Small-Scale X-Ray Variability in the Cassiopeia A Supernova Remnant

A comparison of X-ray observations of the Cassiopeia A supernova remnant taken in 2000, 2002, and 2004 with the Chandra ACIS-S3 reveals the presence of several small-scale features (≤10'') that exhibit significant intensity changes over a 4 yr time frame. Here we report on the variability of six features, four of which show count rate increases from ∼10% to over 90%, and two of which show decreases of ∼30%-40%. While extracted 1-4.5 keV X-ray spectra do not reveal gross changes in emission-line strengths, spectral fits using nonequilibrium-ionization, metal-rich plasma models indicate increased or decreased electron temperatures for features showing increasing or decreasing count rates, respectively. Based on the observed count rate changes and the assumption that the freely expanding ejecta has a velocity of ∼5000 km s^-1 at the reverse shock front, we estimate the unshocked ejecta to have spatial scale variations of 0.02-0.03 pc, which is consistent with the X-ray-emitting ejecta belonging to a more diffuse component of the supernova ejecta than that seen in the optically emitting ejecta, which have spatial scales of ∼10^-3 pc.