The gammas were measured in two energy ranges, from 1.2 to 10 tera-electron-volts (TeV) and from 0.25 to 0.6 TeV. The first group arrived on Earth four minutes later than the second.
We do not know the origin of these gamma bursts, it might have been some type of acceleration process that produces different energies as times goes by.
Here is the original article http://arxiv.org/abs/0708.2889
