GRB 130427A was the brightest gamma-ray burst detected in the last 30 years .
With an equivalent isotropic energy output of 8.5 \times 10 ^ { 53 } erg and redshift z = 0.34 , it uniquely combined very high energetics with a relative proximity to Earth .
As a consequence , its X-ray afterglow has been detected by sensitive X-ray observatories such as XMM-Newton and Chandra for a record-breaking baseline longer than 80 million seconds .
We present the X-ray light-curve of this event over such an interval .
The light-curve shows a simple power-law decay with a slope \alpha = 1.309 \pm 0.007 over more than three decades in time ( 47 ks - 83 Ms ) .
We discuss the consequences of this result for a few models proposed so far to interpret GRB 130427A , and more in general the significance of this outcome in the context of the standard forward shock model .
We find that this model has difficulty in explaining our data , in both cases of constant density and stellar-wind circumburst media , and requires far-fetched values for the physical parameters involved .