How to switch on and off a Gamma-ray burst through a magnetar
  One of the most elusive features of Gamma Ray Bursts (GRBs) is the sporadic
emission prior to the main prompt event observed in at least $\sim 15\%$ of
cases. These precursors have spectral and temporal properties similar to the
main prompt emission, and smaller, but comparable, energetics. They are
separated from the main event by a quiescent time that may be extremely long
and, in some cases, more than one precursor has been observed in the same
burst. Precursors are still a puzzle: despite many attempts none of the
proposed models can account for all the observed features. Based on the
complete sample of bright long GRBs observed by Swift (BAT6), we propose a new
scenario for which precursors are explained by assuming that the central GRB
engine is a newly born magnetar. In this model the precursor and the prompt
emission arise from accretion of matter onto the surface of the magnetar. The
accretion process can be halted by the centrifugal drag exerted by the rotating
magnetosphere onto the in-falling matter, allowing for multiple precursors and
very long quiescent times.
