Thus the inferred isotropic energy loss in g-rays is E G ϳ3 ϫ 10 53 erg: This is remarkably coincident with the gravitational binding energy released during the merger of two neutron stars 24 .Even though E G is much greater than E A , the afterglow measurements pose a clear problem to GRB models. At the time the ␥-rays are emitted, the shocked gas is moving with very large Lorentz factor (Γ). The initial Γ is assumed to be 10 2 -10 3 and expected to evolve as t −3/8 . Due to relativistic beaming, only a small portion, solid angle ϳΓ −2 , of the fireball is visible to us. Hence we remain ignorant of the true shape of the fireball. The estimate of E G can be reduced by assuming that the emitting surface is a narrow jet directed towards the observer. However, Γ is smaller during the optical afterglow emission than during the ␥-ray emission. Relativistic beaming is less of a limitation, and the full extent of the fireball is gradually revealed to the observer. The monotonic power-law decay at optical wavelengths seen in the OTs of previous transients 13-15 is not consistent with the narrow jet-like emitting surfaces. We recognize that more complicated models (in which energy is injected non-spherically) could account for such monotonic decays whilst having nonspherical geometry. However, the main issue that we discuss here is the inferred energetics. It is by no means clear that the energy requirements inferred from afterglow observations (when the Lorentz factor is small) is any lower than the simplest case considered here.According to the models discussed here, the afterglow emission is supposed to arise from non-radiative shocks; that is, the efficiency of the shock in converting E 0 (the total energy in the GRB event) to afterglow radiation is low. Thus, E 0 is significantly larger than the E A reported above. This finding is in direct contrast to the assumptions made by most practitioners in this field, namely E 0 ¼ 10 51 erg. As an aside we note that equation (1) predicts that the break frequency should evolve with time as t −3/2 . Measurement of the break frequency at successive epochs for future bursts offers the possibility of directly confirming or refuting the non-radiative shock assumption.The evidence presented here favours GRB models which produce energy vastly in excess of 10 51 erg; see for example, refs 24, 25, 29. The afterglow emission is similar in nature to the emission from supernovae but is more energetic by two orders of magnitude. Following the naming sequence, nova and supernova, it is only appropriate to refer to GRB afterglow as hypernova 29 .Ⅺ

No abstract

No abstract

No abstract