We present the analysis of the extraordinarily bright Gamma-Ray Burst ( GRB ) 130427A under the hypothesis that the GRB central engine is an accretion–powered magnetar .
In this framework , initially proposed to explain GRBs with precursor activity , the prompt emission is produced by accretion of matter onto a newly–born magnetar , and the observed power is related to the accretion rate .
The emission is eventually halted if the centrifugal forces are able to pause accretion .
We show that the X-ray and optical afterglow is well explained as the forward shock emission with a jet break plus a contribution from the spin–down of the magnetar .
Our modelling does not require any contribution from the reverse shock , that may still influence the afterglow light curve at radio and mm frequencies , or in the optical at early times .
We derive the magnetic field ( B \sim 10 ^ { 16 } G ) and the spin period ( P \sim 20 ms ) of the magnetar and obtain an independent estimate of the minimum luminosity for accretion .
This minimum luminosity results well below the prompt emission luminosity of GRB 130427A , providing a strong consistency check for the scenario where the entire prompt emission is the result of continuous accretion onto the magnetar .
This is in agreement with the relatively long spin period of the magnetar .
GRB 130427A was a well monitored GRB showing a very standard behavior and , thus , is a well–suited benchmark to show that an accretion–powered magnetar gives a unique view of the properties of long GRBs .