Direct Observation of the Cooling of the Cassiopeia A Neutron Star

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.