Upper Limits on the Rapid Cooling of the Central Compact Object in Cas A

The Central Compact Object (CCO) in the Cassiopeia A supernova remnant is most likely a very young (≈300 years) 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 that could cause time-dependent spectral distortions. Employing a different instrument setup that 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 3σ upper limits correspond to <3.3% and <2.4% temperature decreases 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.