Probing a possible vacuum refractive index with γ-ray telescopes

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 similar to E-gamma/M-QG1, where M-QG is some mass scale typical of quantum gravity that may be similar to 10(18) GeV: see [J. Ellis, N.E. Mavromatos, D.V. Nanopolous, Phys. Lett. B 665 (2008) 412] 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 rind 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 similar to 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 ail energy similar to 13.2 GeV, assuming the redshift z = 4.35 +/- 0.15 measured by GROND. In the case of a refractive index depending linearly on the gamma-ray energy we predict Delta t = 26 +/- 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-grarvitational effect and other interpretations of the time-lags observed by the MAGIC, HESS and Fermi gamma-ray telescopes. (C) 2009 Elsevier B.V. All rights reserved.