  Applying our recently-developed generalized version of the high-latitude
emission theory to the observations of X-ray flares in gamma-ray bursts (GRBs),
we present here clear observational evidence that the X-ray flare emission
region is undergoing rapid bulk acceleration as the photons are emitted. We
show that both the observed X-ray flare light curves and the photon index
evolution curves can be simultaneously reproduced within a simple physical
model invoking synchrotron radiation in an accelerating emission region far
from the GRB central engine. Such an acceleration process demands an additional
energy dissipation source other than kinetic energy, which points towards a
significant Poynting-flux in the emission region of X-ray flares. As the X-ray
flares are believed to share a similar physical mechanism as the GRB prompt
emission, our finding here hints that the GRB prompt emission jets may also
carry a significant Poynting-flux in their emitting region.
