Observations of GRBs by the FERMI Satellite and the Challenge to Theoretical Models

Gamma-Ray Bursts (GRBs) are the most violent phenomena observed in the universe. After having been developed for over 40 years, observations of GRBs have become all-time and in all electromagnetic wavelength bands. The Fermi Gamma-Ray Space Telescope (FERMI) was launched into orbit by NASA from the Cape Canaveral on June 11, 2008. It includes two major instruments, the Gamma-ray Burst Monitor (GBM) and the Large Area Telescope (LAT). The GBM is only slightly less sensitive than the Burst and Transient Source Experiment which has 14 detectors designed to study gamma-ray sources in the full sky in the energy band of~8keV to~40MeV. The LAT is a wide field-of-view high-energy gamma-ray imaging telescope. It covers the energy range from~20MeV to more than 300GeV. The two instruments provide an unprecedented spectral coverage of 7 orders of magnitudes in energy (from about 8keV to 300GeV), and can be used to promptly record radiations of GRBs. There are two major accomplishments by the FERMI, which are reviewed in this paper. First, observations with the FERMI have dramatically increased our knowledge of broad-band spectra of GRBs, including the composition and evolution of GRB spectra suggestive of possible physical origins and components of GRBs. Particularly, observations with the GBM yield the result that GRB burst durations are independent of burst energies, which suggests that the bimodal distribution of burst duration found from data is due to certain instrument selection effect. Second, the FERMI has detected radiations of very high energies from GRBs. The detections provide credible data for studying radiation mechanisms and physical conditions of GRBs and their afterglows.