Possible Evidence for Lorentz Invariance Violation in Gamma-ray Burst 221009A

The preliminary detections of the gamma-ray burst 221009A up to 18 TeV by LHAASO and up to 251 TeV by Carpet 2 have been reported through Astronomer’s Telegrams and Gamma-ray Coordination Network circulars. Since this burst is at redshift 

z=0.15
, these photons may at first seem to have a low probability to avoid pair production off of background radiation fields and survive to reach detectors on Earth. By extrapolating the reported 

0.1−1.0
\ GeV LAT spectrum from this burst to higher energies and using this to limit the intrinsic spectrum of the burst, we show that the survival of the 18 TeV photon detected by LHAASO is not unlikely with many recent extragalactic background light models, although the detection of a 251 TeV event is still very unlikely. This can be resolved if Lorentz invariance is violated at an energy scale 

EQG>49EPlanck
 in the linear (

n=1
) case, and 

EQG>10−6EPlanck
 in the quadratic (

n=2
) case (95% confidence limits), where 

EPlanck
 is the Planck energy. This could potentially be the first evidence for subluminal Lorentz invariance violation.

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 previous researches on high-energy photon events from gamma-ray bursts (GRBs) and active galactic nuclei (AGNs) suggest a light speed variation 

v(E)=c(1−E/ELV)
 with 

ELV=3.6×1017 GeV
. From the newly detected 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 Fermi-GBM and HEBS under the above scenario of light speed variation, suggesting that this high energy photon was emitted at the same time with 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 supports the linear form modification of light speed in cosmological space.

It is reported that the Large High Altitude Air Shower Observatory (LHAASO) observed thousands of very-high-energy photons up to 

∼
18 TeV from GRB 221009A. We study the survival rate of these photons by considering the fact that they are absorbed by the extragalactic background light. By performing a set of 

106
 Monte-Carlo simulations, we explore the parameter space allowed by current observations and find that the probability of predicting that LHAASO observes at least one photons of 18 TeV from GRB 221009A within 2000 seconds is 4–5\%. Hence, it is still possible for the standard physics to interpret LHAASO’s observation in the energy range of several TeV. Our method can be straightforwardly generalized to study more data sets of LHAASO and other experiments in the future.