Recent discovery of the year-scale variability in the synchrotron X-ray emission of the supernova remnant ( SNR ) RX J1713.7 - 3946 has initiated our study of multi-epoch X-ray images and spectra of the young SNR Cassiopeia A based on the Chandra archive data taken in 2000 , 2002 , and 2004 .
We have found year-scale time variations in the X-ray intensity for a number of X-ray filaments or knots associated with the reverse-shocked regions .
The X-ray spectra of the variable filaments are characterized by a featureless continuum , and described by a power law with a photon index within 1.9–2.3 .
The upper limits on the iron K-line equivalent width are 110 eV , which favors a synchrotron origin of the X-ray emission .
The characteristic variability timescale of 4 yr can be explained by the effects of fast synchrotron cooling and diffusive shock acceleration with a plausible magnetic field of 1 mG .
The X-ray variability provides a new effective way of studying particle acceleration at supernova shocks .