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

A comparison of X-ray observations of the Cassiopeia A supernova remnanttaken in 2000, 2002, and 2004 with the Chandra ACIS-S3 reveals the presence ofseveral small scale features (<= 10 arcsec) which exhibit significant intensitychanges over a 4 year time frame. Here we report on the variability of sixfeatures, four of which show count rate increases from ~ 10% to over 90%, andtwo which show decreases of ~ 30% -- 40%. While extracted 1-4.5 keV X-rayspectra do not reveal gross changes in emission line strengths, spectral fitsusing non-equilibrium ionization, metal-rich plasma models indicate increasedor decreased electron temperatures for features showing increasing ordecreasing count rates, respectively. Based on the observed count rate changesand the assumption that the freely expanding ejecta has a velocity of ~ 5000km/s at the reverse shock front, we estimate the unshocked ejecta to havespatial scale variations of 0.02 - 0.03 pc, which is consistent with the X-rayemitting ejecta belonging to a more diffuse component of the supernova ejectathan that seen in the optically emitting ejecta, which have spatial scales ~0.001 pc.Comment: 9 pages, 8 figures, to be published in Astronomical Journa