| Abstract : | The compactness problem in GRBs has been resolved by invoking the Lorentz factors associated with the relativistic bulk motion. This scenario applies to GRBs where sufficient energy is converted to accelerate the ejected matter to relativistic speeds. This scenario applies to GRBs where sufficient energy is converted to accelerate the ejected matter to relativistic speeds. It is also commonly thought that this may be essentially the difference between supernovae and GRBs.
In some situations, this may not be possible. For short-duration GRBs, due to the merger of two compact objects, i.e. neutron stars (NS) or tidal break up of an NS by a black hole, it is possible that in some situations the matter is not accelerated to relativistic speeds and the gamma rays are indeed trapped inside the region. In this case, the optical depth is very high. However, as the temperatures could be ~109K, the neutrino pair annihilation process dominates. Hence there could be gamma-ray bursts with no gamma rays, but only neutrinos. So neutrinos should be freely able to escape since there is no associated compactness problem. So there could be many gamma-ray bursts that do not produce gamma rays, but only high energy neutrinos.
Also as the neutrinos drain away energy from the source region, the optical depth could drop steeply (even without relativistic motion) and gamma-ray emission could subsequently follow but with much-reduced intensity. The afterglow would now correspond more to that of a typical SN. It may explain (apart from the beaming factor) why fewer gamma-ray bursts are seen than what is expected. However, in the case of NS mergers, gravitational waves would be detectable, as it is independent of the optical depth. So the signature of such gammaless GRBs could be simultaneous detection of neutrinos and gravitational waves.
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