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Short GRBs with Extended Emission from Magnetar Birth: Jet Formation and Collimation

Approximately 1/4-1/2 of short duration Gamma-Ray Bursts are followed by variable X-ray emission lasting ~ 100 s with a fluence comparable or exceeding that of the initial burst itself. The long duration and significant energy of this `extended emission'(EE) poses a major challenge to the standard binary neutron star merger model. metzger08 recently proposed that the EE is powered by the spin-down of a strongly magnetized neutron star. However, the effects of surrounding material on the magnetar outflow have not yet been considered. Here we present time-dependent axisymmetric relativistic magnetohydrodynamic simulations of the interaction of the relativistic proto-magnetar wind with a surrounding 10^-1 10^-3 M_\odot envelope, which represents material ejected during the merger; the supernova following AIC; or via outflows from the accretion disk. The collision between the relativistic magnetar wind and the expanding ejecta produces a magnetized nebula inside the ejecta. A strong toroidal magnetic field builds up in the nebula, which drives a bipolar jet out through the ejecta, similar to the magnetar model developed in the case of long duration GRBs. We quantify the `break-out' time and opening angle of the jet theta_j as a function of the wind energy flux dot{E} and ejecta mass M_ej. We show that dot{E} and theta_j are inversely correlated, such that the beaming-corrected (isotropic) luminosity of the jet is primarily a function of M_ej. Both variability arguments, and the lower limit on the power of magnetar outflows capable of producing bright emission, suggest that the true opening angle of the magnetar jet must be relatively large. The model thus predicts a class of events for which the EE is observable with no associated short GRB. These may appear as long-duration GRBs or X-Ray Flashes, which may be detected by future all-sky X-ray survey missions.