A Two-Component Jet Model for the X-Ray Afterglow Flat Segment in the Short Gamma-Ray Burst GRB 051221A

In the double neutron star merger or neutron star-black hole merger model for short gamma-ray bursts, the outflow launched might be mildly magnetized and neutron-rich. The magnetized neutron-rich outflow will be accelerated by the magnetic and thermal pressure and may finally form a two-component jet, as suggested by Vlahakis, Peng, & Koenigl. We show in this work that such a two-component jet model could well reproduce the multiwavelength afterglow light curves, in particular the X-ray flat segment, of short GRB 051221A. In this model, the central engine need not be active much longer than the prompt g-ray emission.