GRBs are the most energetic explosions in the Universe. These short bursts of radiation originate in galaxies out to the edge of the visible Universe (at redshifts of 0.03 to 9.4).
There are two observational varieties that are believed to result from two different types of progenitor stars. Long-duration GRBs are due to the death of a rare type of massive star that produces a Type Ibc supernova with powerful relativistic jets. Short-duration GRBs are due to mergers of binary neutron stars, which create a "central engine" with highly collimated bipolar relativistic outflows of ejected material similar to those thought to power long bursts. Short bursts have 90% of the total burst energy detected by the GBM within the first 2 seconds after the trigger.
The two Fermi instruments work together to detect, localize, and characterize the temporal and spectral properties of GRBs.
The Large Area Telescope (LAT) detects the brightest, most energetic GRBs: ~10 per year with spectral coverage from 30 MeV - 300 GeV. Together with the GBM, these observations are stimulating observational and theoretical innovation in both prompt emission and afterglow theory. LAT-detected bursts are not only exceptionally energetic, but they also have bright broadband afterglows and extraordinarily fast ejecta. The brightest of the LAT-detected bursts show several interesting new features. These include the detection of:
Lorentz invariance is a feature of several theories of relativity's effect on space-time. Modern searches for a violation of Lorentz invariance come in many forms, one of which is measuring the speed of light from distant parts of the Universe. GRBs are prime candidates for these tests since they are far away and gamma-rays from a single GRB are all produced at the same time. Fermi-LAT was able to test this using photons from the bright GRB 090510. Two of the photons, one a million times more energetic than the other, traveled over 7.3 billion light-years before arriving at the telescope, arriving at nearly the same time, thus demonstrating that Lorentz invariance holds strong. Fermi scientists will continue to test new theories of Lorentz invariance using Fermi data.
The Gamma-ray Burst Monitor (GBM) has 14 detectors (12 lower-energy sodium iodide crystal scintillators, 2 higher-energy bismuth germinate scintillators) that together cover the full unocculted sky and detect ~240 bursts per year with spectral coverage from 8 keV - 40 MeV.
The GBM detects more GRBs than any other instrument in orbit. Its wide energy coverage for hundreds of GRB observations is providing constraints on the physical emission mechanisms powering these stellar explosions.
An exceptionally bright gamma-ray burst was seen by the Fermi instruments on April 27, 2013. Results from Fermi, Swift, and NuSTAR combined to challenge the most popular model of gamma-ray bursts (see figure below). In particular, the 95 GeV gamma ray seen in the afterglow by the Fermi Large Area Telescope is hard to explain in this model.