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The thermal evolution of young neutron stars (NSs) reflects the neutrinoemission 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 2000and 2009, using archival data from the Chandra X-ray Observatory ACIS-Sdetector in Graded mode. Page et al. (2011) and Shternin et al. (2011)attributed this decay to enhanced neutrino emission from a superfluid neutrontransition in the core. Here we test this decline, combining analysis of theCas 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-SGraded mode observation, using the most current calibrations (CALDB 4.5.5.1).We measure the temperature changes from each detector separately and test forsystematic effects due to the nearby filaments of the supernova remnant. Wefind a 0.92%-2.0% decay over 10 years in the effective temperature, inferredfrom HRC-S data, depending on the choice of source and background extractionregions, with a best-fit decay of 1.0+/-0.7%. In comparison, the ACIS-S Gradeddata indicate a temperature decay of 3.1%-5.0% over 10 years, with a best-fitdecay of 3.5+/-0.4%. Shallower observations using the other detectors yieldtemperature 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 of2.9+/-0.9 (stat) +1.6/-0.3 (sys) % over 10 years. The complexity of the brightand varying supernova remnant background makes a definitive interpretation ofarchival 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 beregulated by superfluidity of nucleons in the stellar core.

Measuring the Cooling of the Neutron Star in Cassiopeia A with all Chandra X-ray Observatory Detectors