GRB 130831A: Birth and death of a magnetar at z = 0.5

We present observations of GRB 130831A and its afterglow obtained with Swift, Chandra, and multiple ground-based observatories. This burst shows an uncommon drop in the X-ray light curve at about 105 s after the trigger, with a decay slope of aX ≃ 7. The standard Forward Shock (FS) model offers no explanation for such a behaviour. Instead, a model in which a newly born magnetar outflow powers the early X-ray emission is found to be more viable. After the drop, the X-ray afterglow resumes its decay with a slope characteristic of FS emission. The optical emission, on the other hand, displays no clear break across the X-ray drop and its decay is consistent with that of the late X-rays; we thus believe that the optical and late X-ray emission are both FS. We model our data to derive the kinetic energy of the ejecta and, in conjunction with the study of SN 2013fu associated with GRB 130831A, we work out for the first time the energy break-down of a supernova with a central engine into non-relativistic ejecta, relativistic ejecta that power the afterglow, and emission from the magnetar outflow.