The effects of Dense Medium on the Electron Acceleration in Kepler's SNR: A Multiwavelength Exploration.

Supernova remnants (SNRs) frequently exhibit electron acceleration within their shocks, with the maximum achievable energy constrained by radiative losses. This limitation arises when the synchrotron cooling time scale matches the acceleration time scale. Conversely, the slow speed of a shock propagating through a dense medium is expected to lead to a higher acceleration time scale, leading to a diminished maximum electron energy and nonthermal X-ray flux for a given SNR age. We investigated the case of Kepler's SNR, a young remnant which exhibits a shock evolving within diverse environments, employing various independent diagnostics spanning different wavelengths. Leveraging a spatially resolved spectral analysis in the X-rays, we identified an enhanced efficiency of the acceleration process, approaching the Bohm limit in the north of its shell, where the shock is impeded by a dense circumstellar medium. Our findings were validated by scrutinizing radio polarization fractions; we indeed retrieved a lower radio polarization fraction in the north of the remnant, associated with a more turbulent magnetic field, hence a lower Bohm factor. Specifically focusing on synchrotron filaments using Chandra, we identified a region with a low shock velocity and a large acceleration time over cooling time ratio. In this area, we observed a significant decline in flux from 2006 to 2014, marking the initial evidence of fading synchrotron emission in Kepler's SNR. Ultimately, by combining hard X-ray spectra with radio and gamma-ray observations of Kepler's SNR, we constructed a comprehensive spectral energy distribution. Our results suggest that the observed gamma-ray emission is compatible with being predominantly hadronic and originating in the northern part of the shell.