It is postulated in Einstein's relativity that the speed of light in vacuum is a constant for all observers. However, the effect of quantum gravity could bring an energy dependence of light speed, and a series of studies on high-energy photon events from gamma-ray bursts (GRBs) and active galactic nuclei (AGNs) suggest a light speed variation $v(E)=c\left(1-E / E_{ \mathrm{ LV}}\right)$ with $E_{ \mathrm{ LV}}=3.6 \times 10^{ 17} ~\mathrm{ GeV}$ or a bound $E_{ \mathrm{ LV}} \ge 3.6 \times 10^{ 17} ~\mathrm{ GeV}$. From the newly observed gamma-ray burst GRB 221009A, we find that a 99.3~GeV photon detected by Fermi-LAT is coincident with the sharp spike in the light curves detected by both Fermi-GBM and HEBS under the above scenario of light speed variation, suggesting an option that this high-energy photon was emitted at the same time as a sharp spike of low-energy photon emission at the GRB source. Thus this highest energy photon event detected by Fermi-LAT during the prompt emission of gamma-ray bursts might be considered as an optional signal for the linear form modification of light speed in cosmological space.
Categories: astro-ph.HE gr-qc hep-ph