  Very long and precise follow-up measurements of the X-ray afterglow of very
intense gamma ray bursts (GRBs) allow a critical test of GRB theories. Here we
show that the single power-law decay with time of the X-ray afterglow of GRB
130427A, the record long and most accurately measured X-ray afterglow of an
intense GRB by the Swift, Chandra and XMM Newton space observatories, and of
all other known intense GRBs, is that predicted by the cannonball (CB) model of
GRBs from their measured spectral index, while it disagrees with that predicted
by the widely accepted fireball (FB) models of GRBs.
