Long optical plateau in the afterglow of the short GRB 150424A with extended emission

Evidence for energy injection by a magnetar?

¹ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
e-mail: fknust@mpe.mpg.de
² University of Bath, Bath BA2 7AY, UK
³ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁴ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁵ University of California, 1156 High Street, Santa Cruz, CA 95064, USA
⁶ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK

Received: 8 February 2017
Accepted: 3 July 2017

Abstract

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.