Signature of a Magnetar Central Engine from the Multiband Afterglow of GRB 151027A Associated with a Supernova

Abstract We report multiwavelength observations and theoretical modeling of the afterglow of gamma-ray burst (GRB) 151027A. The object exhibits a complete evolutionary sequence, including prompt gamma-ray and optical emission, an initial optical onset bump, an optical rebrightening, a late-time supernova bump, an X-ray flare, and two distinct X-ray plateaus. GRB 151027A exhibits nearly all characteristic features observed in GRB afterglows. Such a rich set of features within a single event is exceptionally rare. We show that the first X-ray plateau followed by a rapid decay can be attributed to the collapse of a newly formed magnetar to form a black hole. The second X-ray plateau can be explained by fallback accretion. We find that the energy-injection model successfully reproduces the optical rebrightening of GRB 151027A. The late-time optical bump is well described by a 56 Ni-powered supernova light curve. GRB 151027A presents a complete physical picture for GRB phenomenology, whereas reconstructing such a comprehensive picture previously required the statistical analysis of multiple GRBs.