event and its energy are confirmed and possibly even more photons above 10 TeV have been observed, the photon-pair production process would have to be suppressed by mechanisms predicted in extensions of the Standard Model of particle physics. We consider the possibilities of photon mixing with a light pseudo-scalar (e.g., axion-like particles; ALPs) in the magnetic field of the host galaxy and the Milky Way and Lorentz invariance violation (LIV). In the case of photon-ALP mixing, the boost factor would reach values $\sim10^6$ for photon couplings not ruled out by the CAST experiment, but limited by other astrophysical constraints. Viable scenarios would require either very efficient mixing in or near to the GRB or that the largest part of the total luminosity is radiated at TeV energies, different from previous GRB afterglows. In the case of LIV, required boost factors are achievable for a LIV breaking energy scale $\lesssim 2\times 10^{29}$~eV ($\lesssim 4\times 10^{21}$~eV) for the linear (quadratic) modification of the dispersion relation. A more simple explanation would be a misidentification of a charged cosmic-ray air shower.Comment: revised version, submitted to JCAP. Revised VHE afterglow model, added two figures. Numbers update