Two X-Ray Plateaus Of Gamma-Ray Bursts: Energy Injection From Nascent Magnetars With An Evolving Magnetic Inclination Angle

As the central engine of gamma-ray bursts (GRBs), a nascent magnetar can release its rotational energy by magnetic dipole radiation. This energy is injected into the outflow of the GRB jet. With the rapid evolution of the magnetic inclination angle, the perpendicular component of the magnetar's surface magnetic field decreases rapidly. The energy loss rate of the magnetar is characterized by two different timescales. This peculiar two-stage energy injection model will affect the shape of the X-ray afterglow light curve. It is shown that a new shallow decay phase will emerge following the initial shallow decay phase. As a result, there could be two plateaus in the X-ray light curve. The effects of various physical parameters on the light curves are studied in detail. It is found that the observed afterglows of GRB 161217A and GRB 190114A, both having two plateaus in the light curve, can be well fitted in our framework.