In the news this month.... Fermi spies a record-breaking GRB
So it's time for the March edition of the Jodcast, and the News. For a change, I thought I'd do something with the script, so here is the first story.
Gamma ray bursts are enormously energetic events, thought to be signatures of massive stellar explosions in distant galaxies. On September 16th last year, a spectacular event was recorded by the Gamma-ray Burst Monitor, or GBM, on board the Fermi Gamma ray satellite.
Launched on the 11th of June 2008, the Fermi gamma ray space telescope searches for these events using sensitive instruments, as well as surveying the sky to investigate the high energy emission from other interesting objects such as active galaxies, pulsars and solar flares, as well as unidentified sources of high energy gamma rays. Since it's launch, Fermi has triggered follow-up observations of 58 GRBs detected with the Large Area Telescope, another science instrument on board the satellite.
Located in the southern constellation of Carina, the burst of September 16th, given the designation GRB 080916C, was quickly observed with other telescopes on the ground - a team using the the 2.2-metre telescope at La Silla in Chile calculated that the object was at a redshift of 4.35, a distance of 12.2 billion light years. Analysis of the results, published in Science Express on 19th February, show that this burst is the most energetic observed to date, releasing more than twice the estimated energy as the previous record holder. Knowing both the distance and the brightness of a burst means that the energy of the explosion can be estimated. Although GRB's emit their energy in very narrow jets, estimates of their energies are usually based on how much energy would have been involved if energy was emitted equally in all directions, a quantity known as isotropic energy. In this case, the isotropic energy of the burst was almost 9000 times the power of a single regular supernova explosion, and the gas emitting the initial gamma ray flash must have been travelling at almost the speed of light.
Credit: NASA/DOE/Fermi LAT Collaboration
As well as being record breaking, this burst displayed another unusual characteristic – a five second time delay between the arrival of the highest and lowest energy photons. Such a delay has been seen only in one previous burst and the explanation is not yet certain. One idea is that the delay is caused by the nature of the material surrounding the explosion, the low and high energy gamma rays could be coming from different parts of the jet or created through different mechanisms, according to Peter Michelson, the principle investigator for Fermi's Large Area Telescope. Another suggestion comes from the speculative idea of quantum gravity – if correct, then at its smallest scales space is not smooth but turbulent, and this turbulence would have stronger effects on higher energy photons, slowing them down slightly compared to other photons. This effect would be very small and only visible over huge distances – such as that to a distant GRB.
Further observations of other GRBs at different distances should be able to distinguish between the two ideas. If the environment around the GRB is causing the time delay then the signal should be similar for GRBs at any distance. However, if quantum gravity is correct then the time delay should be more pronounced for more distant GRBs. Fermi is designed to operate for at least five years, and should collect a large sample of GRB events over that time.