  GRB 130427A was the brightest gamma-ray burst detected in the last 30 years.
With an equivalent isotropic energy output of $8.5\times10^{53}$ 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.
