Long optical plateau in the afterglow of the short GRB 150424A with extended emission. Evidence for energy injection by a magnetar?

Context. Short-duration gamma-ray bursts (GRBs) with extended emission form a subclass of short GRBs, comprising about 15% of the short-duration sample. Afterglow detections of short GRBs are also rare (about 30%) because of their lower luminosity.
Aims: We present a multiband data set of the short burst with extended emission, GRB 150424A, comprising of GROND observations, complemented with data from Swift/UVOT, Swift/XRT, HST, Keck/LRIS, and data points from the literature. The GRB 150424A afterglow shows an extended plateau phase, lasting about 8 h. The analysis of this unique GRB afterglow might shed light on the understanding of afterglow plateau emission, the nature of which is still under debate.
Methods: We present a phenomenological analysis made by applying fireball closure relations and interpret the findings in the context of the fireball model. We discuss the plausibility of a magnetar as a central engine, which would be responsible for additional and prolonged energy injection into the fireball.
Results: We find convincing evidence for energy injection into the afterglow of GRB 150424A. We find that a magnetar spin-down as the source for a prolonged energy injection requires that at least 4% of the spin-down energy is converted into radiation.