  We report on the study of six Chandra observations (four epochs) of the
Central Compact Object (CCO) in the Cassiopeia A supernova remnant with the
ACIS instrument in the subarray mode. This mode minimizes spectrum-distorting
instrumental effects such as pileup. The data were taken over a time span ~ 14
years. If a non-magnetic carbon atmosphere is assumed for this youngest known
CCO, then the temperature change is constrained to be $\dot{T}=-2900\pm 600$ K
yr$^{-1}$ or $\dot{T}=-4500\pm 800 $ K yr$^{-1}$ ($1\sigma$ uncertainties) for
constant or varying absorbing hydrogen column density. These values correspond
to cooling rates of $-1.5 \pm 0.3$% per 10yr and $-2.3 \pm 0.4$% per 10yr,
respectively. We discuss an apparent increase in the cooling rate in the last
five years and the variations of the inferred absorbing hydrogen column
densities between epochs. Considered together, these changes could indicate
systematic effects such as caused by, e.g., an imperfect calibration of the
increasing contamination of the ACIS filter.
