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The origin of ultra-high energy cosmic rays (UHECR) has been widely regardedas one of the major questions in the frontiers of particle astrophysics. Gammaray bursts (GRB), the most violent explosions in the universe second only tothe Big Bang, have been a popular candidate site for UHECR productions. Therecent IceCube report on the non-observation of GRB induced neutrinos thereforeattracts wide attention. This dilemma requires a resolution: either theassumption of GRB as UHECR accelerator is to be abandoned or the expected GRBinduced neutrino yield was wrong. It has been pointed out that IceCube hasoverestimated the neutrino flux at GRB site by a factor of $\sim 5$. In thispaper we point out that, in addition to the issue of neutrino production atsource, the neutrino oscillation and the possible neutrino decay during theirflight from GRB to Earth should further reduce the detectability of IceCube,which is most sensitive to the muon-neutrino flavor as far as point-sourceidentification is concerned. Specifically, neutrino oscillation will reduce themuon-neutrino flavor ratio from 2/3 per neutrino at GRB source to 1/3 on Earth,while neutrino decay, if exists and under the assumption of normal hierarchy ofmass eigenstates, would result in a further reduction of muon-neutrino ratio to1/8. With these in mind, we note that there have been efforts in recent yearsin pursuing other type of neutrino telescopes based on Askaryan effect, whichcan in principle observe and distinguish all three flavors with comparablesensitivities. Such new approach may therefore be complementary to IceCube inshedding more lights on this cosmic accelerator question.

Constraining GRB as Source for UHE Cosmic Rays through Neutrino Observations