Magnetar powered GRBs: explaining the extended emission and X-ray plateau of short GRB light curves

Abstract

Extended emission (EE) is a high-energy, early time rebrightening sometimes\nseen in the light curves of short gamma-ray bursts (GRBs). We present the first\ncontiguous fits to the EE tail and the later X-ray plateau, unified within a\nsingle model. Our central engine is a magnetar surrounded by a fall-back\naccretion disc, formed by either the merger of two compact objects or the\naccretion-induced collapse of a white dwarf. During the EE phase, material is\naccelerated to super-Keplarian velocities and ejected from the system by the\nrapidly rotating ($P \\approx 1 - 10$ ms) and very strong ($10^{15}$ G) magnetic\nfield in a process known as magnetic propellering. The X-ray plateau is\nmodelled as magnetic dipole spin-down emission. We first explore the range of\nGRB phenomena that the propeller could potentially reproduce, using a series of\ntemplate light curves to devise a classification scheme based on phenomology.\nWe then obtain fits to the light curves of 9 GRBs with EE, simultaneously\nfitting both the propeller and the magnetic dipole spin-down and finding\ntypical disc masses of a few $10^{-3}$ $M_{\\odot}$ to a few $10^{-2}$\n$M_{\\odot}$. This is done for ballistic, viscous disc and exponential accretion\nrates. We find that the conversion efficiency from kinetic energy to EM\nemission for propellered material needs to be $\\gtrsim 10\\%$ and that the best\nfitting results come from an exponential accretion profile.\n