Observability of the Very-High-Energy Emission from GRB 221009A

The LHAASO Collaboration detected the gamma ray burst GRB 221009A at energies above 500 GeV with a tail extending up to 18 TeV, whose spectral analysis has presently been performed up to 7 TeV for the lower energy instrument LHAASO-WCDA only, with no indication of a cutoff. Soon thereafter, Carpet-2 at Baksan Neutrino Observatory reported the observation of an air shower consistent with being caused by a photon of energy 251 TeV from the same GRB. Given the source redshift z=0.151, the expected attenuation due to the extragalactic background light is very severe so that these detections have proven very hard to explain. In this Letter, we show that the existence of axionlike particles with mass m_{a}≃(10^{-11}-10^{-7})  eV and two-photon coupling g_{aγγ}≃(3-5)×10^{-12}  GeV^{-1} strongly reduce the optical depth of TeV photons, thus explaining the observations. Our ALPs meet all available constraints, are consistent with two previous hints at their existence, and are good candidates for cold dark matter. Moreover, we show that Lorentz invariance violation can explain the Carpet-2 result but not the LHAASO observations.