Magnetized GRB outflow model:weak reverse shock emission and energy transfer timescale

摘要

We show that the absence of the bright optical flashes in most Swift Gamma-Ray Burst(GRB)afterglows can beexplained,if the reverse shock region is magnetized with aσ~1,or the emission spectrum of the electrons accelerated inthe mildly magnetized(0.1<σ<1)reverse shock front is very soft,or the reverse shock of a non-magnetized ontflow issub-relativistic,where σ is the ratio of the magnetic energy flux to the particle energy flux．We also find that forσ》1,theenergy transfer between the magnetized ejecta and the forward shock may be too quick to account for the shallow declinephase that is well detected in many Swiyt GRB X-ray afterglows．