The maximum energy of electrons accelerated by supernova remnants (SNR) is typically limited by radiative losses.
In this scenario, the synchrotron cooling time scale is equal to the acceleration time scale.
On the other hand, the low propagation speed of a shock in a dense medium is expected to result in an extended acceleration time scale, thus inducing a decrease in the maximum electron energy for a given SNR age and in the X-ray nonthermal flux.
The young Kepler's SNR shows an enhanced efficiency of the acceleration process, which is close to the Bohm limit in the north of its shell, where the shock is slowed down by a dense circumstellar medium.
Conversely, in the south, where no interaction with a dense medium is evident and the shock speed is high, the acceleration displays a higher Bohm factor.
To investigate this scenario, we studied the temporal evolution of the non-thermal emission, taking advantage of two Chandra X-ray observations of Kepler's SNR (performed in 2006 and 2014).
We analyzed the spectra of different filaments both in the north and south of the shell, and measured their proper motion.
We found a region with low shock velocity where we measured a significant decrease in flux from 2006 to 2014.
This could be the first evidence of fading synchrotron emission in Kepler's SNR.
This result suggests that under a certain threshold of shock speed the acceleration process could exit the loss-limited regime.