Nanjing Univ, Dept Astron, Nanjing 210093, Peoples R China
; Chinese Acad Sci, Inst High Energy Phys, Particle Astrophys Lab, Beijing 100039, Peoples R China
The shock model has successfully explained the observed behaviours of afterglows from long gamma-ray bursts (GRBs). Here, we use it to investigate the so-called early afterglows from short GRBs, which arise from blast waves that are not decelerated considerably by their surrounding medium. We consider a nearby medium loaded with e(+/-) pairs. First, the temporal behaviours show a soft-to-hard spectral evolution, from optical to hard X-ray, and then a usual hard-to-soft evolution after the blast waves begin to decelerate. The light curves show variability and consist of two peaks. The first peak, owing to the pair effect, can be observed in the X-ray, although too faint and too short in the optical. The second peak will be detected easily by Swift. We show that detections of the double-peak structure in the light curves of early afterglows are very helpful in determining all the shock parameters of short GRBs, including both the parameters of the relativistic source and the surroundings. Besides, from the requirement that the forward-shock emission in short GRBs should be below the Burst and Transient Source Experiment (BATSE) detection threshold, we give a strong constraint on the shock model parameters. In particular, the initial Lorentz factor of the source is limited to be no more than similar to10(3), and the ambient medium density n is inferred to be low, n less than or similar to 10(-1) cm(-3).