Probing a Possible Vacuum Refractive Index with Gamma-Ray Telescopes

TL;DR

This paper explores the possibility that vacuum acts as a refractive medium affecting gamma-ray propagation, using observational data from gamma-ray telescopes to test quantum gravity models predicting energy-dependent light speed delays.

Contribution

It combines observational results from MAGIC, HESS, and Fermi telescopes to estimate quantum gravity scale effects on gamma-ray time-lags, proposing a method to distinguish quantum effects from source-intrinsic delays.

Findings

Time-lags are compatible with a linear energy-dependent vacuum refractive index with M_QG1 ~ 10^18 GeV.

Predicted time-lag for high-energy photons from GRB 080916c is 25 ± 11 seconds.

Quadratic energy dependence would result in negligible time-lags.

Abstract

We have used a stringy model of quantum space-time foam to suggest that the vacuum may exhibit a non-trivial refractive index depending linearly on gamma-ray energy: eta -1 ~ E_gamma/M_QG1, where M_QG1 is some mass scale typical of quantum gravity that may be ~ 10^18 GeV: see Phys. Lett. B 665, 412 (2008) and references therein. The MAGIC, HESS and Fermi gamma-ray telescopes have recently probed the possible existence of such an energy-dependent vacuum refractive index. All find indications of time-lags for higher-energy photons, but cannot exclude the possibility that they are due to intrinsic delays at the sources. However, the MAGIC and HESS observations of time-lags in emissions from AGNs Mkn 501 and PKS 2155-304 are compatible with each other and a refractive index depending linearly on the gamma-ray energy, with M_QG1 ~ 10^18 GeV. We combine their results to estimate the time-lag Delta t to be expected for the highest-energy photon from GRB 080916c measured by the Fermi telescope, which has an energy ~ 13.2 GeV, assuming the redshift z = 4.2 \pm 0.3 measured by GROND. In the case of a refractive index depending linearly on the gamma-ray energy we predict Delta t = 25 \pm 11 s. This is compatible with the time-lag Delta t <= 16.5 s reported by the Fermi Collaboration, whereas the time-lag would be negligible in the case of a refractive index depending quadratically on the gamma-ray energy. We suggest a strategy for future observations that could distinguish between a quantum-gravitational effect and other interpretations of the time-lags observed by the MAGIC, HESS and Fermi gamma-ray telescopes.