Fallback accretion on to a newborn magnetar: short GRBs with extended emission

Abstract

There are a subset of short gamma-ray bursts (SGRBs) which exhibit a\nrebrightening in their high-energy light curves known as extended emission.\nThese bursts have the potential to discern between various models proposed to\ndescribe SGRBs as any model needs to account for extended emission. In this\npaper, we combine fallback accretion into the magnetar propeller model and\ninvestigate the morphological changes fallback accretion has on model light\ncurves and fit to the afterglows of 15 SGRBs exhibiting extended emission from\nthe Swift archive. We have parameterised the fallback in terms of existing\nparameters within the propeller model and solved for the disc mass and angular\nfrequency of the magnetar over time. We then apply a Markov chain Monte Carlo\nroutine to produce fits to the data. We present fits to our extended emission\nSGRB sample that are morphologically and energetically consistent with the data\nprovided by Swift BAT and XRT telescopes. The parameters derived from these\nfits are consistent with predictions for magnetar properties and fallback\naccretion models. Fallback accretion provides a noticeable improvement to the\nfits of the light curves of SGRBs with extended emission when compared to\nprevious work and could play an important role in explaining features such as\nvariability, flares and long dipole plateaux.\n