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

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. © ESO, 2017.H.J.v.E. was supported by the Alexander von Humboldt foundation at the time of this work. D.A.K. acknowledges financial support from MPE, from TLS, from the Spanish research project AYA 2014-58381-P, and from Juan de la Cierva Incorporacion fellowships IJCI-2015-26153 and IJCI-2014-21669. P.S., T.W.C., J.F.G., M.T. acknowledge support through the Sofja Kovalevskaja Award from the Alexander von Humboldt Foundation of Germany. S.K. and A.N.G. acknowledge support by DFG grant K1 766/16-1. S.S. acknowledges support by the Thuringer Ministerium fur Bildung, Wissenschaft und Kultur under FKZ 12010-514. Part of the funding for GROND (both hardware as well as personnel) was generously granted from the Leibniz-Prize to G. Hasinger (DFG grant HA 1850/28-1).Peer Reviewe