DELAYED GeV EMISSION FROM COSMOLOGICAL GAMMA-RAY BURSTS : Impact of a Relativistic Wind on External Matter

Sudden collapse of a compact object, or coalescence of a compact binary, can generate an unsteady relativistic wind that lasts for a few seconds. The wind is likely to carry a high magnetic field; and its Lorentz factor depends on the extent to which it is ’loaded’ with baryons. If the Lorentz factor is ∼ 100, internal dissipation and shocks in this wind produce a non-thermal gamma-ray burst, detectable in the range 0.1 MeV <∼ Eγ <∼ 0.1−1 GeV out to cosmological distances. The cooled wind ejecta would subsequently be decelerated by the external medium. The resultant blast wave and reverse shock can then give rise to a second burst component, mainly detectable in the GeV range, with a time delay relative to the MeV burst ranging from minutes to hours.