New constraints on the cooling of the Central Compact Object in Cas A
  To examine the previously claimed fast cooling of the Central Compact Object
(CCO) in the Cas A supernova remnant (SNR), we analyzed two Chandra
observations of this CCO, taken in a setup minimizing instrumental spectral
distortions. We fit the two CCO X-ray spectra from 2006 and 2012 with hydrogen
and carbon neutron star atmosphere models. The temperature and flux changes in
the 5.5 years between the two epochs depend on the adopted constraints on the
fitting parameters and the uncertainties of the effective area calibrations. If
we allow a change of the equivalent emitting region size, R_Em, the effective
temperature remains essentially the same. If R_Em is held constant, the
best-fit temperature change is negative, but its statistical significance
ranges from 0.8sigma to 2.5sigma, depending on the model. If we assume that the
optical depth of the ACIS filter contaminant in 2012 was +/-10% different from
its default calibration value, the significance of the temperature drop becomes
0.8sigma to 3.1sigma, for the carbon atmospheres with constant R_Em. Thus, we
do not see a statistically significant temperature drop in our data, but the
involved uncertainties are too large to firmly exclude the previously reported
fast cooling. Our analysis indicate a decrease of 4%-6% (1.9-2.9sigma
significance) for the absorbed flux in the energy range 0.6-6keV between 2006
and 2012, most prominent in the 1.4-1.8 keV energy range. It could be caused by
unaccounted changes of the detector response or contributions from unresolved
SNR material along the line of sight to the CCO.
