An extraordinarily high-energy photon detected in the aftermath of a distant gamma-ray burst should not have reached Earth, according to standard physics. Now, two Italian astrophysicists have proposed a theoretical framework that could allow it to survive its journey across the cosmos, invoking two hypothetical phenomena: axion-like particles and a possible violation of Lorentz invariance.
The event in question began on 9 October 2022, when a gamma-ray burst designated GRB 221009A sent a massive pulse of high-energy radiation toward Earth. The Large High Altitude Air Shower Observatory in China detected thousands of photons from the burst, including some with energies above 10 TeV. More than an hour later, the Carpet cosmic ray detector in Russia recorded a photon-like signal with an estimated energy of 300 TeV, a reading that challenges conventional astrophysics.
At that energy, the photon should have been annihilated during its journey. The space between galaxies is filled with background light, including the cosmic microwave background, the faint afterglow of the early universe. A 300 TeV gamma ray can collide with these low-energy photons and produce an electron and a positron, effectively absorbing the gamma ray. «The CMB is extremely dense,» said Giorgio Galanti of the National Institute for Astrophysics in Italy, who conducted the study with Marco Roncadelli of the National Institute for Nuclear Physics. The mean free path of such a photon is far shorter than the distance to GRB 221009A, making the universe effectively opaque to it.
Galanti and Roncadelli calculated that under conventional propagation, the expected number of photons in the Carpet energy range would be about 10-96, a vanishingly small probability. The Carpet event itself is also unusual because the detector did not find the muon signal expected from a hadronic cosmic-ray shower. The collaboration estimated the probability of a hadron being misidentified as the event at roughly 3 × 10-4. Galanti said the researchers are taking the published Carpet result at face value while leaving open the question of whether it will ultimately prove to be a signal of new physics.
Their explanation, published in Physical Review Letters, involves two separate but complementary effects. The first is axion-like particles, or ALPs, hypothetical particles that can mix with photons in magnetic fields. A photon could convert into an ALP, travel through a region where photons would be absorbed, and later convert back into a photon. This mechanism could help explain the LHAASO photons detected between roughly 10 and 20 TeV. But it runs into trouble at 300 TeV. «ALPs alone fall short by roughly two orders of magnitude at 300 TeV,» Galanti said. Within the range of ALP parameters considered, the expected number of Carpet photons stays below about 10-4, lower than the 0.0513 required at the 95% confidence level.
That led the researchers to consider a second effect: a possible violation of Lorentz invariance, one of the foundations of Einstein's theory of relativity. In the scenario they studied, modifications to photon propagation change the threshold for photon–photon absorption. A 300 TeV photon could then interact with higher-energy background photons, which are much less abundant than the CMB photons responsible for ordinary absorption. The universe would become more transparent to the photon. The researchers calculated upper limits of 1.22 × 1021 GeV for the linear Lorentz invariance violation scale and 2.03 × 1013 GeV for the quadratic case, at 95% confidence.
Crucially, the model does not use Lorentz invariance violation as a replacement for ALPs. As Galanti explained, ALPs function at LHAASO energies but not at Carpet energies, while Lorentz violation operates at Carpet energies but not at LHAASO energies. Their framework incorporates both effects into a single theoretical model. A separate analysis by Dmitry Ofengeim and Tsvi Piran found that the more-than-one-hour delay between the LHAASO and Carpet events can also be explained with quadratic Lorentz invariance violation, and their estimated scale is consistent with the limit obtained by Galanti and Roncadelli.
Despite the intriguing fit, Galanti is careful about what the result means. «A single candidate photon from a single cosmic event cannot be claimed as an undisputed discovery,» he said. Confirmation would require repeated observations from multiple distant sources showing the same energy-dependent signatures. Future observations from facilities including the ASTRI Mini-Array, the Cherenkov Telescope Array Observatory, the Southern Wide-field Gamma-ray Observatory and an expanded LHAASO could test whether similar high-energy signatures appear in gamma-ray bursts and distant active galaxies. For now, the 300 TeV event remains an unusual observation with a possible explanation that goes beyond standard photon propagation.
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