Gamma-ray Bursts: the Four Crises

We discuss some open problems concerning the origin and the emission mechanism of gamma-ray bursts (GRBs) in light of recent developments. If GRBs originate at extragalactic distances, we are facing four crises: (1) an energy crisis, models have to account for more than 10 ergs of energy emitted in the gamma-ray band; (2) a spectral crisis, emission models have to account for the surprising ‘smoothness’ of GRB broad-band spectra, with no indication of the predicted spectral ‘distorsions’ caused by inverse Compton scattering in large radiation energy density media, and no evidence for beaming; (3) an afterglow crisis, relativistic shock models have to explain the complexity of the afterglow behavior, the longevity of optical transients detectable up to six months after the burst, the erratic behavior of the radio emission, the lack of evidence for substantial beaming as indicated by recent searches for GRB afterglows in the X-ray band; (4) a population crisis, from data clearly indicating that only hard and long GRBs show a strong deviation from an Euclidean brightness distribution, just the opposite of what expected from extragalactic models without substantial cosmological evolution. All previously proposed cosmological models are challenged: in particular, the neutron star-neutron star coalescence model most likely will not survive the resolution of the problems raised by points (1) and (4).