  The cooling rate of young neutron stars gives direct insight into their
internal makeup. Although the temperatures of several young neutron stars have
been measured, until now a young neutron star has never been observed to
decrease in temperature over time. We fit 9 years of archival Chandra ACIS
spectra of the likely neutron star in the ~330 years old Cassiopeia A supernova
remnant with our non-magnetic carbon atmosphere model. Our fits show a relative
decline in the surface temperature by 4% (5.4 sigma, from 2.12+-0.01*10^6 K in
2000 to 2.04+-0.01*10^6 K in 2009) and observed flux (by 21%). Using a simple
model for neutron star cooling, we show that this temperature decline could
indicate that the neutron star became isothermal sometime between 1965 and
1980, and constrains some combinations of neutrino emission mechanisms and
envelope compositions. However, the neutron star is likely to have become
isothermal soon after formation, in which case the temperature history suggests
episodes of additional heating or more rapid cooling. Observations over the
next few years will allow us to test possible explanations for the temperature
evolution.
