  The thermal evolution of young neutron stars (NSs) reflects the neutrino
emission properties of their cores. Heinke et al. (2010) measured a 3.6+/-0.6%
decay in the surface temperature of the Cassiopeia A (Cas A) NS between 2000
and 2009, using archival data from the Chandra X-ray Observatory ACIS-S
detector in Graded mode. Page et al. (2011) and Shternin et al. (2011)
attributed this decay to enhanced neutrino emission from a superfluid neutron
transition in the core. Here we test this decline, combining analysis of the
Cas A NS using all Chandra X-ray detectors and modes (HRC-S, HRC-I, ACIS-I,
ACIS-S in Faint mode, and ACIS-S in Graded mode) and adding a 2012 May ACIS-S
Graded mode observation, using the most current calibrations (CALDB 4.5.5.1).
We measure the temperature changes from each detector separately and test for
systematic effects due to the nearby filaments of the supernova remnant. We
find a 0.92%-2.0% decay over 10 years in the effective temperature, inferred
from HRC-S data, depending on the choice of source and background extraction
regions, with a best-fit decay of 1.0+/-0.7%. In comparison, the ACIS-S Graded
data indicate a temperature decay of 3.1%-5.0% over 10 years, with a best-fit
decay of 3.5+/-0.4%. Shallower observations using the other detectors yield
temperature decays of 2.6+/-1.9% (ACIS-I), 2.1+/-1.0% (HRC-I), and 2.1+/-1.9%
(ACIS-S Faint mode) over 10 years. Our best estimate indicates a decline of
2.9+/-0.9 (stat) +1.6/-0.3 (sys) % over 10 years. The complexity of the bright
and varying supernova remnant background makes a definitive interpretation of
archival Cas A Chandra observations difficult. A temperature decline of 1-3.5%
over 10 years would indicate extraordinarily fast cooling of the NS that can be
regulated by superfluidity of nucleons in the stellar core.
