  The maximum energy of electrons in supernova remnant (SNR) shocks is
typically limited by radiative losses, where the synchrotron cooling time
equals the acceleration time. The low speed of shocks in a dense medium
increases the acceleration time, leading to lower maximum electron energies and
fainter X-ray emissions. However, in Kepler's SNR, an enhanced electron
acceleration, which proceeds close to the Bohm limit, occurs in the north of
its shell, where the shock is slowed by a dense circumstellar medium (CSM). To
investigate whether this scenario still holds at smaller scales, we analyzed
the temporal evolution of the X-ray synchrotron flux in filamentary structures,
using the two deepest Chandra/ACIS X-ray observations, performed in 2006 and
2014. We examined spectra from different filaments, we measured their proper
motion and calculated the acceleration to synchrotron time-scale ratios. The
interaction with the turbulent and dense northern CSM induces competing effects
on electron acceleration: on one hand, turbulence reduces the electron mean
free path enhancing the acceleration efficiency, on the other hand, lower shock
velocities increase the acceleration time-scale. In most filaments, these
effects compensate each other, but in one region the acceleration time-scale
exceeds the synchrotron time-scale, resulting in a significant decrease in
nonthermal X-ray emission from 2006 to 2014, indicating fading synchrotron
emission. Our findings provide a coherent understanding of the different
regimes of electron acceleration observed in Kepler's SNR through various
diagnostics.
