  We investigate time variations and detailed spatial structures of X-ray
synchrotron emission in the northeastern limb of SN 1006, using two Chandra
observations taken in 2000 and 2008. We extract spectra from a number of small
(about 10") regions. After taking account of proper motion and isolating the
synchrotron from the thermal emission, we study time variations in the
synchrotron emission in the small regions. We find that there are no regions
showing strong flux variations. Our analysis shows an apparent flux decline in
the overall synchrotron flux of about 4% at high energies, but we suspect that
this is mostly a calibration effect, and that flux is actually constant to
about 1%. This is much less than the variation found in other remnants where it
was used to infer magnetic-field strengths up to 1 mG. We attribute the lack of
variability to the smoothness of the synchrotron morphology, in contrast to the
small-scale knots found to be variable in other remnants. The smoothness is to
be expected for a Type Ia remnant encountering uniform material. Finally we
find a spatial correlation between the flux and the cut-off frequency in
synchrotron emission. The simplest interpretation is that the cut-off frequency
depends on the magnetic-field strength. This would require that the maximum
energy of accelerated electrons is not limited by synchrotron losses, but by
some other effect. Alternatively, the rate of particle injection and
acceleration may vary due to some effect not yet accounted for, such as a
dependence on shock obliquity.
