The flat X-ray segment of GRB051221A: two component jet in short gamma-ray bursts

Recent observation to the afterglow of GRB051221A found a flat segment in the X-ray light curve around 0.1 day after the burst. This flat segment caused the flux increasing by a factor of 5 over the extension of a single power law decrease. It may be caused either by a strong energy injection or by contribution of a second outflow component. Such a two component jet could be formed if the initial ejecta launched in the double neutron stars merger or neutron star-black hole merger is neutron rich and magnetized. In this work we show that the multi-band afterglows of GRB051221A could be well fitted by a two component model, provided that the shock physics parameters in different components are much different. This result suggests that GRB051221A might be the first detection of two component jet in short gamma-ray burst outflow