Quantum Gravity in GRB 080916C ?

The Fermi Collaboration has an interesting article in recent issue of Science

Fermi Observations of High-Energy Gamma-Ray Emission from GRB 080916C
Science 27 March 2009, Vol. 323. no. 5922, pp. 1688 - 1693

Abstract: Gamma-ray bursts (GRBs) are highly energetic explosions signaling the death of massive stars in distant galaxies. The Gamma-ray Burst Monitor and Large Area Telescope onboard the Fermi Observatory together record GRBs over a broad energy range spanning about 7 decades of gammaray energy. In September 2008, Fermi observed the exceptionally luminous GRB 080916C, with the largest apparent energy release yet measured. The high-energy gamma rays are observed to start later and persist longer than the lower energy photons. A simple spectral form fits the entire GRB spectrum, providing strong constraints on emission models. The known distance of the burst enables placing lower limits on the bulk Lorentz factor of the outflow and on the quantum gravity mass.

The article presents details about the gamma ray burst 080916C observed on Sep 16 2008 with the recently launched Fermi satellite (formerly GLAST). Follow-up x-ray and optical observations measured a redshift of z approx 4.35. They have estimated an enormous energy release of ~4.9 times the solar mass, suggesting that the outflow was directed and occurred only into a narrow jet. The burst covered many orders of magnitude in energy, with the highest detected photon being at approx 13 GeV.

The interesting thing is that the higher energetic photons seem to be arriving later, with the MeV band peaking some seconds after the keV band, and the 13 GeV photon arriving approx 16.5 seconds after the onset of the burst. The article discusses various astrophysical reasons for this delay, such as spatially distinct regions of origin, a delay of high energetic photons through not well understood opacity properties of the source, or additional time needed to accelerate protons or ions sufficiently.

An alternative explanation they investigate is that the delay is due to an energy dependent speed of light caused by Planck-scale corrections to the dispersion relation, which could be a signature for quantum gravitational effects. With the assumption that the delay is entirely explained by astrophysical effects, they obtain a lower limit on the scale of quantum gravity, that is M_QG > 1.3 x 10¹⁸GeV. Note that this is about one order of magnitude smaller than the Planck scale.

I would have found it more useful to think about it the other way round: if the delay is not caused by astrophysical effects but entirely by quantum gravitational effects, the scale must be below this limit to reproduce the observed delay. Since the scale is one order of magnitude below what one would have expected and thus the effect quite strong, I would then be concerned should this effect be due to a modified dispersion of photons it should already have been observed previously, for example in MAGIC's observation of a gamma ray flare in 2007.

In addition to this one has to keep in mind that one photon doesn't make a particularly great statistic. This gamma ray burst seems to be quite unusual in its luminosity anyway and an astrophysical origin of the delay is a more conservative explanation. To make a case for quantum gravity, much more data would be needed, in particular one would need to establish a dependence of the delay on the distance. If it is a quantum gravitational effect, the delay should increase with increasing distance. If the delay is of astrophysical origin, the delay should be independent of the distance.

Giovanni Amelino-Camelia gave a seminar at PI last week (I could not attend since I was in Stockholm as you know), in the first half he talks about the above discussed observation of GRB 080916C, see PIRSA: 09030039. (He then talks about the "mystery noise" in the GEO600 gravitational wave interferometer, previously discussed here.)