  The Central Compact Object (CCO) in the Cassiopeia A supernova remnant is
most likely a very young ($\approx 300$ yr) neutron star. If a previously
reported decrease of its surface temperature by 4% in 10 years could be
confirmed, it would have profound theoretical implications for neutron star
physics. However, the temperature decrease was inferred from Chandra ACIS data
affected by instrumental effects which could cause time-dependent spectral
distortions. Employing a different instrument setup which minimizes spectral
distortions, our 2006 and 2012 Chandra spectra of the CCO did not show a
statistically significant temperature decrease. Here, we present additional
observations from 2015 taken in the same instrument mode. During the time span
of 8.5 years, we detect no significant temperature decrease, using either
carbon or hydrogen atmosphere models in the X-ray spectral fits. Our
conservative $3\sigma$ upper limits correspond to $<3.3$% and $<2.4$%
temperature decrease in 10 years for carbon atmosphere model fits with varying
or constant values of the absorbing hydrogen column density, respectively. The
recently revised model for the ACIS filter contaminant has a strong effect on
the fit results, reducing the significance of the previously reported
temperature and flux changes. We expect that a further improved contaminant
model and longer time coverage can significantly lower the upper limits in the
future.
